1. My BRUSA NLG5 charger gives me “STD Batt Volt Hi” error (overvoltage error) while the battery never exceeds its preset output . Why?
– Please check the settings in the “Emergency Shutdown If” section of the ChargeStar GUI supplied with charger. There is “Battery voltage above, [V]” shutdown criteria responsible for preventing too much voltage on the charger DC output in case of circuit failure: battery disconnect, fuse blow, loose wire etc. This setting is not meant for any kind or regulation or limiting during normal operation and therefore margins have to be wide enough so only obvious circuit problems will trip protection.
The way charger works is it initially injects current into the battery circuit and waits for voltage or current regulation (whichever limit comes first) to get established. Therefore initial voltage overshoot can be significant. Lasting for just a few microseconds voltage peaks present at the output of any switching device are harmless to the battery, but if occur, will trip fast hardware based overvoltage detection circuitry. Therefore this emergency shutdown voltage must be at least 50VDC higher than maximum battery voltage on charge; 75 to 100V higher is safe enough margin. Should the battery disconnect, even small current injected into the open circuit will cause voltage to shoot to the charger hardware limit (520 VDC for NLG513) and trip the protection, which is whole purpose of the circuit. Setting this voltage very conservatively just 10…20V higher than max. battery voltage trying to protect battery from “run away” charger is wrong – it will certainly trip overvoltage detector and shut down charger as soon as you plug it in the mains or enable it – initial short voltage spike caused by current injection before regulation is established, especially for multi-cell battery, will certainly exceed this small margin. If this error still occur, check your fuse holders, service disconnects, contactors if any, anything between charger output and battery terminals. There is likely either poor contact somewhere in circuit or internal resistance of the battery (especially at low temperatures) has become so high than even mild current injection causes momentary overvoltage to occur. Older batteries are especially prone to this. If all connections are OK, you can slow down initial current ramp by splitting it in several steps or commanding lower rate of increase over CAN, or connect large electrolytic capacitors in parallel to the battery – it should absorb any such voltage spikes. Capacitance should be around 200…500 uF per each amp of charging current. The older the battery and worse potential condition of external circuitry – the more capacitance will be needed. Connect the capacitor directly to the output of the charger (before fuses or service disconencts), not at the battery terminals. Please remember: the charger is designed to be permanently wired to the battery for the life of its service, and its output subjected to the battery voltage 24/7. If you use contactors to connect and disconnect battery from the charger, precharging its output internal capacitors (and along – the capacitor you may place in parallel as discussed above) is mandatory. In any case, even without your own extra cap, FAILURE TO PRECHARGE OUTPUT OF NLG5xx CHARGER EVERY TIME HIGH VOLTAGE BATTERY IS CONNECTED TO IT HAS BEEN KNOWN TO DESTROY INTERNAL C-CONVERTER CIRCUIT AND RENDER CHARGER INOPERABLE.
2. Do I need a BMS (Battery Management System) for my LiFePO4 battery pack in my EV?
Short answer – YES.
The topic is too important to be satisfied with short answer only, therefore here is
I recently had a couple of customers willing to install EVision system in their home built EV conversions equipped with LiFePO4 cells. What’s alarming – both were convinced that the BMS is totally optional and they can live without it. Evidently this idea that the main purpose of BMS manufacturers is to extract money from naive or ignorant customers while there is no real value in having BMS in place, being promoted by some people on the internet. While money extraction aspect certainly does not apply to Metric Mind Corporation (we don’t sell BMS’es as generic boxed product, so we won’t benefit from debunking those money extraction ideas), let’s stick to technical aspects of the issue. I should only mention that publishers of such [mis]information no doubt have no credibility whatsoever and evidently have no technical background to know what they are talking about. That would be OK if these people would implement their ideas in their own garages, but evidently they feel urge to advise others to do the same. Advocating to adopt such ideas without credible evidence that it is acceptable and safe is great disservice to novices and EV enthusiasts who yet haven’t learned more about the subject.
Other popular notion is that if one just restricts operating range of a stack of lithium cells and will not allow them to get overcharged and overdischarged by just tracking total amount of energy in and out, it will take so many cycles for the cells to drift apart to dangerous levels, that having sophisticated BMS is just not worth it – once in a while re-balancing can be done manually and risk of something going wrong in the middle of such period is acceptably low. Sort of thinking like if you drive slow (slower than people around you), you have less chance to get in the accident. If you one of believers in this concept, please read on.
