Experience charging batteries with SEPIC buck/boost?

I’m trying to convert dead DeWalt XRP+ battery packs from nicad to Li-Ion using 5 18650 batteries, a 5S BMS, and a CC/CV charging circuit. I could very cheaply feed in 24V to charge through CC/CV buck converters (about $2.50 each), or for $6 I could almost not care about the input voltage and charge from practically anything in the 12V-30V range, if I can fit it in the pack with the BMS and cells:

The larger, more expensive, more flexible board is also less efficient, it seems. But the idea of having the flexibility to be able to charge this straight off a car, or more efficiently from a 24V power supply, is attractive.

It says it uses a “single-ended primary-inductor converter (SEPIC)” topology, which isn’t something I know about. Any experience/advice?

Edit: As best as I can tell from pictures and advertised size, neither unit will fit with the BMS and 18650 cells into the existing packs. On to plan C.

I could just use a 24V power supply with an external step-down with CC/CV settings for the charger:

What’s annoying is that the BMS I have doesn’t start balancing until 4.18V and overcharge protection doesn’t kick in until 4.25V, but I guess that will be good enough. The cells I have are recycled from a device that died before its cells, and I haven’t yet found a datasheet, and I won’t have a fan inside the pack to keep them cool, and the temperature protection doesn’t kick in until 50⁰C, so while I’m guessing these are fairly high-rate cells I’m unlikely to want to charge at more than 2A. Maybe I should have a balancing charger (set to 21V so that it engages balancing mode in the BMS) that charges at up to 2A, and a fast undercharger that will charge to 20.25V at (say) up to 5A, and use the fast charger to recharge while I’m working and the balancing charger occasionally.

This seemed so simple when I started!



Chargers made for operational readiness, or standby mode, often let the battery voltage drop to 4.00V/cell and recharge to only 4.05V/cell instead of the full 4.20V/cell. This reduces voltage-related stress and prolongs battery life.

Looks like my 20.25V idea wasn’t crazy. Also:

Prolonged charging above 4.30V on a Li-ion designed for 4.20V/cell will plate metallic lithium on the anode.

So 4.25V individual cell safety cutoff is not crazy.

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Rather than going into my own interpretation …:slight_smile: