In an email, Kirk said that he has found that ‘dead’ laptop batteries often have only a single cell that’s bad. I have torn apart some Dell laptop batteries, and found that they have a PC board that has some kind of battery management capability. I also found that the five or six cells are often the same size as the lithium rechargeable cells commonly known as 18650. This number is the size of the battery in millimeters. However, the 18650 cells may have a button on one end signifying that it’s the positive end. The cells in the laptop battery may have no button, and are typically spot welded to metal straps that connect them in series. These straps may be peeled off to remove the single cells. Then the cells can be recharged and tested to find if they hold a charge.
A couple of warnings
First off, the battery management board may keep in its memory how many times the battery has been charged. Even though the cells may seem to have some life in them, the management may have shut down the battery because it reached a certain recharge limit, and what life that’s left in the cells is very limited. So don’t expect to get much from the cells.
Another point that I want to strongly emphasize is that these cells are lithium cells, and have been known to explode or catch on fire if they are charged too fast or overcharged. That is another reason why the battery management board is used – to monitor the charging. So I highly recommend that the correct charger be used to recharge these cells. I bought mine from Dealextreme.com for under ten dollars U.S. It will also charge RCR-123 cells.
I have seen lithium rechargeables charge up to more than 4.2 volts. On discharge, they may fall to as low as 3 volts. So it may be necessary to use a DC to DC converter to get a stable output.