The objective of this FAQ is to educate you so you understand what is going on. I feel it is my social responsibility to explain facts that can impact well being of my fellow EVers. Intent is not to sway you one way or the other – ultimately to BMS or not to BMS will be up to you. Intent is only to to supply critical information so your decision is based on the knowledge what the risks and rewards are and not on someone’s opinion or reputation. If you don’t understand the substance, gambling by following advises found on the internet without checking credibility of the source is at least unwise and at most dangerous. However if you do understand what really happens and why and still decide to do unwise thing, I have no problem with that – there is no one to stop you. Just like with safety seat belts: no one can MAKE you click your belt, one can only let you know what eventually will happen if you don’t as it did happen to others. Again, natural selection in action.
Back in 2003 when no one had lithium batteries in their hobby conversions I made contact with former Thunder-Sky company (currently – Winston battery – an English name I suggested to the inventor of the battery while visiting factory in China). Pioneering in many aspects of EV technology for EV enthusiasts (making first AC drives available for amateur conversions since 2003, first use of supercapacitors, first lithium ion batteries (Honda ACRX), first informational LED lighting, first use of fiber optics in BMS communication and other innovations), I wanted to stay ahead and adopt new at the time technology. The only large enough lithium cells being made at that time and available for individuals were LiIon with LiMn cathode material made by Thunder-Sky in China. After buying and successfully evaluating a couple of samples (which were hand picked for me) I wanted to make it available to people, but still was skeptical that cells and the factory can actually deliver as expected. Before plunging I decided to visit Thunder-Sky factory in China. All looked legitimate and soon I arranged a group buy getting first batch of 504 cells types TS-LP50, TS-LP90 and TS-LP100. People quickly discovered issues with them – inconsistency in production and purity of materials made cells *very* non-uniform. Few years later Thunder-Sky admitted selling to us substandard cells meant for a dumpster (that’s why you won’t see TS cells in my EVs), but that’s whole another story. Some cells died mysterious death – voltage collapsed to 0V in a few weeks without any apparent reasons, while others even today (10 years later) still have 3.3V on them (!) and while internal resistance is high and most of capacity is lost, they can sustain some load. Cycling cells back then made few of them swell and if unrestrained, plastic case would bulge from both sides, cell would loose all capacity and die. If the swelling on charge was restrained using supplied aluminum bookends and steel straps, bad cells would still die at the same rate, only confined. I dug into the literature, talked to chemical engineers and while exact formulation of these batteries was never revealed, I understood reasons for internal outgassing causing internal pressure built and inner shorts up and as a result – bulged cells as well as internal heating and capacity loss.
Today manufacturing is much improved. Newly emerged manufacturers such as A123, K2, Electrovaya, as well Chinese manufacturers – Thunder-Sky and their copycats (like CALB) no longer offer LiMn manganese cathode cells – they replaced their product line with popular LiFePO4 type. Why? LiIon ones are too delicate and if mishandled – are too dangerous, especially in amateur projects. Numerous documented events can be found online describing lithium battery violently releasing its energy in uncontrollable fashion, outgassing at best and catching fire at worst; that includes some early OEM designs.
Here is where problem starts. In order to ensure sales these batteries are marketed by their respective manufacturers as “inflammable”. (Very much reminds me of Titanic business – the ship was marketed to its investors as “unsinkable”…). You might notice that while the conclusion of the Corvallis university team leader that lithium battery deserves more respect was right, the solution to just replace the cells with “safe” LiFePO4 type [and everything will be taken care of], is totally wrong. Why? Not a word to equip vehicle with better and decent BMS. (By the way, we at Metric Mind Corp. met with couple of team members and offered professional help designing BMS, but evidently the team decided to keep experimenting on their own).
There is something common in both cases – battery manufacturers and Titanic story. That is – many people have bought that notion of almost immunity to destruction, because of lack of information and blind reliance on authority of respective manufacturer. (As far as batteries go, you probably have heard that saying about a battery salesman, haven’t you?). Well, I hate to disappoint you but the real life suggest the opposite – LiFePO4 cells are destructible and do burn quite well. I’ll cite just one example here because it is described in detail and hopefully it will make you think twice before accepting anyone’s ideas that LiFePO4 is magically so safe that no BMS is OK.
Make no mistake: THERE IS NO SUCH THING AS SAFE BATTERIES as song as they contain energy. Lead acid battery included. Some types are unsafe even totally discharged just because of active chemicals inside ready to react with environment if exposed.
What makes lithium battery volatile? Simple: it contains finite amount of energy in finite volume. It is no different than notion about gasoline – we use to think that cars are safe yet, no one will dispute that a gasoline in a tank everyone is sitting on is very volatile substance. Precisely because of very high energy density which, if released, has to go somewhere. Physics tell us that energy does not disappear, it gets transformed in different form. In case of gasoline or batteries this form is *only* heat, nothing else it can get transformed to.
