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Over-discharged LiFePo4 BMS damage

Over-discharged LiFePo4 BMS damage

Over-discharged LiFePo4 BMS damage

Aloha, and thanks for this opportunity! I have a pair of Simpliphi 3.8kw 48v batteries that have apparently been over-discharged. I say apparently because they are rated 3k discharge, but could potentially have seen 4K from inverter.

Symptoms are a battery that remains at full charge (51.4v),but when connected to a load (even <3watts) they protect and reduce output to an unusable 10v.

The batteries are equipped with a BMS, so I could assume that a component of that BMS was damaged.

I would love to get any insight in trouble shooting the BMS. I have yet to open up the batteries, and I am willing to share photos of the batteries guts if that would serve the forum. The Bats are 3 grand each, new and under warranty, unfortunately $800 to return and the manufacturer is not really helping.

Could it be a relay of some sort?

RE: Over-discharged LiFePo4 BMS damage

Sorry to hear about your trials. Especially with such an over-priced battery.

If you've truly tried to pull 4kwhr out of that 3.7kwhr battery it's destroyed. Don't spend a nickle more on it. Buy a reasonably priced replacement and take this one apart to see what you find. Test each cell manually.

Last fall I picked up 7.2kWhr of LiFePO4 for $1,600. You can add a BMS for ~$100 that you can actually touch.

Have you checked out the forum:

Keith Cress
kcress - http://www.flaminsystems.com

RE: Over-discharged LiFePo4 BMS damage

Thanks, what Manufacturer did you decide for your batteries? I have checked out DIY forum and will continue thanks. BTW I was going to test cells and try a New BMS with these Simpliphi cells.

RE: Over-discharged LiFePo4 BMS damage

Seems odd that your BMS would allow the battery to get into that state; I would recommend ditching, and staying away from, that BMS and all its ilk.

At your risk: https://www.electricrcaircraftguy.com/2014/10/rest...

TTFN (ta ta for now)
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RE: Over-discharged LiFePo4 BMS damage

BLS Battery

Took about a month from China. Batteries are spectacular. Came individually packed in static bags and locking boxes in engineered foam in four large boxes. So nice you don't want to get finger prints on them. It appears for a bit more money you can get them out of USA warehouses now quicker. They now sell Daly BMSes to go with them too. If you're in a cold place make sure the Daly model has undertemp charging cut-off.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Over-discharged LiFePo4 BMS damage

Glad you like you batteries,I’ll give look, thanks.

With regards to BMS not doing its job;


“Every battery protection circuit has two electronic switches called "MOSFETs." MOSFETs are semiconductors used to switch electronic signals on or off in a circuit.

A battery management system typically has a Discharge MOSFET and a Charge MOSFET.

If the protector detects that the voltage across the cells exceeds a certain limit, it will discontinue the charge by opening the Charge MOSFET chip. Once the charge has gone back down to a safe level then the switch will close again.

Similarly, when a cell drains to a certain voltage, the protector will cut off the discharge by opening the Discharge MOSFET.”

My assumption is that something went wrong here, perhaps what I called a relay, is in actuality a MOSFET chip which is toast.

RE: Over-discharged LiFePo4 BMS damage

That would make more sense. Relays are pretty rare in that service these days.

You description is not very specific either.

BMSes come in two flavors.

1) They have a bunch of MOSFETs the entire battery output travels thru. If a SINGLE CELL gets discharged below a limit or above a limit using the MOSFETS the BMS disconnects the BATTERY from everything.


2) They have two bunches of MOSFETs. You hook the charge system to the charge set and the load to the 'load' set. If a single cell exceeds a high limit the charge source is disconnected. If a single cell is discharged blow a limit the 'LOAD' is disconnected.

When using the term 'discharge MOSFETs' with respect to BMSes it means the BMS provides 'leveling'. With leveling there are yet more MOSFETs, one across each single cell. If while the battery is being charged a particular cell reaches a high limit the discharge MOSFET will turn ON 'discharging' the high cell(s) marginally while waiting for other cells with more capacity to catch up with their cell voltages.

Keith Cress
kcress - http://www.flaminsystems.com

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