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I have a 4-cell 18650 battery pack in a 2S configuration. I have a 2S BMS for the cells. However, I am curious if it would be possible to charge this pack from a 5 V USB charger?

Since this battery is primarily meant to be used on the road, I can use a step-down regulator to charge from the 12 V power in the vehicle. However, charging from a USB charger would be more convenient. Additionally, this would allow me to charge the pack from my backpacking solar rig.

Just to ensure I am thinking this through properly, here are my thoughts for the pack:

  • 4x matched 18650 cells.
  • 1x 2S BMS.
  • 1x 5 V USB step-up 8.7 V regulator.
  • 1x 5 V buck w/ USB ports for outgoing voltage.
  • 0.15x5 nickel strip for cell interconnect.
  • 2in Kapton tape.
  • Awg 20 for wiring (outgoing current should never exceed 3 A)

Extraneous odds and ends:

  • 1x ammeter to monitor draw.
  • 1x 8 V battery level indicator.
  • 1x external interrupt switch to open outgoing circuit.

The assembly is more or less straight forward. I will design and print an enclosure in ABS, or PETG, for the pack. I just want to ensure my thinking is on track for the BOM and for the design.

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  • \$\begingroup\$ If your boost converter features constant current (CC) limiting, then sort of yes, with some caveats. If pure CV, then no. \$\endgroup\$
    – winny
    Commented Dec 28, 2021 at 9:28
  • \$\begingroup\$ Cool, that's what I was wondering. I know that the incoming voltage needs to be managed and current throttled as the cells fill. Almost like a resistive voltage limiter, but I wasn't sure if a steady incoming filtered power would do the first part with the BMS keeping it from killing the cells. (Yes terrible design, but a starting point for the idea). \$\endgroup\$ Commented Dec 29, 2021 at 16:50
  • \$\begingroup\$ The idea is to create my own charging element for the pack, as compact as possible. Also making the charging element dynamic enough to handle recharging via incoming 14v from a vehicle (alternator is going to need a buck to keep the power safe and keep concert speaker level noise down). I also want to be able to charge from a USB wall wart, and a backpacking solar panel as well. Once I have this smaller powerbank complete. I am planning on making a much larger 16v(buck'd to 14v) platform for my Ham radio and laptop power extender.... Yes I could but all these... But what fun is that?! \$\endgroup\$ Commented Dec 29, 2021 at 16:55
  • \$\begingroup\$ “variable voltage limiter” will make your own understanding much harder. People have tried before. Think about constant current and constant voltage. Look at charge curves for batteries and understand them. \$\endgroup\$
    – winny
    Commented Dec 29, 2021 at 17:09

2 Answers 2

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As long as the available input power minus the boost regulator efficiency exceeds your required output power, then generally speaking, yes. A 5V 2.1 Amp usb charger would be just about topped out if your charging the batteries at 8.7 V 1 Amp at 90% efficiency.

You seem to be missing a lithium charging circuit (unless your bms includes that). In that case I would go with a dedicated step up 2s lithium charging module instead of a generic step up regulator.

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No, it is not safe if you don't have a charger.

Please understand that a regulator is not a charger. Also the BMS is not a charger either.

A buck or boost regulator is a power supply and batteries can't be connected to a power supply. Batteries can be connected only to a charger which safely charges the batteries with the correct current and voltage and stops charging when battery is full. A power supply does not do that. BMS will cut at overvoltage but it is a safety feature if the charger malfunctions and does not stop charging.

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  • \$\begingroup\$ I was under the impression that this is a charging unit as well as an under voltage cut off? HX-2S JH20. amazon.com/dp/B07P2X59J3/… \$\endgroup\$ Commented Dec 28, 2021 at 12:24
  • \$\begingroup\$ I was considering using the buck to change the incoming voltage to the specs for the above mentioned BMS. So stepping 14v alternator down to 8 or 9 v for the cells behind the BMS. \$\endgroup\$ Commented Dec 28, 2021 at 12:25
  • \$\begingroup\$ It isn't a charger. It never claimed to be a charger. Don't assume, as improper handling of lithium cells will degrade and damage them and they are dangerous as they can explode or burst into fire. I hope you have not damaged them already, the safest thing would be to discharge and recycle the improperly handled cells and buy new ones and handle them safely and properly. If you connect discharged cells to 8V buck module the buck module will output as much current it possibly can trying to make the output 8V. It likely results into overcurrent condition and hopefully the buck module shuts off. \$\endgroup\$
    – Justme
    Commented Dec 28, 2021 at 12:34
  • \$\begingroup\$ I haven't even soldered the cells yet... I didn't think that "Balance Function Charger Protection Circuit Board" was making a wild assumption about it's abilities. From the limited documentation I've read, it takes an incoming voltage supply, balances, and provides over charge protection, but who knows. Maybe that's not what it means... \$\endgroup\$ Commented Dec 29, 2021 at 5:54
  • \$\begingroup\$ I can definitely understand the concern about Li batteries, they don't typically like heat and O2. Rapid oxidation can be fun in the proper circumstances however. Typically, I don't have a habit of playing around with Alkali metals willy nilly. There was that one time, I made tried eating a Popsicle made out of Rubidium, but it tasted funny... LOL At any rate, I'm still researching which BMS I want to work from. I know some do support charging the cells as well as managing current, and voltage regulation. I just was working on something of a principle for the time being. \$\endgroup\$ Commented Dec 29, 2021 at 6:01

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