Energy density of the most advanced batteries today is still thousands times less than that of gasoline, and it makes it much harder to trigger release of it. Key word here is *harder* and not impossible and link above is your proof. A lithium battery *will* ignite at around 200’C and evidently burns very well. In case of LiFePO4 manufacturers formulated chemistry such that it raised flash point temperature to 300’C so it is even harder to ignite this battery. This is the source of safety claims. The fact is, modern battery no doubt became safer, but not safe. The battery will never be as safe as ANY other hardware component of an EV simply because components do not possess energy. They can dangerously USE energy supplied to them, but without energy source they are pieces of steel, silicon and plastic. In contrast, batteries are never off. If shorted, they will release energy and you will only have to pray that energy converted to the temperature does not make it exceed 300’C. If not, the battery itself may not ignite, but may ignite surroundings causing fire, again as described per link above. How you may ask who in their right mind will short or heat the battery to 300’C? Nobody on purpose, but that comes back to discussion about seat belts. No one wishes the belts will ever have a need to get to work, yet everyone has them just in case, because while chances are slim, consequences may be deadly. Very same concept regarding the BMS. It is quietly sitting on the background taking care of any abnormality and until abnormality happens it may seem like waste of resources. Only in case of batteries and BMS, unlike seat belts, it is known fact that trouble will happen as individual cells performance diverge (just ask naysayers – what exactly otherwise is the mechanism of self-balancing of individual cells? How do they know about existence of each other let alone each other’s parameters without supervision by a BMS and see these “experts” mumble their expert opinion).
There are classes of failures a BMS cannot and will not prevent. A BMS will not fix a bad cell. If a cell is not made right, it will eventually loose capacity, bulge, short or fail. But the BMS can warn you that such event is approaching so you can remove such cell(s) before actual burst, rupture, terminals meltdown, out-gassing or similar event will take place. That’s the value a good and smart BMS brings to you. A BMS can adjust operating conditions of every *good* cell such that they are all kept within narrow window. All good cells diverge during cycling – again, because manufacturing tolerances and working conditions (mainly temperature) prevent them work in unison. There are simply no means to make them all work in unison (contain about the same amount of energy at all times) other than by the BMS. None. No matter who claims what. And when, therefore, cells naturally drift apart, always lowest and highest energy containing cells will limit pack performance on discharge and charge respectively. This is something BMS can correct. Same applies to internal resistance – something a BMS cannot correct but can warn so you will not wait until bad cell gets reversed and its safety valve blows off releasing stinking gasses.
Now, final words on the topic.
So, knowing all this, is BMS really necessary, can you use an EV without it? Of course you can. You can drive without safety belts too. It would be really stupid thing to do since you’re only cheating yourself, but you sure *can*. Probably better analogy here will be an engine thermostat. It watches for the engine temperature for you and adjusts amount of water flowing through the radiator depending not only on the engine temperature but the ambient temperature, and vehicle speed. Can you use the car without thermostat? Sure! You just have tot stop every half mile, stick thermometer in the radiator, measure, manually adjust water valve, run for another half mile keep doing just that. Else you risk to ruin your engine. But you can do it if you consciously choose to accept that risk. Automatic thermostat removes that risk and you don’t have to worry about engine temperature anymore. If the thermostat cannot cope with extremes, it will lit “idiot light” on the dash so you are aware of the issue. Exactly the same goes for the BMS. You CAN run your EV without it. But if you don’t want to ruin your battery and possible your EV, *you* will be the BMS – once in a while determine capacity of each cell and manually charging or discharging it to bring to the level of average cell. Doable, but extremely tedious, therefore automating this necessary process is delegated to the BMS, just like water temp regulation – to the thermostat. Without these systems AND without you being a substitute for these systems you WILL ruin your batteries (as you would engine). Now, you’re equipped with knowledge to decide.
I’m standing responsible for advising to have a BMS. I wish whoever is advising to use lithium battery of any kind without a BMS would also be held responsible for such advice! But I’m positive if you follow such advice and damage your pack or burn down your EV, publisher of such info will not pay for the damage nor even acknowledging any disservice or doing anything wrong at all. This is free country so anyone can say anything – people are free to misinform others. At the end such “experts” will just ignore you or blame you for your own failures because *you* made your choice!
3. I want to convert my (…insert your car make and model here…) to an electric. I want 70 mph top speed, and about 50 miles range. Which drive system and battery do I need?
You have to realize that there is no answer to this question without knowing much more about your conversion and its purpose. It is similar to asking when buying conventional car “which engine option – gasoline or diesel do I need and what fuel tank size do you recommend?” The answer always will be “it depends”. There is no one to tell you that need exactly 100 hp gas engine and 5 gal tank. Same for electric drive system, it’s not as simple as “50kW drive and 10kWh battery will do it”.
– How quickly you want to be able to reach that speed
– How much you’re willing to pay for that advantage (you don’t expect Porsche to cost the same as Metro even though they both can go faster than legal speed limit and cover 400 (or whatever) miles per tank full, do you?)
