# How to charge Lithium Ion with protection circuitry

I am trying to build a charger for the Ctr 003 battery for the Nintendo DS. This battery has 3 outputs, which I call +, 0, and - for simplicity. When I measure voltage from + to -, or from + to 0, I get the same reading of 2.54V. However, when I apply arbitrarily small current from + to -, the voltage jumps arbitrarily high, and then trickles back down to 2.54.

I've had this happen in protected laptop batteries. The protection circuit kept the batteries from charging, unless it thought they were inside a laptop. This problem could be overcome by running an appropriate resistor between two non + or - ends of the battery. However, in this case, there is only one end that's not + or -. What other means do there exist, to fool the protection circuitry into thinking that the battery is inside a Nintendo DS?

I'm not expecting a full answer; it would be great even just to get some ideas of what I could try with other three-output protected batteries.

• Usually the third pin is a connection to a thermistor inside the battery pack (usually 10k), are you sure this isn't the case? E.g. try measuring resistance from 0 to -. – anrieff Jan 2 at 7:17
• The resistance is as close to 0 as my meter will measure, so it looks like that's not the case here... Unless it also has something to do with the fact that the voltage is currently too low, and the protection mechanism is preventing it from activating. – Alex Jan 2 at 11:23
• You can also try 0 to +. In any case, your suspicion about the voltage now being too low is likely correct, and it is advisable to charge it to at least 3.6V before proceeding to investigate. – anrieff Jan 2 at 12:23
• @anrieff And 0 to + gives 59.4K, the resistance of the battery. Unfortunately I don't have a charger for this battery. – Alex Jan 2 at 12:32
• If you have a lab PSU, you can apply CC/CV charge (e.g. 400mA / 4.2V). If you don't, then you need to borrow a compatible charger from someone. – anrieff Jan 2 at 13:27