Leaf cell & BMS

I just found these. If I am interpreting it correctly, that’s 48VDC * 45 AHr ~= 2 kWh with BMS for under $500 ? Does anyone have experience with these?

It would make a good battery replacement for my APC Smart-UPS, where the current battery pack is over 10 years old (40 Ahr * 48V). But I still need to figure out if the old “lead acid” charging output of the APC will be bad for the BMS, or is it just plug and play? Given that the battery is way bigger than the UPS ships from the factory with (but it’s an XL model, is designed to take up to 48VDC * 198 AHr total in batteries), I think it’ll be fine (read: slow charge, but we only need it 3-4 times a year. Also if the pack degrades by 50% capacity, that’s fine by me! Our fridge, the intended “essential load”, is ~1kwh/day, where our average outage is like 4 hours.

No, but I’m tempted now!

Your math looks correct enough. It’s a 14S pack, so 51.8V nominal, and ~2.3kWh for the 45Ah modules.

A “float charger” for lead acid is going to be around 13.7V per 6S, so it’d be about 54.8V (you could measure what it floats at if you want when fully charged), or 3.9V/cell.

In other words, it wouldn’t charge it fully. You’d need ~58.8V for full on that pack.

But it would probably live a very long time at that lower state of charge!

Leaf battery seems to be NMC (LiNiMnCoO2) which is spec’ed for 4.2 - 2.5 V (absolute min/max)

I measured today and the UPS floats at 54.5 V and is said to cut off at 4*10.3 = 41.2 V.

54.5 V / 14 = 3.893 V per cell or about 80% SOC
41.2 V / 14 = 2.943 V per cell or under 0% SOC?

so 80% of 2.3 kwh ~= 1.84 kwh

Several places say that my APC uses a resistor based voltage divider, which means a simple resistor replacement with a trim pot can let you tweak the float voltage. Presumably the cutoff voltage will also rise, but with the lower voltage being so low, that would probably be a win to raise the upper (float) voltage to 4.1.

However, the firmware has been removed by Schneider Electric (who bought APC), so I’m stuck without any network monitoring on this thing.

4.0 or 4.1 per cell is fine for float, though as you note, you’re not losing a ton of energy by going with 3.9 - and they’ll last about forever there. If it were me, I wouldn’t mess with the voltages much. See if it’s trivial to do, otherwise don’t worry about the bit of extra storage up higher. You’re still above the center hump of capacity, and they won’t be sitting stressed.

I wouldn’t run a UPS pack much above 4.1V max anyway, and I’d probably go with 4.0 for one of mine if it were on constant standby service.

Some of the threads also mentioned that the voltage divider resistors also drift with age, so that’s another reason to replace/refresh them. This UPS is several decades old! But I may just be lazy and not do anything about it.

I’ve been fighting all day trying to find firmware for it, because Schneider Electric pulled the ftp site with all the old firmware down, refuses to support the ancient hardware anymore. But I think I might have just found a working set, so I can monitor via snmp…

Ohh! Hadn’t seen GreenTec have a BMS designed for those Leaf cells.

I’ve long thought about buying some of those used (but tested) old Leaf cells, but always worried about a BMS. But if they have a BMS that can sit right on top that easily…

Hmmm… That’s not all that expensive either, for G2 cells, 40Ah is is $463. That actually might make feasible my Burning Man camps plan to try and do solar/battery.

Victron 100v/20a 48v MPPT is $90 right now, add a couple of old house panels in parallel for cheapish, could have an entire 40a 48v system for ~$600-700 or so. Hmmmm