11 April 2012

NiMH laptop battery from 2001

I got hold of an old laptop from 2001 with the battery dead: what a better opportunity to have a look inside? If possible I wanted to preserve the battery case as much as possible.

A close inspection revealed that the battery case was composed of two parts very closely coupled together. Poking around with a small flathead screwdriver showed that the "weakest" point was the area around the battery connector. And from that point I started pushing and pulling. Few "crack" and some more "ziiipppp" were released before I could remove the lid, which was glued to elements.

The battery, being a 9.6V 4500mAh NiMH looked as shown in the picture: 8 cells in HR-4/3FAU format. All cells measure 1.265V each, which is a sign or them being still balanced somehow, and not dead. A small two-wire component was also found in close contact with two batteries. This could be a simple TC resistor or a more complex device which had also kept count of the number of recharge cycles.

I recharged the battery in place and it got very hot to touch, but charging stopped normally (with the two-leaded component in close contact with two elements). Final voltage was about 11V.

Drawing some heavy current allowed to evaluate then internal resistance developed during the years. At 3A the voltage drop across battery terminals suggested an internal resistance of 3.3 ohm, that is about 0.4 ohm per cell. This is high resistance for a high capacity battery! I wonder if there is a way to reduce it with slow discharge/charge cycles?

One week after the full charge the pack still exhibits 10.6V, about where I have left it, so apparently no cell is dead or leaking current.

Besides being a good paper weight, this pack could be used to power some light-current load despite the internal resistance, with an acceptable and known voltage drop. I am thinking of some power LEDs in series. Other usage ideas welcome.