– How heavy is your vehicle while doing this (is it a truck hauling cargo trying to merge on the freeway or it’s a sport 2 seater?)
– How long are you planning (in average) to run it at 70 mph – is it really mostly a local commuter or a freeway flyer?
– What is typical terrain where you drive it (flat or hills)?
– What is the shape of your vehicle (how aerodynamic body it has)
– What is primary purpose of the vehicle – a workhorse truck, commuter/errands runner, show off dragster, sipping amps long ranger or somewhere in between? As for gasoline vehicles, you can’t have them all combines in one vehicle. Choose and compromise, build more than one vehicle for its primary purpose.
Continuous power required to move fairly slippery mid size sedan down a freeway at 60 mph is about 15kW. For a truck it is about 25-30kW. At lower speeds the difference is not that drastic.
To climb 6% incline you need 2x of power vs needed on flat terrain. For 10% it is about 4x.
To accelerate a 1500 kg (3300 lb) vehicle to freeway speeds comparably to average gasoline vehicle will require 80…100kW of power.
4. Fundamentally, why would I choose AC system over comparable power DC system?
Probably to say “Because OEMs exclusively use AC drive systems) will not be a good enough answer… OK, few main reasons:
Reason 1 – of course natural regenerative braking without extra hardware. Deceleration during regen can be the same as acceleration during drive – the system is symmetrical in this respect – you can supply into the battery as much current as you can take out of it. It should be mentioned that DC systems with specialized controller and [typically] SepEx DC motor also offer regen (at extra complexity and cost).
Reason 2 – favorable torque – RPM characteristics providing constant torque for wide range of RPM. This provides constant acceleration regardless of the speed (within certain limits), and often allows driving without shifting gears. To get fair comparison, a DC system can be set up to provide constant torque if the controller is programmable – it has to work at the current limit at stall and low RPM. This way constant torque can be achieved as well; however top rotor RPM speed of a typical DC motor remains about twice as low as for an AC motor requiring to shift gears in this case, thus loosing torque at the wheels. Normally vehicles using DC systems avoid need for shifting by starting on 3rd or even 4th gear so the RPM at freeway speeds remains manageable. This, however, aside requiring larger than otherwise necessary motor (to provide high starting torque at the wheels while on 3rd gear), greatly stresses transmission components normally not intended for such abuse, sometimes resulting in broken gear teeth, stripped splines, twisted shafts, damaged CV-joints etc. AC setups don’t have these issues.
Reason 3 – no motor brushes. Sure, unless abused, the brushes can last very long time, and yet there are all these issues with brush advancing, seating, commutator arcing in reverse, self-destroying at high RPM etc. If electric reverse is used, the requirements for brush advance for forward and reverse rotation direction are contradictory. If you want to avoid these problems all together, consider AC motor.
Reason 4 – programmability. Strictly speaking, this is not the property of AC system in oppose to a DC one. A DC controller can be made programmable and be as sophisticated as any AC one. But normally DC controllers cannot optimize motor performance as they are not designed with particular DC motor in mind as a system.
Reason 5 – safety. Well known fact that if a DC controller’s power stage fails, entire pack voltage is applied to the motor and you better have good circuit breakers, fuses, kill switches and good reaction time if partial failure happens to be at the intersection while you are waiting for your green light. In contrast, power stages of AC inverter are used to *generate* excitation for the motor, not regulate power, so in case of a failure AC waveform synthesis just stops and so AC motor just looses power.
Reason 6 – easier (smaller gauge, more flexible) battery wiring. This actually applies only to high voltage AC systems (typically using higher voltage and lower current than DC systems of the same power). Since resistive power losses equal I2R where I is current through a conductor and R is its resistance, the lower current, the lower losses. Note, the *battery* voltage U is not part of equation for copper losses. So with typically lower voltage and thus higher current systems you’re forced to use heavy gauge wire – typically with crossection area ~100 mm2(US gauge 2/0) or larger. Even fine stranded welding cable of that size is not very flexible, heavy duty lugs are large and harder to crimp.
Reason 7: Excellent thermal reserve characteristics of EV AC motors. Technically, it is not AC vs. DC feature, but you will be hard pressed to find a water cooled DC motor. This also means no large fan which takes extra space, consumes extra energy and produces extra noise.
Reason 8: Naturally, electric reverse accomplished by adding only a small toggle switch on the dash. All it takes for an inverter is to swap sequence of 2 phases, so the rotor runs in opposite direction. A DC system requires reversing contactors, not to mention that the brush advance is far from optimal when DC motor runs in reverse and commutator is easily damaged in reverse. Ask me or John Wayland how do we know.
