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Unknown Li-ion battery packs... How to charge them? Answered

Someone gave me several Li-ion battery packs (BobS can use anything...). They are all different, but all put out 7.6 V. Apart from the cells (4 of them, 2 x2 parallel), they are stuffed with electronics. They have 2 input wires (sockets), without a voltage rating on them. I already fried one circuit board with 15 V (thought it was 12 V....!!!). A model number Google search did not reveal anything. What would be the voltage to supply the battery packs??

I would like to take the cells out and charge them just like ordinary rechargables. Put them in battery holders for power tools and portable sound system (all have different volatge ratings).

Can I use the on board electronics (as is) to charge several ( more) cells in parallel?


As mentioned by frollard the electronics attached to the battery handle voltage monitoring and sometimes cell balancing and temperature monitoring. You do not want to bypass this protection. These type of batteries can get very hot and can explode.

The typical method to charge Li-ion batteries is with a constant voltage of 4.2 volts times the number of cells in series. In your case 4.2 x 2 = 8.4 volts. The current is initially limited to .7 to .9 times the capacity. For example if you had a single cell that was rated at 1000mAh, you would use a constant voltage of 4.2 volts with a current limit set to 700mAh (.7 x 1000). If you monitor the current you will see that it is steady for about an hour then slowly drops to 0. That means that the cell voltage is 4.2 volts and fully charged. Most protection circuits have an over-voltage protection of 4.25 volts per series cell. This may seem like a small difference, but the cell make up will limit the voltage to 4.2 max unless there is a problem. That is why the over-voltage protection.

+1. Put too much current through one of these and you're looking at possible explosion and/or metals fire (which can't be put out with water).

If you insist on trying to charge them without getting the specs, low current and long charging time is the safer way to go.

You need a couple different things in mind when charging lithium -- voltage monitoring, cell balancing (taken care of by the electronics hopefully) -- and temperature monitoring. I can't offer specifics, but a simple search will get you where you need to go.