DIY high-current BMS for LifePO4 batteries - Page 1
Forum section: electronics / projects

author: kallisti5
author_rank: Contributor
timestamp: July 15, 2022, 03:28:59 am
content: Background:* I have a big'ol DIY 2P 4S 12V 200Ah LifePO4 battery bank for my off-grid solar well house which is charged with a LifePO4 aware MPPT charge controller.* I also have a small jelly-bean LifePO4 balancing circuit which can balance the pack at ~1.2A (which I think is enough to keep things in order)I'm rapidly realizing that full BMS's that can handle the 150A max draw of my inverter are pretty pricey, and pretty sketchy.The question is, since the MPPT charger is aware of normal operating ranges of LifePo4 cells (and will stop charging when the batteries are full), the balancing circuit should be able to keep cells balanced with the 400W of solar I have.... would throwing in a 200A relay with a well protected BD679 transistor and resistors to shutoff the relay at a low-voltage cutoff be a crazy thing to do instead of a full-blown BMS?I think I have all the bases covered for basic LifePO4 protection...1) LifePO4 charging and cut-off via the MPPT solar charge controller2) LifePO4 balancing (not pictured, that's just a cell monitor)3) Low-Voltage cutoff to protect the cells via the DIY BD679 low-voltage shutoff.

author: kallisti5
author_rank: Contributor
timestamp: July 15, 2022, 03:34:17 am
content: side note, I do know about the potential damage from "below freezing charging". My plan is to flip the solar array off in the dead of winter since the whole system isn't super essential. We don't get many freezing days around here. Most BMS's on the market miss the low-temp cut-off anyway.

author: kallisti5
author_rank: Contributor
timestamp: July 15, 2022, 01:04:03 pm
content: QuoteThanks! I use it to top off my EV some days. My goal is 8A (900-1000w) sustained at 120v via an inverter. Then I should be able to trickle charge with a level 1 charger for essentially free.The rest of the time it keeps all my power tool batteries charged.QuoteThanks for confirming these. Yeah, I feel a bit weird considering no BMS, but the 150A inverter pull is what is killing the BMS idea and making me consider trade-offs. I honestly should have sized the system at 24v 8S to cut my inverter amperage down... but then the rest of the equipment gets less "common".QuoteGood point. I guess I could start plotting to add a high-voltage cut-off as well... however there's no easy recovery strategy there.QuoteHmmmmm. My original plan when I posted this was to put the voltage sense on the opposite side (non-battery) of the relay. Then the power from the MPPT could "recover" the system. Now that I spell that out though... I could see things potentially interacting poorly and in unknown ways. (and hell, the inverter trying to run off of the MPPT controller.)Maybe automatic recovery isn't in the design if it is this simple. I could add a manual switch to close the relay and "recover" when sufficient solar is present.QuoteGood point. I do have a 200A fuse in the middle of the pack for an extra "oh shit" safety.. But that's really intended to be a last line of defense.QuoteGreat idea! That would be pretty simple to add to be honest.QuoteGreat points, I was thinking bare minimum on the voltage cut-off... but there are multiple supplemental things I could add without getting too crazy. (and without getting more "digital")Thanks for these! I'm glad to get a little validation that i'm not *too* crazyI got the LifePO4 batteries from AliExpress for dirt cheap. Testing has shown the capacity is reasonable "real" vs "advertised". I needed to replace my aging Pb batteries and the LifePO4's were technically around the same price (minus all the supporting stuff above.. and the need to compress them in a fixture)