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 (as of this writing, 2011) 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 unvise 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 automatically diverts right amount of water to the engine block 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 to stop every half mile, stick thermometer in the radiator, measure, manually adjust water valve, run for another half mile and keep driving like 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 a 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 your engine).
Now, you’re equipped with some 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!