Recovering a "dead" lithium battery

[jdurand]

Most tools that use lithium batteries have built in battery protection circuits and sometimes even a microprocessor.

The protection circuit and possibly microprocessor draw power from the battery, even when it's "off". It's not a lot of power, just a very slow drain.

Well, if you leave the tool/battery pack laying around long enough, especially if it wasn't fully charged, it's going to run down.

Herein lies a problem. There's 4 major trip points for lithium cells, for lithium polymer (common ones), 4.2 V = full charge, stop all charging.

4.0 V is typically the "start recharging" if the charger was left plugged in.

around 3V is where the protection circuit cuts off the power to the tool. So, to the tool the battery is dead, the user grumbles and goes to find where he left the charger.

But, there's one more very important trip point. It can vary but let's say it's 2V. Remember the protection circuit is still drawing a little power, even if it cut off power to the tool. The battery will eventually drain below this point.

Now, even if you plug in the charger, the protection circuit will NOT allow you to charge it. It's seen as an error. The user curses, tosses the battery pack out and buys a new one.

However, if you are not accident prone and have the appropriate tools, you can fix this if it hasn't gone too far.

*** WARNING! DANGER! DO NOT TRY THIS AT HOME! TRAINED PROFESSIONAL ON CLOSED TRACK! ***

You can open up the battery pack, verify the low voltage condition of the battery (note, if there's multiple cells this gets more complicated), and then with a constant current power supply apply a charge directly to the battery cell. The charge current should be low, no more than 10-25% of the rating of the cell (a 1000mA cell would be charged at 100-250mA). The power supply voltage should have been set to the correct multiple of about 3.5V per cell.

MONITOR THE VOLTAGE, it shouldn't take long for the cell voltage to climb up close to 3. Disconnect the power supply and connect the regular charger. If all goes well, the charger will take over and bring the battery back to life.

I just had to do this with a battery pack spot welder. Can't make a battery pack if the battery in the welder is dead!