I made a 4s battery pack made of 18650 LiFePO4 cells that I'm planning to use for a mobile computing hobby project.

My plan for charging it is to use a salvaged power supply from a computer along with a boost converter on the 12 V rail to push the pack pack to around 14 V at "full" charge. Max charging voltage according to spec is 14.6 V (3.65 V/cell) but I've read that slightly under-charging dramatically increases lifespan, which is desirable in my use case.

When initially assembling the pack I charged it on a bench power supply directly, until the pack was 13.3 V. Thanks to the power supply's gauges and dials I felt comfortable manually dialing in the current, etc for the initial charge.

After that, I finished the pack by hooking up a 4s protection circuit (https://www.amazon.com/4Series-Protection-Balancing-LiFePO4-Battery/dp/B07511RJ9S) and connecting all of my sensor wires and load-bearing wires according to the diagram pictured here. Now that I'm trying to charge through the protection circuit, nothing seems to be happening.

I've set the open-circuit voltage on the boost converter from 14 V to 14.6 V and no matter what I do I'm always seeing 13.3 V (pack's current resting voltage) across the boost converter's terminals. Waiting a while doesn't change anything, and I'm paranoid that I'm going to fry something just letting it sit powered too long or without supervision.

I don't have anything connected to the BMS load terminal. The charge terminal of the BMS is connected to the Vout- terminal of the boost converter, and the battery pack's positive lead is connected to the Vout+ of the boost converter. The boost converter's Vin- and Vin+ are connected to the PSU's GND and +12v rails, respectively.

I haven't tried going back to the hobby shop and directly charging through the BMS with the bench power supply. Would that be a good step to eliminate some problem with the BMS board? I'm not sure how often those come out bad, and I'm pretty certain I've got it wired correctly. If nothing else it would let me see if I've got any current flowing. The ammeter setting on my tester is finicky, and I don't trust it.

Is this inherent to how a boost converter would work ordinarily? If being connected to a power source causes the boost converter to freeze at that voltage then I'm going to have to figure out some other way of putting together a charger. I was under the impression that by pushing a higher voltage than the battery had, it should cause current to flow back into the battery.

I realize I'm going about this the hard way, but building it from scratch is part of the fun. Please no "buy a charger" replies. I may end up going that route, but until then I'm here specifically to troubleshoot my efforts at making one myself.

I can try some different things, including hooking up a 12v fan as a load, if anybody needs me to in order to eliminate potential issues.

like this, but with an open circuit where the load would be

  • \$\begingroup\$ I found that unplugging the BMS's sensor wires jumps the voltage back up to where I have it set. Looking again, the wires are definitely set in the same order as shown in the diagram. Is it just a bad unit? \$\endgroup\$
    – J T
    Commented Feb 16, 2019 at 6:32
  • \$\begingroup\$ Have you checked what the charge current is? You may just be overloading the boost converter. \$\endgroup\$
    – Simon B
    Commented Nov 13, 2020 at 13:22

1 Answer 1


Looks like you'r boost converter is succsessfully charging the battery pack at whatever current it can provide and battery pack itself can absorb that current with very little of a voltage increase. LFP (LiFePO4) batteries do have VERY flat charge (and also discharge) curve beetween 20% SOC and 90% SOC. I do suggest using ammeter to confirm charging and verify charging current is suitable for you.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.