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12v Lifepo4 spec question - max discharge amps

Boomtown

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Edit: sunshine_eggo found issue, the spark while I was hooking up the batteries activated the PCB's shutdown process. I still don't know the max discharge amps, but will test to discover.

I have 4 Tenergy 12.8v 40aH batteries, connected them in series (56.4v, looks good, fuses fine), and connected them to my LV6548 hybrid inverter. The battery is not detected. The voltage drops about -1volt/second for 10 seconds, then quickly down to about 30v.

I don't have a BMS, but the batteries have PCBs (power control board). The max battery charge rate is 15amps, but I don't know what the max discharge rate is.

I have my LV6548 set to a "maximum battery discharge current" of 30amps, the lowest setting (edit: same voltage drop result when I set this to "disable")

1) should I get 4 more? Then run 4S 2P and assuming the max discharge is the same as the max charge, 15*2=30 amps

2) is there something obviously wrong here, like the batteries are way too small or a BMS is a necessity?
 
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Discharge is usually higher than charge but on so small a battery, I would be hesitant to assume so. 15A charge is surprising for a 10Ah battery.

Did you fully charge each battery to 14.4V individually and then in parallel prior to connecting in series? This should always be done when connecting 12V in series for higher voltage.

There's no "BMS" for this scenario. There are "balancers" that seek to keep each 12V at the same voltage by transferring charge from higher voltage batteries to lower voltage batteries.

Have you managed to miss the need for a pre-charge circuit when connecting LFP batteries to an inverter? When you connect the batteries, there is a MASSIVE surge of current that the battery BMS registers as a short circuit and engages protection cutting off discharge and yielding an arbitrary voltage notably lower than normal.

Here's Will:


1) No. See #2.
2) Too small to an absurd degree even if multiple parallel strings are installed.
 
It sounds like your batteries are dead/damaged. If you're reading 30v for four 12v batteries in series, you have too low a voltage on at least one of them. Are these new? Are you sure they are all connected in series?
 
Have you managed to miss the need for a pre-charge circuit when connecting LFP batteries to an inverter? When you connect the batteries, there is a MASSIVE surge of current that the battery BMS registers as a short circuit and engages protection cutting off discharge and yielding an arbitrary voltage notably lower than normal.

1

I'm not precharging the circuits... yep! That was it! Thank you!! Also, my typo, they are 40Ah, not 10Ah

I had a bunch of breadboard resistors (5w) that I combined to make a 25w and ~27ohm resistor, worked right away! Now I'll buy the proper gauge wires and I have another 4 of these batteries to create a 4S2P system.

I still haven't found the max discharge amps, I guess I will need to test them and monitor with a voltmeter to see when the PCB shuts them down.

The are used batteries from a seismic monitoring system. I think used while searching for oil in canada.
 
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It sounds like your batteries are dead/damaged. If you're reading 30v for four 12v batteries in series, you have too low a voltage on at least one of them. Are these new? Are you sure they are all connected in series?
Thanks! Yeah the 30v was after I hooked up the inverter, before that the voltage was correct. Sunshine_eggo identified the issue: the spark while connecting caused a shutdown
 
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