Easiest way of charging two 18650 batteries in series?

Underi · April 16, 2019

Was thinking about doing a bluetooth speaker project and using a couple of 18650 batteries in parallel and series, but I was wondering what would be a proper charging solution. If I were to order a 2S 18650 charging protection circuit from aliexpress, what would provide an adequate charging voltage of ~8.4V? Should be as small as possible in size so step-up converter wouldn't work. Or something that is outside the casing and simply plugged in with a connector on the case.

Hackentosher · April 16, 2019

12 minutes ago, Underi said:

Was thinking about doing a bluetooth speaker project and using a couple of 18650 batteries in parallel and series, but I was wondering what would be a proper charging solution. If I were to order a 2S 18650 charging protection circuit from aliexpress, what would provide an adequate charging voltage of ~8.4V? Should be as small as possible in size so step-up converter wouldn't work. Or something that is outside the casing and simply plugged in with a connector on the case.

You could use a buck converter set to output 8.4v at 2 amps and a 12v wall wart. That should provide adequate charging current to the batteries, and it should provide the proper CC CV charging cycle that a typical lipo charger would. Just be sure to get a proper BMS, and maybe install a permanent volt meter to keep an eye on it.

mariushm · April 16, 2019

I would say just connect them in parallel and use a cheap charger, for example something using MCP73831 : http://ww1.microchip.com/downloads/en/DeviceDoc/20001984g.pdf

You can make the circuit yourself on a prototyping board or buy a ready made board from ebay .. for example: ebay link

You can buy the chip yourself: https://www.digikey.com/product-detail/en/microchip-technology/MCP73831T-2ATI-OT/MCP73831T-2ATI-OTCT-ND/1979803

or boards made with it if you don't want ebay: https://www.digikey.com/products/en/development-boards-kits-programmers/evaluation-and-demonstration-boards-and-kits/787?k=MCP73831

Yes, it's not a super duper ideal way to charge 2 batteries in parallel, but laptop batteries charge batteries like this all the time and it's an acceptable tradeoff (laptop batteries often have 3 or 4 groups of 2 batteries .. so 3-4 S - 2P and the laptop battery only controls each group, they don't control individual batteries.

If your audio amplifier needs more than 4.2v you can always add a step-up regulator to boost 4.2v or 5v to whatever voltage you need, like 12v or higher.... here's a 1$ board on eBay that would do that : https://www.ebay.com/itm/DC-3-24V-to-5V-28V-9V-12V-24V-2A-Adjustable-Boost-Step-Up-Converter-Power-Module/112619032791?hash=item1a389e10d7:g:NE8AAOSwvDpbBoGe

You could use a couple diodes to make it possible to use either DC in or battery to power your circuit... something like this :

For diodes like the picture says, use something with very low drop at 1-2A of current, or whatever the maximum power you'll pull from batteries or wallwart adapter/usb charger

For example 95SQ015 can do up to 9A and has a voltage drop of only around 0.1v at 1A and it's around 60 cents each if you buy 10 : https://www.digikey.com/product-detail/en/smc-diode-solutions/95SQ015/1655-1922-1-ND/8021507

or mbrs410 , with 0.33v at 4A but only ~ 0.15v at 1A : https://www.digikey.com/product-detail/en/on-semiconductor/MBRS410LT3G/MBRS410LT3GOSCT-ND/1967073

or pmeg3050 , with 0.3v at 5a but only ~ 0.2v at 1A : https://www.digikey.com/product-detail/en/nexperia-usa-inc/PMEG3050EP115/1727-5317-1-ND/2676793

and you could use multiple in parallel to reduce the voltage drop slightly.

Basically whatever voltage is higher will go through the diode, and the other diode would block ... so when you have 5v adapter plugged in, the 5v goes through the diode and the other diode blocks, preventing 5v from going straight into the battery... but the 5v still goes the other way through the battery charger and charges your battery to 4.2v

When you remove the adapter, there's no more 5v so the diode on the battery side becomes unblocked and lets whatever voltage is there go through..

If you use a big enough adapter, like let's say a 5v 1A - 2A usb charger, the charger circuit will take up to 500mA to charge the battery and the rest will be used to power the audio board and everyone's happy.