5. Well, but it all comes to the cost. I’ve heard AC systems are much more complex and thus expensive than simpler DC systems providing the same power. Is that true?
That is true, though *much more* is relative. Currently EV AC inverter is more expensive than a DC controller for 2 main reasons: because it is more sophisticated device offering more, and naturally you pay premium for it. AC inverter is always matched to the motor it runs, thus achieving optimal performance and highest efficiency. A software model of the motor is normally stored in the inverter’s memory (therefore AC systems are only sold as matching kits). A DC system does not care (to a degree) what motor is connected to it – it fundamentally it only chops the battery power modulating it, without regard of motor;s magnetic characteristics. Whether all of this is relevant to you is your decision. As with everything, you get what you paid for.
6. Since AC requires high (typically higher than DC) voltage, my battery will be twice as big, heavy and BMS twice as complicated/expensive now.
No. First, and AC drive can be made for any arbitrary low voltage, for instance see an OEM EV with 96V Siemens system – VW CitySTROMer. Recent fork lifts and golf carts with AC drives use typical 36 or 48 V AC systems. High voltage is used to take advantage of high speed AC drive and ability to cover all drive speeds on one gear if desired. Any motor is also a generator and produces voltage as it spins (at the same time as being supplied with voltage). It’s called back EMF (electro motive force) voltage. To make motor produce torque (and so power) you need to supply current in. You only can supply current in if the voltage from the battery exceeds back EMF voltage (which is being subtracted from the battery voltage). So the higher battery voltage – the more RPM AC motor can afford still making torque up there as the battery voltage stay above back EMF longer (toward higher RPM). That is the same concept for any motor, AC or DC, it’s just DC motor can’t handle high RPM because of mechanical limitations of commutator and brushes, so can’t take advantage of higher voltages. Now, back to battery question – the vehicle range only depends on the battery capacity – amount of Wh stored on board. For the same range the amount of Wh (and the battery weight) is the same and has nothing to do with type of the drive system – AC or DC. If you use twice as many batteries to gain voltage for AC drive, each battery can be twice as small (and light and cheap), so total battery weight and cost in theory remains the same. In fact, the battery for AC driven vehicle can be a bit smaller than for typical DC one because of regen. In reality, retail cost of the battery is not exactly proportional to its capacity, if you buy 70Ah lead acid battery for $200, this doesn’t mean you will be able to find the same brand 35Ah one for $100. But it should be close to that proportion, certainly twice as much voltage DOES NOT mean twice as big, heavy and expensive battery (for the same range). One of main disadvantages is indeed – the price of battery regulators or BMS will be higher, although not twice as much.
7. How do I size the motor? How much power do I need? What horse power engine equates to, say, 30 kW motor?
These are very valid and important questions to answer right. First, don’t get obsessed with raw power unless you plan to drag race and enjoy breaking and fixing things.
First – formal relationship. On paper, one horse power equals 746 watt. So, say 200 hp amounts to 147,200 watts, or ~147 kW. This, however, doesn’t mean you need 147 kW electric motor for your car to perform the same as 200 hp gasoline counterpart, and this is why: the power of ICE (Internal Combustion Engine) is always specified at max horse power (or kW) it can produce at the best spot – certain RPM, let say 3000 RPM. Maximum power almost never achieved because engine does not spend any length of time running at 3000 RPM at full power (unless you clime up a steep hill), you will certainly accelerate. So typically the RPM ramps up from ~500 RPM to ~5000 RPM, thus having peak power output just for a fraction of second when crossing 3000 RPM mark. To cruise with steady speed on a freeway it takes about 12 hp and the only reason we have huge 200 hp engines is to be able to accelerate or pass quickly. So vast majority of the time your 200 hp engine really runs at 12 hp. If you ask what *average* power an ICE engine makes in useable RPM range, it will be about 30-40 hp for 200 *peak* hp engine.
In contrast, electric motors are always rated at maximum continuous power output, say 30 kW (which is 40.2 hp). Typically good EV motor can output peak power at least 5 times of rated power (provided, inverter and battery are capable of supplying it), so a 30 kW motor can give you 150 kW (or >200hp) for acceleration or passing. So in a first approximation 30 kW motor might give you at the same *acceleration* performance as 200 hp engine in the same car. Practically, electric motor is even better than that because it develops full torque instantly right from the start (0 rpm) while ICE doesn’t make any power until the shaft ramps up to at least or so 500 RPM. In essence you have far more area under torque curve for electric motor, even though high peak number for engine is higher. Practically this means it feels more powerful than number of kW would suggest. So as a rough rule of thumb, an ICE engine with X hp power will equate to the electric motor with X/4 hp electric motor rating. In our example, 200 hp engine (making 30-40 hp average) will need to be replaced with 200/4=50 hp motor. Since 1 hp is 0.746 kW, 50 hp motor is 50*0.746=37.3 kW. This is what you have to choose for substitution to keep the same performance.
