Charging 3.6V battery packs from 5V supply

[rab]

Hi,

I've been helping out today on a project where they're using Li-Ion 3.6V battery packs ( 3 x 18650 Tenergy cells in parallel) with protection circuits (one circuit per pack) to drive LED units.

These had been charging straight off of the 5V rail of a switch mode power supply and perhaps unsurprisingly a number of the pack's protection circuits have fried.

I've given them some high wattage / low value resistors I had lying around to place between the power supply and cells but would really appreciate some advice as to the values we should be shooting for as to the optimal and maximum charging voltage and current ?

Thanks,
Rab


www.rainnea.com/cnc.htm

 

[SilverFox]

Hello Rab,

Welcome to CPF.

Li-Ion cells are supposed to be charged with a constant current/constant voltage algorithm.

A constant current of ideally 0.8C (maximum 1.0C) is applied until the cell voltage gets to 4.2 volts. Then the voltage is held at 4.2 volts as the current drops off. When the current drops below about 100 mA, the charge is complete and the current should be terminated.

Some chargers are set up to utilize the protection circuit to terminate the charge. These chargers charge at a constant voltage of 4.6 volts but the protection circuit needs to be robust enough to handle this. If the protection circuit fails, you ruin the cell.

You can probably get by if you can accurately drop your voltage to 4.2 volts +-0.05 volts and simply charge using constant voltage. You may have problems in the long run, because Li-Ion cells don't take well to trickle charging. With the voltage held steady at 4.2 volts, the charge should be terminated when it drops to around 100 mA, but in this type of set up the current will continue to trickle charge as the current drops to 0 mA. This can leave the cell damaged and there is a possibility of explosive results during the next charge. It also reduces your cycle life.

You should probably get a proper Li-Ion charger, but if you are watching what is going on, you can use a variable power supply to charge. The risks are that the voltage will get bumped, and you will miss the end of charge termination and end up trickle charging a little.

Charging (and discharging) rates are related to the capacity of the battery. If you have 3 cells with 2200 mAh of capacity hooked together in parallel, a 1C charging rate would be 6600 mA.

Tom

 

[rab]

Hi Tom,
Thank you for your very informative answer.
I appreciate the help,
Rab