Lithium Battery Charging

[cpfeiffer55]

Hi All,
   I am building a battery pack with a 3s-2p BMS using 21700 batteries. I would like to be able to recharge the pack with another portable lithium battery pack instead of a wall charger. Any info on what configuration I should use to be able to fully recharge the 3s-2p pack would be greatly appreciated. Thanks in advance.

[CaptDon]

The cells in your pack should have their charging controlled by a BMS circuit. You can't safely simply 'charge' them. Three in series would equal around 12.3 volts fully charged so you would probably require around 15 volts driving the BMS circuit. Can you describe the 'other' lithium battery you expect to charge from?? It should probably be 10X the A.H. capacity of the pack being charged, otherwise you end up with two half charged batteries. Why on earth would you charge one battery from another?? Why not just charge the first battery from a suitable mains powered device? Doesn't sound well thought out. The only thing close to what you want is the R.C. car or boat chargers which charge 'on location' from a car battery. You could check that market and what is commonly available. They have plenty of pit stop battery fires and explosions. Sort of the direction you're headed without proper BMS charge control.

[tunk]

AFAIK a BMS does not control charging: It has overcurrent
and over/under-voltage protection. Sometimes it can also do
balancing. The standard charging for li-ion is:
- constant current (CC) until max voltage is reached
- constant voltage until 10% of the CC is reached
- stop charging

[CaptDon]

Balancing during the charge is important for good life and equalized cell capacity. Ending discharge at 2.5 to 3.0 volts is also important. The flip side is the R.C. racers who beat the cells to death in a "win at all costs" environment. My friend has run his cells so hot you can't hold them with bare hands, and yes, the parallel battery packs are known to flame out / explode just sitting dormant on a shelf. He stores his in a metal fishing tackle box with a latched lid. Some of the R.C. chargers that hook to a car battery are nothing more than a low ohm resistor, an amp gauge, and some have a spring wound timer to cut off the charge. I wonder if you find the 'time to explode' and set it for 1 minute less?

[indeterminate]

Lookup the manufactures data sheet a  21700 cell is just its external dimensions , you need to use the correct charging perimeters for your specific cells.
on the spec sheet you are looking for the following  information

Standard charge CCCV, 2.5A, 4.20 V, 250mA cut-off
Maximum continuous charge CCCV, 6A, 4.20 V, 100mA cut-off

so a normal charge would be
"cc" constant current charge at 2.5 amps until the cell reaches 4.2 volts  then switch to constant voltage charge mode
maintain 4.20 volts until the current drops to 250ma

you should also monitor the cell temperature
in my case the max charge temperature is 60c
so the charger should switch off before the cell gets to 60c

there are many charge control ic's available to manage this for you.
Do it properly or they will explode and set fire to anything around them