Granted electric motors have their limitations and in some circumstances are not as much superior compared to gasoline engines. When it comes to long runs uphill or towing heavy loads, situation is different. Engine becomes relatively more efficient (say 20% instead of 10%) but most importantly spend most of their running time near peak power RPM. They can supply 200 hp continuously (many minutes or hours). if they have to. Granted, 37kW rated motor cannot provide that power for longer than couple of minutes, it will overheat. You’d need ~150 kW *rated* power motor (and battery and controller/inverter also capable of sustaining this demand). Such motor will likely be as large as heavy as 200hp engine. For such applications (where continuous power demand is close to peak power demand) EVs are not as good alternative to conventional vehicles. Luckily, for typical passenger cars and typical driving pattern transporting people, this is not the case.
8. One of typical emails with typical questions I get quite regularly:
“First, I’m still trying to figure out the motor-transmission question. Will I still need to make my own adapter plate?
Second, What kind of performance can I expect, roughly. Is there any “quick-n-dirty” correlation that can be made between ICE hp and DC (or AC) kW? Even if it’s as simple as 1kw roughly give the equivalent as between 1.5 and 3.6 hp, that would help a lot.
Third, Why would I pay so much for your charger when there are plenty of chargers out there under $500?
The short answer:
There are two kinds of components: Real OEM EV components and components useable in an EV. For example, Siemens, UQM, BRUSA and other AC drive systems as well as BRUSA chargers are designed ground up to be used in OEM EVs.
Except for specific applications like a fork lift or a golf cart, unfortunately there is no OEM EV DC components industry. Manufacturers produce DC motors know they are usable in EVs but the motors, even made by ADC or NetGain are not specifically designed to be EV traction motors for highway capable . That does not mean at all that DC motors out there are not usable for a on-road vehicles, especially for conversions done by hobbyists. I’ve heard that with AC drive no gear switching is needed, making it possible to lock on one gear or going “direct drive” eliminating gear box all together. Is this a good idea?
First, proper definition: it is customary to call fixed (singe) gear transmission “direct drive”. Technically, “direct drive” applies to wheel hub motors or if the motor shaft directly linked to the wheel shaft, no gears, no chains and sprockets, no belts and lulleys. More common understanding, however, is that the presence of any fixed ratio gear box between motor shaft and the wheel shaft still called direct drive as long as no gear switching is possible. For example, connecting the motor directly to the drive shaft of the RWD car would be called “direct drive”, even though final differential with fixed reduction ratio gear set is present.
Due to much wider RPM range with useable torque than DC systems can offer, indeed entire driving speeds range can be covered with one, usually second, gear. People have done it and it works very well. Many OEM AC drives (notably made by MES) even come with AC motor already integrated with single gear reduction gear box for all driving conditions. However one should consider following:
1. The torque at the wheels is what moves your car. Obviously, by retaining the stock gearbox, you retain the option to increase this torque shifting to the lower gear. There can be situations when you need that extra torque:
– Climbing hills. With depleted battery you may be unable to climb 25% grade at all if your transmission is locked on the second gear. You may be unable to climb steep spiral entry to an airport multi-level parking garage or even steep driveway. Having low gears available solves these problems.
– Safer driving. There might be situations when while cruising on the third gear you are squeezed between huge SUVs and they don’t even see you. Getting out of this situation as quick as possible may be critical to avoid an accident. On the second gear you can leap forward in no time leaving those SUVs in the dust – your acceleration will be as much greater than on the third gear as the ratio between these gears (provided you’re in linear torque region of the drive AC motor). Also, for long descents: if your battery is not full you can use continuous regen and will never had chance to overheat disc brakes.
– Fast take offs. If you like to race on the streets and be ahead of everybody when the light turns green, the first gear is an answer. Caution: high torque at the wheels can cause loosing traction on wet surfaces easier than you might expect, be careful.
2. Energy savings. You may find out that on freeway driving on the third gear puts you in most efficient operation spot compare to any other gear. Thus this is preferred gear to spend least amount of precious Wh stored in on-board battery. Ability to select gears on freeway vs. around the town would be very beneficial. This impacts regenerative braking ability and efficiency as well.
3. Lifetime of the motor shaft bearings. The bearings are the only parts of the motor that wears out and determines its lifetime. Depending on RPM it is rated in several thousands of hours of operation which usually far exceeds the life of the vehicle. Obviously, the lower RPM, the longer the motor will last, and likely can migrate to your second conversion car. So if the torque at the wheels is enough to maintain desirable speed on the freeway using third gear, it is advisable to do that rather than stick to the second gear. Running on the second is doable, the motor will likely run at around 5000 RPM (AC motor limit is usually about 10,000 RPM), but it may not be the most efficient spot for the motor. Also on the lower gears the gearbox will get unusually hot increasing its wear and wasting battery energy. So while driving on the lower gears all the time is technically possible, it makes no practical sense and usually is more wasteful.
