High current BMS suggestions

Hello

I'm currently building a 28s5P pack for my ME1507 and Sevcon Gen4 Size6 controller for the EV motorcycle I'm building. I've got the batteries on the way (Headway 38120 LiFePo4 cells) and thanks to their phenomenal discharge capacity, I'm fully expecting to be maxing out the amperage of the controller.

My question is on the BMS. I'd like to keep all these cells in good shape and balanced, but I'm confused on the BMS options. Many of them are smaller, only rated for 50A or 100A, but they're capable of my 28 cells in series. Do I need a BMS rated for 600A or can I get away with one that's rated for my continuous use, which is closer to 50A.

Or, can I just not use the high current leads on these BMS's and just use the BMS for cell balancing?

Thanks everyone.

JKBMS has units that are good up to 300 amps. Frequently at that high of discharge levels though, you'll see either:
4-8s units, 600 amp rated, connected in series to get to your 28s. Teslas do something like this, you could read into their systems more. Or,
28s units, rated for 100 amps or so, connected in parallel, fused at 100 amps on each BMS's positive lead. You said you were going for 5p, so theoretically, you could use 5ea 28s BMS, say 150 amp rated each. That would give you 5 strings, they just connect them in parallel. It's a wiry mess, but its doable.

I personally wouldn't choose to bypass the BMS for discharge and just keep it around for balance charging. But that's just me. Other people have done it.

May I ask: you have a controller rated for 600A, and 5p of headway cells (carefull, they're 200A-rated specs are a little exaggerated, but you're using 5p so you should be ok). Why do you mention that you're only expecting 50A continuous? If that's the case, is your controller a bit overkill? Just asking, you may have a good reason for it. I myself like to select components overspecced for my use to prolong their lifespan, but that does seem a bit excessive.

harrisonpatm said:

May I ask: you have a controller rated for 600A, and 5p of headway cells (carefull, they're 200A-rated specs are a little exaggerated, but you're using 5p so you should be ok). Why do you mention that you're only expecting 50A continuous? If that's the case, is your controller a bit overkill? Just asking, you may have a good reason for it. I myself like to select components overspecced for my use to prolong their lifespan, but that does seem a bit excessive.

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I got the size 6 because it came with the motor. Motenergy sold them to me together for $900 back in 2018. It was oversized, but also a steal so I took it.

I'm guessing 50A because that's what the cruise amperage is around. Heavy gassing is up by 600A, but that's only about 1% of the time I'll be using it.

Basically, I just wanna balance cells but I don't know what my options are. Everything I see from what I've searched for is BMS that you wire the actual load through the BMS. (except for Tesla, which you mentioned) LiFePO4 cells can take one hell of a beating, and I'm supplying more amperage than the controller can take so the controller is the weak link.

Ok, so option 3, is that you could get the JKBMS or Daly 300amp, which is about as high as you can get for off the shelf. Then you can go into the bms and adjust settings. The JKBMS has an overcurrent protection delay feature. Meaning, it will allow current to flow above its rated load for 10 seconds. Or 5, or 20, however long you set. If you're sure that you're only going to cruise at 50 amps and only see high current for very short times, this is one path you could take. Just know that the more you increase that delay, the more you risk damaging the BMS.

I use a 200-amp rated bms, and I see peaks up to 300. I have my delay set to 10s. I also use LiFePO4, so I know my pack can handle something like 400-600, in short bursts. Haven't had an issue yet. But again, it's a calculated risk.

harrisonpatm said:

Ok, so option 3, is that you could get the JKBMS or Daly 300amp, which is about as high as you can get for off the shelf. Then you can go into the bms and adjust settings. The JKBMS has an overcurrent protection delay feature. Meaning, it will allow current to flow above its rated load for 10 seconds. Or 5, or 20, however long you set. If you're sure that you're only going to cruise at 50 amps and only see high current for very short times, this is one path you could take. Just know that the more you increase that delay, the more you risk damaging the BMS.

I use a 200-amp rated bms, and I see peaks up to 300. I have my delay set to 10s. I also use LiFePO4, so I know my pack can handle something like 400-600, in short bursts. Haven't had an issue yet. But again, it's a calculated risk.

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Good answer, thank you for your time. :thumb:

There are bms:es that control a contactor that can handle more amps, I have only seen it in another recommendation.
Another option is to use external contactors, and just let the bms control the coils (trough a dc dc converter) like I do on my yamaha.

j bjork said:

Another option is to use external contactors, and just let the bms control the coils (trough a dc dc converter) like I do on my yamaha.

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Would you be able to diagram that? I've also read about that a couple of times, but I can't quite wrap my head around it.

One way to do the BMS-controlled-contactor:

https://endless-sphere.com/forums/viewtopic.php?f=2&t=117890&p=1735095#p1735095

harrisonpatm said:

j bjork said:

Another option is to use external contactors, and just let the bms control the coils (trough a dc dc converter) like I do on my yamaha.

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Would you be able to diagram that? I've also read about that a couple of times, but I can't quite wrap my head around it.

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It is almost exactly like the description Amberwolf linked to, except I have a switch in the line between the bms and contactor coil so I can turn off the contactors without having to turn off the bms.
I also have my contactors on the positive side, but that is mostly a question of taste

Dont forget to add a pre charge circuit in parallel with the contactor.