41C battery pack HP82120

[John Robinson]

I had one of these rebuilt within the past 12 months with the correct N75P cells. I haven't used it for three months, but when I went to charge it the other day, it wouldn't charge (only had a about 2V after charging). I check the charger voltage - seems OK. Does anyone have any thoughts on this ?. Or does any one have a circuit diagram for the battery pack ?. Thanks.

[Yani]

Hmm... In other circumstances I would say it sounded like shorted cells. This is very common where fine hair like wires grow inside the cells between the electrodes and short out the negative and positive electrodes. It usually shows up as a decrease in 1.2V on the overall battery pack. In this instance it looks like two cells may have developed shorts.

Calculators are particularly susceptable to this since they are very low current devices. The high currents in childrens toys for instance do not allow the growth of these hairs. I have seen these hairs grow in thin air with no more than a low voltage between a steel plate and components seperated by about 1/2 and inch.

To test this check each individual cell. The good cells should be about 1.2V and the dead cells should be 0V.

If this is the case there are two alternatives. Replace the cells or "thump" thenm with a very high current to burn off the hairs.

If you replace the cells replace them all and use better quality cells. Any 1/2 N type should suffice. I would use NiMH. They are far better than NiCd. To thump the cells use a high current source (a car battery is good) to generate several amps for a second or so. It does not take long. Remember positive to positive and negative to negative. DO NOT I repeat DO NOT LEAVE THE CELL CONNECTED TO THE CURRENT SOURCE FOR MORE THAN A COUPLE OF SECONDS otherwise the cell will EXPLODE!!!!!!!

[Yani]

To thump the cells use a high current source (a car battery is good) to generate several amps for a second or so.

BTW do this to the individual faulty cells only - tot the whole pack.

[Tony Duell]

No, DON'T use a car battery. The short-circuit current of a car battery is somewhere around 1000A (!) and the risk of the NiCd exploding is fairly high.
I normally use my bench PSU (which has an adjustable current limiter), with the voltage control set to about 3V. Try it with a 2A limit first, if that does no good, try 5A, then 10A. If 10A won't clear the short then you need a new cell.
And only connect the bench PSU briefly. I normally clip one lead onto the appropriate side of the cell and just 'flash' the other lead onto the other terminal. Do this to one cell at a time (i.e. take the pack apart), you can't do several cells in series.
My experience suggests that cells that have once been internally shorted are damaged inside (probably the separators get damaged) and are likely to short again. I'll try to recover a cell to make sure the device it's used in works (and that it's worth buying some new cells for it), but if I intend to use said device
then I replace all the cells.

[Yani]

You are probably right Tony. I have never used a car battery myself but I have a friend who uses this technique. I can see however that if the cell was internally damaged then there would be a high risk of explosion. Thanks for pointing this out ;-\

I do use a current limited supply of about 2 amps myself but not everybody has one of these.

The alternative is to use a car battery (or even better a car battery charger) with a series resistor of 5 ohms 10W. This is a little under rated but would do the job for short bursts, and would limit the current to about 2 amps.

[John Robinson]

Thanks for the suggestion guys, I'll take the pacl apart and give it a go. Cheers - John