4. Extra conversion work. The stock gearbox is already there for “free”, assembled, tested and operating reliably and efficiently. It will take extra effort to disassemble the box, remove all “unneeded” gears, lock the shift lever on one remaining gear and put it all back together. As the result, the weight saving is negligible, and you end up with the same result as someone who retained original gearbox but just don’t use other gears. Besides, you no longer *have choice* to switch gears if for whatever reason you want to.
For RWD cars eliminating fear box entirely trying to link the motor directly to the drive shaft doing to differential won’t work because the gear ratio in differential (usually 1:3 to 1:4) is not high enough to get the motor to spin at normal RPM. It’s the same as having it on the forth gear all the time (which is usually close to 1:1 or no gear box at all), but on the forth gear your take offs will be very sluggish and modest incline (6%…8% grade) will be difficult to overcome.
At the highway speeds a wheel makes about 600 RPM, so it’s almost 10 times slower than the optimum motor shaft speeds. Only special multi-pole AC motors (often Brussels DC ones which are technically AC motors) can handle that.
So the conclusion is you want to retain your gears.
9. To clutch or not to clutch?
This is also highly debatable question and really is a matter of preference. Any manual gearbox will allow shifting without disengaging the clutch as long as motor and transmission shafts RPM are nearly the same. After short practice is possible to shift with no clutch fairly quickly. Since in general shifting is not needed, you can drive on the second gear from dead stop to freeway speeds and then, when the situation on the road allows, pick the time and shift to the third gear. This may take 3-5 seconds, but no longer matters much as you already move at steady speed along with the traffic. Test your donor car before conversion – for some cars it is difficult to shift without clutch unless you stop.
In other words, if you want to eliminate the clutch, no problem with the driving, just retain the gear box and ability to shift (don’t remove any gears).
10. How does AC drive systems compare to DC drive systems?
There are two main components in the drive system working together: a motor and a DC power converter (or AC inverter) allowing adjusting amount of power delivered to the motor per driver’s demand. Looking at each component separately most significant differences are:
Most commonly used AC induction motor – in general simpler construction and lower cost than comparable power most common series DC motor. Very reliable, no commutators to fan out, comm bars to lift or brushes to wear or spark. Available in great variety of the sizes and working voltages. Usually higher power models have higher voltage and lower current than DC motors. Torque is constant regardless of RPM (up to a point determined by the voltage, which is usually limited by the inverter). Siemens motors, however will work at as low as 140V or so with high voltage inverters and at just 70V or so (same motors) with low voltage inverters, just performance and efficiency will be poor. Whether it will be good enough, only simulation can tell. Low voltage inverters (65-180V) are not offered for sale in the US and Canada.
Regenerative braking is native for AC motors. Separately excited DC motors also offer regen (with somewhat more complex controller), but most widely used series DC motors are practically not suitable for regen operation mode.
A few examples of the conversions done with Siemens systems at different voltages:
DC motor – in general reliable, but the presence of commutator and carbon brushes is the most troublesome part of a DC motor. Bottom line – brush assembly and commutator ire mechanical parts, and it is known fact that reliability of mechanical parts is not as good as properly designed solid state equivalent. It is similar comparison of a hard drive with solid state RAM disk. Hard drives are very reliable these days, but once in a while you hear that an HD fails, doesn’t read, doesn’t boot and it’s not a matter of whether, but when. AS far as DC motors: brushes spark and wear out. It sure may take long time – thousands of miles. Well, make an error advancing them wrong (or run backwards fast) – brushes will be destroyed in 50 miles. Spring tension have to be within specifications. With increased tension excessive friction and wear of the brushes and copper bars occur; if tension is insufficient, excessive sparking, bouncing and overheating as well as increased EMI level become a problem. For higher power models working voltage is lower and current is higher than that of an AC motor. Torque is highest at 0 RPM and from that point is approximately inversely proportional to RPM (provided, the current has room to increase). Talking about cost, EV specific AC motors are more expensive than industrial fixed frequency motors (due to the special magnetics design to handle variable frequency and high RPM, presence of the shaft speed encoder, and other differences), and mainly because of very low production volumes. Also, very low thermal reserve of DC motors often presents cooling problems.
AC power inverter – far more complex and sophisticated than DC converter. Complexity comes from the need to generate 3 phases of the output voltage with simultaneous frequency and amplitude changes, which is constantly calculated (with high speed CPU) based on the shaft rotation sensor signal, instant output demand and other information. The sine wave shape is usually synthesized using PWM technique. Usually therefore larger and more expensive than PWM DC converters. However, as semiconductor technology advances and control functions are integrated into couple chips, the price difference diminishes rapidly. Most of the complexity and cost of an AC inverter is in its software. In case of inverter failure, AC motor will simply stop since AC voltage for it no longer will be generated.
