Help With Ni Cad Charger For 8.4v 80mah Pp3

[Damo666]

Hi Guys,

I've not been here for a while.

I recently picked up some old but unused 8.4V Nickel Cadmium PP3 batteries.

They are rated at 8.4V at 80mAH, 7 cells.

I tried to charge 2 of them on a dedicated NiMh/NiCd charger rated at 16mA, approx 0.2C, but after 4 Hours the batteries went bang and ruptured the casing.

I'm now looking to build my own NiCd charger to charge these batteries at 0.1C, 7 - 8mA.

I just ordered some LM317T regulators, but upon checking the spec' they're only capable of supplying a minimum load current of 10mA. This is no good.

Can anyone please be so kind as to provide a schematic for a simple 8.4V Ni Cad charger at 8mA?

Also, can anyone shed any light on why my Ni Cad PP3's ruptured their lids? Like I say, they are rated at 8.4V and 80mAH, but they were nearly 30 Years old but unused. They had been cycled a couple of times before being shipped to me, though.

All the best,

Damian

[AwesomeMatt]

You have to use a charger that is built for that size of battery in most cases. That is because the charger is blind to the battery.

NiCads are charged with constant current but are ruined (or will explode) if their voltage climbs too high, because all excess voltage is turned into heat. Some chargers in fact only measure end of cycle by when the batteries get hot. Some do it on a fixed time duration and presume batteries were fully drained when they started. Some drain the batteries before giving the fixed time charge. Some smarter ones detect voltage drop (from getting hot at the end of cycle).

I'd suggest that building your own NiCad charger is complicated and a waste of time.

Or, do what I do, slap 'em on a blind power supply and wait a few minutes until you think they're charged, stop before they get warm.

[CWB]

yep ... 30 years old ...
there is a chance that one or two of the cells in a pack were shorted .
this would cause the "extra" voltage to be passed on to the other cells .
they may have cycled them a couple of times but cells of that age can fail at any time .
i would measure the cells in the packs and look for dead or "reverse charged" cells .