Ni-Cad 3.6V "barrel" battery replacement.

[Druid6900]

I'm planning on replacing the Ni-Cad batteries on all the motherboards I have in stock with rechargeable Ni-Mh ones the same size and voltage.

They are 80mAh, 14h 8mA units with spade solder tabs and I was wondering if anyone has used them to replace leaking or dead Ni-Cads. The long charging time concerned me, but, since each board would be tested for 24 hours prior to shipping, I figured that it would be mostly just trickle recharging after that.

Further, does anyone foresee a problem using them? It seems like they would be a "drop-in" replacement, but, we know how that goes sometimes.

TIA

Druid

 

[Dwight Elvey]

I believe you may have issues with constant current charging with Ni-Mh batteries. I think it best to stay with Ni-Cads. What I do is take portable phone Ni-Cads and put them in a zip lock bag. I run insulated single solid strand wire from the battery to the board. This has worked well, even when the cells eventually leak. 7 strand wire simply forms a path of the alkali to get to the board.
Dwight
Here is a quote from one of the web pages on Ni-Mh:

NiMH dislikes overcharge, and the trickle charge is set to around 0.05C. NiCd is better at absorbing overcharge and the original NiCd chargers had a trickle charge of 0.1C. The differences in trickle charge current and the need for more sensitive full-charge detection render the original NiCd charger unsuitable for NiMH batteries. A NiMH in a NiCd charger would overheat, but a NiCd in a NiMH charger functions well. Modern chargers accommodate both battery systems.

 

[drdanj]

Varta MemPac NiMH batteries are designed as drop-in replacements for NiCd cells and can cope with the trickle charging perfectly well. Still advisable to keep them off the board.

Unless you have a really specific high-drain application where NiMH won't cut it, I really wouldn't use NiCds - cadmium is horrible stuff and the less of it about the better.

 

[Druid6900]

That excerpt is something to take into consideration, Dwight, but I was going more to retain the original look of the board. Getting them off-board could be done just using a 4 x AA holder with the appropriate Berg connector.

As for the MemPac modules, same basic problem.

I tried finding vertical mount 2032 coin type sockets (single or dual), but the pin spacing wasn't right for the holds that would have been left when the barrel Ni-Cad was removed and I'd have to add a diode to block the charging voltage.

With the NiMH, it looks like I'd have to add a divider resistor or a Zener clamp to keep the trickle under control.

It looks like, to achieve what I want, I'm just going to have to go back to the Ni-Cad barrels and install them just before the pre-ship burn-in testing to charge them up.