DC power converter – simpler, lighter and less expensive than comparable power AC inverter. Exclusively use proven Pulse Width Modulation technique (changing duty cycle of relatively high frequency output pulses per driver’s demand) to control amount of energy delivered to the motor. Motor acts as an inductor filtering out (averaging) output current. There is significant draw back – in case of the converter failure, usually full battery power will be applied to the DC motor and vehicle can suddenly get in motion with highest possible motor torque. This potentially dangerous shortcoming is one of the major reasons why large auto makers chose AC systems for their vehicles. High end models are microprocessor controlled while more common models are not programmable and are hard wired pieces of hardware.
Therefore there are very few established mass production (full size EV) manufacturers which will use a DC drive system (mostly Sep-Ex motors). Think about it: trustful large scale manufacturers in Europe like BMW, Mercedes-Benz, Volkswagen, Fiat, and Opel all have unmatched technical experience, they know what they are doing. And they all have chosen industrial strength Siemens drive systems for their production electric cars. Why? There must be a good technical, economical and safety reasons for that, not to mention that Siemens have designed and built its systems specifically for these manufacturers responding to their performance and reliability requirements. In the US Ford and GM also have chosen to use an AC drive system in their pilot production lines. In fact most of the hardware components are directly based on Siemens designs.
For the lack of better analogy AC system vs. DC system comparison is similar to fuel injection vs. carburetor comparison. Both sure work, and it is not about power – a large quad barrel carburetor can result in more raw power than fuel injection system for given engine displacement. In essence carburetor is a hole with a shutter controlled by the driver’s foot, a variable valve. Similarly, DC controller is a valve between battery and DC motor. Carburetor does not generate fuel flow, it controls existing flow. That is why if return spring breaks, you get full throttle flow, just like failed DC controller. On the other hand, AC system is like fuel injection system – it generates the flow. If it breaks – no flow and no runaway situation.
Yes, the simplicity is an advantage, no one chooses more complex (and so potentially less reliable) solution without benefits which outweigh drawbacks of complexity. It is easy to understand carburetor and troubleshoot it in the garage, and much more difficult to understand fuel injection system unless its schematics and firmware source code is provided. Yet, fuel injection systems practically took over. Why? Because potential benefits such as extreme flexibility, getting inputs from multiple sensors and ease of changing characteristics – it’s all in software. Upgrade the chip and you get totally different behavior. And, reliability of more complex fuel injection system is the same or better than that of simple carburetor. When was last time your fuel injection computer broke?
Don’t get wrong impression. For years DC systems have been successfully used in EVs, especially small types (fork lifts, golf carts, airport shuttles, and such). Both system types have their uses, and low budget DC systems may be better to start with. Once you taste an AC solution though, you won’t go back.
11. What is the best way to increase the power of the system?
The mechanical power of the motor really represented by the torque of its shaft (for given RPM). The torque depends on the motor current, and the current primarily depends on the voltage applied (Ohm’s law). if you want more power and about to pick the motor, for given power go with higher voltage. Of course in any case applied voltage must exceed so-called back-EMF so the battery amps still can flow into the motor. One nice side effect – reduced heat losses on the connectors and wires which are less bulky than the ones designed to handle high currents (the power lost on the cables and terminals is proportional to square of the current, P=I2R).
12. Which motor do you recommend ?
This is similar to asking the salesman: “Should I get this Honda with V4 or V6 engine in it?” No one can answer this question of personal preference for you. I can tell you exactly what the performance will be (0-60 mph acceleration and top speed) with the motor of your choice, but I can’t make or recommend that choice, it really depends how much fun *you* want driving and how well you tolerate not-so-sporty acceleration. Obviously, with more powerful motor performance will be better, but still the judgment call is yours: the motor should fit your expectations, driving style and purpose of the vehicle.
13. Is the regenerative braking current adjustable?
Yes, in two different modes. The regen current by off throttle is proportional to the pedal position (between completely off throttle and pre-definable point (as default – last 10% of the pedal travel). Beyond that there is either predefined regen current controlled by the brake light switch, or proportional control if optional brake potentiometer is used. These modes are described in detail in downloadable
14. Can it regen rolling backwards?
Yes, it can, same as forward.
15. What will happen if I press accelerator and regen pedal simultaneously?
You set regen or acceleration pedal priority in the software. If you choose accelerator, then if both pedals pressed simultaneously, regen input is ignored, and vice versa.
For other specific questions welcome to call or e-mail to Metric Mind Corporation