I am making a high capacity battery bank that uses a 4s Li-ion cells from a car manufacturer. With Li-ion there is no 12v direct drop in replacement as it ranges from 16.8 full to 12.8 empty. So here is the problem. As part of the build I want to use an off the shelf 12v 300w inverter. This inverter can handle from 10.3v to 15.7v. I believe the limitations for the inverter is universal to any inverter in that 12v class as 16v will damage components used in the inversion circuit. When my battery is not full, everything is happy. As soon as I fill the batteries, the inverter doesn't work until it drops below the threshold. The question is what is the best thing to use to drop the battery output voltage on the inverter supply circuit to the needed 15v from 16.8v? I would think a resistor would work, but it seems to heat up, and of course the voltage change is dependent on the load amps. I am not trained in EE so I could be getting this wrong. I have looked into buck converters but most of them want to drop more than the needed 1.5v and have efficiency and cost side effects. I could use a regulator at say 12v or something. What is the implications of that? Heat? Efficiency? Any help would be appreciated. I know this is probably easy for you guys, but I have been banging my head on the wall over this one.
2 Answers
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Your first mistake is using the wrong lithium chemistry. 4S LFP is a direct drop-in replacement for 12-volt Pb systems. The voltage range is identical, 10.5 to 14.2 volts.
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Charge batteries to 4.1V only (16.4V of all pack) since there is not much capacity above this voltage and place one schottky diode (25A rating) in series with pack to drop about 0.8V.
answered May 22 at 23:37