Need to slightly reduce charging voltage from inverter/charger to LiFePO4 battery bank

Thread Starter

ischonfeld

Joined Jun 22, 2019
15
I put together a standby power source from a pair of LiFePO4 batteries (200Ah each, 12V DC nominal), and a charger/inverter. When power goes out, I have a source of 120VAC to keep a fridge running for a while. It's all manual - wheel the unit over and plug in the fridge or whatever.

After each use (infrequent), I need to recharge the batteries. Problem is, while the inverter/charger is set to limit maximum voltage during bulk charge to 14.0V, for whatever reason it exceeds that and will go to 14.3-14.4V or so. Each LiFePO4 battery has it's own BMS, and that BMS self-protects by disconnecting its battery at a little over 14.0V. So the batteries never get fully charged since the inverter/charger never gets to the next stage of charging.

I was thinking about putting a rectifier between the inverter/charger and the batteries when trying to charge them. This would introduce a voltage drop across the rectifier, and prevent the batteries from seeing more than around 14.0V. For example, if the voltage drop across the rectifier was 0.6V, then they'd only see around 13.7V max and would keep charging until the inverter/charger dropped to float charge - at which time I'd stop charging anyway. As a side note, I believe I can get the inverter/charger to max out higher than 14.3-14.4V, just not lower. So, for example, if I can nudge it up to max at 14.6V, then the 0.6V drop across the rectifier would limit what the batteries see to 14.0V.

When actually using the batteries to supply the inverter, I'd clamp on a short across the rectifier to the inverter would see full battery power with no loss.

Question: Can someone suggest a rectifier to use? Charging current through the rectifier would be 54 Amps during max charge (a setting in the inverter/charger). And the lower the voltage drop across the rectifier, the better, since (1) the rectifier will have to dissipate 54 Amps * voltage-drop in Watts, and (2) I'd like to get as close to 14.0V dropped down from about 14.3-14.4V.

Also, how much of a heat sink would you recommend?

Thanks in advance for any help.
 

ericgibbs

Joined Jan 29, 2010
12,927
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