hp41c negative battery

[hugh steers]

i have acquired a 41c in which the batteries were old and tired, but not leaky. the unit would power up with BAT for a short time only. i put in new batteries and it works fine (new N-cells appear to be 1.4v which is odd).

out of interest i tested the voltage of the old cells. three read out 1.2v and one -0.3v (yes negative). is this just a figment of my test meter's imagination. ie no current, or is it likely for one cell to be hammered more than the others.

just curious?

[Wayne Brown]

Yes, that's quite possible. When batteries get low on charge, minor differences in manufacturing cause some batteries to run completely out faster than others. If one goes completely dead then the others that still have a little charge will begin to "reverse-charge" the dead one. That's why you always should replace all the batteries at the same time with fresh ones. Mixing old and new batteries can produce a situation where a significant portion of the new batteries' power is going to reverse-charge the old ones, causing the overall voltage to drop (perhaps lower than the device's requirements) as well as shortening the life of the new batteries.

This is a major cause of death for NiCd battery packs. Letting the pack run all the way down can produce this reverse-charging situation in one or more cells, causing damage to those cells. Often a dead NiCd pack will have several perfectly good cells and one bad one.

[hugh]

thanks wayne, that makes good sense.

[Matthias]

In addition to the good explanation Wayne gave, let me tell you, that in most of the cases you can revive this cell by using a charged polarized capacitor which you connect (pos to pos and neg to neg, of course) to the (isolated!) battery. The strong but very short current should remind the cell of its original polarity.

Matthias