Basic Nissan Leaf 7 module Lithium setup with Generic Chinese BMS

[Murby]

Originally Posted by Volt_Ampere

My home designed BMS is completely isolated from the pack when the power is off. (either key off or not charging) In addition when I left the cart for months I completely isolated the pack. It was exactly at the same voltage when I returned 5 months later.

How did you isolate the cell taps?

[Murby]

Originally Posted by DaveTM

I did a quick search of the REC BMS and from what I see this unit will cost about $1,000.

I have been using a ZEVA BMS for (I'm gonna guess) 3-4 years now with no real issues. (Pouring rain in the monitor....but that dried out and all was good again) at a cost of $500.

I don't see the benefit of spending another $500 for a unit that does the same thing.

Maybe that's me.

Zeva is a decent BMS.. Only reason I don't use it is because the balance current is jokingly small. But then so is a golf cart pack so they match up.

I haven't converted my cart from lead acid yet, and a Zeva would never be able to balance my 25kWh off-grid system.

But yup. Zeva is acceptable.. not some Chinese piece of junk.

The REC BMS itself is about $383 USD.. (315 Euros) + shipping. By the time you purchase the coms cable, external relays, etc, you're up in the $500 range.

The biggest difference is that the REC will balance at up to 1.3 amps, which is important on large packs.

[Volt_Ampere]

Originally Posted by Murby

How did you isolate the cell taps?

With Optical FET switches.

[Murby]

Originally Posted by Volt_Ampere

With Optical FET switches.

Good idea..

I isolate the negative terminal on my battery bank, but I leave the cell taps plugged in to the BMS.

I've always wondered if that would cause a drain and if so, how much. I shut the system down in late November 2020 and put it into storage mode at 51.56 volts (3.68v per cell)
Created a log to monitor the voltage and check it once a month with a Fluke meter.
After 6 months, the meter bounces between 51.55 and 51.56. Checked it last night and was at 51.56 volts.

Seems that the ambient temperature of the basement has a small effect on the meter reading. During the winter, the basement is around 45 to 50 degrees, as the summer heats up, it can climb to as high as 65 and 70 down there.

[Volt_Ampere]

I don't know how well most BMS isolate themselves from the pack. Some use some of the pack voltage to power themselves. Mine is powered from the 12V reducer so it's unpowered when the cart key switch is off - and it's not charging. I used to leave my cart for 5 months in the summer - and when I did that, I unplugged my BMS from the cell taps and I also disconnected the positive terminal so the pack was completely isolated. (I don't go to Colorado in the summer anymore so I am using the cart all summer now)

[Murby]

Originally Posted by Volt_Ampere

I don't know how well most BMS isolate themselves from the pack. Some use some of the pack voltage to power themselves. Mine is powered from the 12V reducer so it's unpowered when the cart key switch is off - and it's not charging. I used to leave my cart for 5 months in the summer - and when I did that, I unplugged my BMS from the cell taps and I also disconnected the positive terminal so the pack was completely isolated.

When my off grid system was powered by Chevy Volt cells, that's exactly what I did as well. I'd unplug the cell taps because it was so easy to do.. just a simple 12 pin connector. The new battery isn't so easy to do that.. I could unplug them at the BMS, but its a far more difficult because the connector isn't nearly as easy, so I don't do it.

I always wondered how much juice a dormant BMS would draw.. Well, so far, after 6 months on my new battery, it doesn't look like much.. I'm guessing less than 5 uA of current.. if even that much. Not enough for my Fluke meter to detect yet.

I would have thought the natural self-discharge of the batteries would show itself, but even that hasn't happened.

Now I'm wondering if the stable cool temps of the basement are playing a big part in this.

The Optical FET idea is a good one.. that should be internal to all BMS's. Was thinking to email REC with that idea, but what am I going to say? "After 6 months I've had no detectable drain so you should improve the design?"

[Volt_Ampere]

Self discharge on Lipo's is extremely minimal. I have some RC packs that I have stored for several years that are still sitting at storage voltage. As long as nothing is draining them, they are good for a long time in storage.

[yensidcamper]

I got the pack to charge back up. Stopped at 55v to hopefully not over charge any single module.

I am going to order another Cheap BMS unless I can stomach buying a better one. I paid $450 for the leaf cells hard to just justify a very expensive BMS. Any suggestions on a moderately priced BMS?


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[DaveTM]

Originally Posted by yensidcamper

I got the pack to charge back up. Stopped at 55v to hopefully not over charge any single module.

I am going to order another Cheap BMS unless I can stomach buying a better one. I paid $450 for the leaf cells hard to just justify a very expensive BMS. Any suggestions on a moderately priced BMS?

Well, ....and this is just MHO......when purchasing a new rifle, the general consensus is to pay as much or more for a new scope as you have for the rifle. Why? You want the rifle to be as accurate as possible.

You just spent $450 on batteries. If a cheap BMS cost $150....you're only $300 away from a good BMS that has a good track record and does the job it's suppose to.

[S-N-R Customs]

Originally Posted by yensidcamper

Thanks for all the replies. Here is what I have right now for voltage, all batteries isolated and measured between center taps and then outer taps.

Module 1 2.8v 2.8v 5.6v
2 2.8 2.8 5.6
3 2.9 2.9 5.8
4 2.9 2.9 5.9
5 2.8 2.8 5.6
6 2.8 2.8 5.6
7 2.8 2.8 5.6

I think I may have avoided disaster but clearly the BMS is not functioning. I will worry about that replacement after vacation.

For now I am going to replace the bus bars and try to attach my charger to the main pack negative and positive and see if I can get it to charge. If not, do you guys think this pack will sit okay for the next two weeks until I can figure out how to get it charging?

Thanks again for all the helpful replies.

Your charger will most likely not do the trick... you’ll need to “wake up” the batteries with a much higher voltage.
Then you should be able to balance them and charge them


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