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| LiFePO4 balancing. |
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| paulca:
I have seen people white smoke BMS's by testing the overcurrent protection. I believe the device had 20 mosfets (some odd back to back "bridge" config) 10 x 170A brutes in parallel and claiming something like 300A breaking current. Turns out it did break the current path, but unfortunately for one poor little mosfet at the end, it was the slowest. Puff! Cracked through the case, skid marked the board. Chinese electronics, halve the stated ratings. If you "need" 300A breaking current, buy 600A. I personally plan on only really using 40A, but technically the battery is 100A capable and should be cabled and fused accordingly. Thus a 100A BMS. The only time the over current protection which I believe is 300A rated is going to be a question is in a short circuit or serious overload scenario and the BMS popping a few mosfets trying to halt that, I'm ok with that. £50 BMS is cheaper than the potential damage a short will ultimately lead to. To be honest, I might consider setting the overcurrent protection on the BMS above the battery DC breaker rating, which is probably cheaper still to replace if it dies breaking 300A short circuit. I don't need to worry about overcurrent under charge, my solar panels have known limits. On the "balancing" aspects, I am picking a BMS that has fine grained settings over the balancer. When it comes on, when it goes off, what imbalance it will tolerate etc. Having it come on as soon as one cell hits 3.65V and remaining on while there is a >20mV imbalance, using it's full 2A active balancing path... ONLY for top balancing. Then disconnecting 99% of the time. The BMS would also have solved todays 3.9V mishap. The "charge mosfets" would have shut down the incoming current from the solar panel as soon as a single broke a 3.75V limit (by example). |
| SiliconWizard:
--- Quote from: Zucca on March 22, 2023, 05:54:24 pm ---I mean instead to built clever protection in the battery we should use clever chargers and clever loads. --- End quote --- Uh, no. I certainly would not second that. While some BMS in cheap (read: crap) batteries may not work well, adding a reasonable level of protection inside the battery shell itself is a must IMO. LiFePO4 are not as dangerous as standard LiIon, so they can take a good amount of beating, but still. That said, for sure if you buy "random" chinese batteries from shaky sources, the odds of getting something reliable is pretty low, and I'm not surprised about your experience with the built-in BMS. Generally speaking, you protect what needs to be protected (here the battery) as close to it as possible, rather than rely on "clever" stuff connected to it. Besides, your clever stuff is never going to be as clever as you thought. Maybe you can also rely on clever users rather than add any protection to a product, in the same vein. Just my 2 cents. |
| paulca:
--- Quote from: SiliconWizard on March 22, 2023, 09:09:32 pm ---Maybe you can also rely on clever users rather than add any protection to a product, in the same vein. Just my 2 cents. --- End quote --- I tried this recently. I put zero protection on a PCB I had printed. Willingly. Believing, "It's a dev board, only I will be using it. I'm not stupid." Well. Guess what? 2 voltage regulators and an Optical->I2S module later, I'm regretting that decision. On my other battery builds, most appropriately the one in context is the "growing, budding, house battery style" with LiFePO4 cells. It's pay day in a few days. The shopping cart contains * a BMS (the best reviewed one from AliExpress) JK-BMS. Which includes RS-485 monitoring and control. * a small dual way din enclosure * a 100A DC compatible breaker (not from aliexpress!) * 2 large 100A capable 'female' bulkhead style screw terminals, ideally with safety caps. The DC Breaker will be overcurrent protection and (assuming I can find a dual pole breaker) the isolator switch. I know using a breaker RCBO as an isulator switch is frowned upon, but much like your air-cond or heating system the battery will very, very rarely get turned off. As I have the mechanical skills of a lettuce that has been microwaved for 30 seconds, I am placing the battery inside a "Really Useful Box" with the only exposed bits being the female + and - terminals and... the BMS display/rs-485 cables. |
| Zucca:
--- Quote from: SiliconWizard on March 22, 2023, 09:09:32 pm --- --- Quote from: Zucca on March 22, 2023, 05:54:24 pm ---I mean instead to built clever protection in the battery we should use clever chargers and clever loads. --- End quote --- clever stuff is never going to be as clever as you thought. --- End quote --- for "clever" I meant KISS or appropriate solution. Example to limit the current and prevent fire, a simple fuse is a very clever solution. To me better than any µP or high tech BMS. Why the closer to the cells the better? I mean a good protection circuit will work either if inside the battery pack as well as integrated in the load or charger. Of course a temp sensors on the cell can`t be avoided in some application, but why this general concept to put a protection or BMS or whatever in the cells pack? A battery pack is a consumable device, we should be able to replace it throwing away the less components as possible.... |
| Zucca:
--- Quote from: paulca on March 22, 2023, 09:11:39 pm ---zero protection on a PCB I had printed --- End quote --- IMHO the problem here is the zero protection of course. |
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