Battery charger question

[Syke]

I have a Uniross CX2000 battery charger for Ni-cads. Can this be used to charge Ni-MH batteries?

Regards and thanks in advance

Syke

[Tabby]

You've not given us the charging characteristics of the charger or the specs of the ni-mh. Generally its possible to use nicd chargers on nimh or nimh chargers on nicd /if/ the charge current matches what the battery needs. In no way can one assume it will though, it frequently wont. For fast charging there is partial swappability, IIRC nimh are fussier about overcharging.

NT

[Grimly Curmudgeon]

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember Syke saying something like:

It will put a partial charge into them, but without knowing what NiCads and what NiMHs, it's anyone's guess how much and when /if it stops or overcharges.

[Peter Parry]

The CX2000 is a low output charger (about 120mA max) which takes 14 to

16 hours to fully charge a battery. That charging regime is pretty benign and should be fine for NiMH as well as NiCd as long as you don't leave the battery on charge for several days.

A fairly simple, but reliable, test is the battery temperature. If they are getting quite warm to hot when held in the hand they are being overcharged and damaged.

[pcb1962]

Why take the risk of damaging them when you can get a NiMH charger for

4 quid delivered, eg

[Andy Dingley]

Chargers do two things:

• They shove some current in to charge.

• They measure the terminal voltage and use this to determine when to stop charging (better ones look at rate of change)

Crude NiCd chargers only do the first and rely on you switching them off manually after a time. These chargers are now obsolete. Given the high cost of many cells and the low cost of a single good charger (Aldi, 13 quid, still the best deal around) it's worth switching to a decent charger. Most rechargeable radio-control toys use this sort. It's worth replacing those, as specialised battery packs aren't cheap.

Charge current is roughly the same for NiCd and NiMH of about the same cell size and capacity, so the first part is no problem. Assume C/10 for starters, i.e. charge a 700mAh cell with 70mA for about 12-15 hours. Anything more precise than this needs knowledge of particular cells (mostly because "C" should be the one-hour capacity, not the "C/

10 for 10 hours" that they're labelled with, because it's bigger).

Now for automatic chargers. If you have a crude auto NiCd charger that measures terminal voltage (i.e. anything over 5 years old) then it works adequately for NiCds, but poorly for NiMH. Not because it's using the wrong target voltage (it is), but because simple voltage measuring isn't such a good algorithm for NiMH. A simple fix for this is a switchable NiCd / NiMH setting on the charger, and a selectable target voltage (i.e. early NiMH chargers of 5 years ago). Avoid these chargers for NiMH use! They use a crude algorithm that doesn't control well.

Better automatic chargers support NiMH, and don't need to be switched between battery technologies. This is because they use delta-V sensing instead of just an absolute voltage and delta-V can work for both technologies. Delta-T is also good, but only used on powertools with dedicated battery packs with inbuilt temperature sensors. These are the only sorts of charger really worth using these days.

If you have a non-automatic, non-NiMH, NiCd charger, then it _can_ be used to charge NiMH cells. But treat it like a dumb manual charger - only leave it on for 5 hours at a time, switch off when the cells become warm, don't expect it to fully charge your cells, expect reduced service life on your cells, and look at upgrading to a modern charger.

Fast-charging NiMH is usually better and easier on the cells, because it's done with better chargers than the older slow chargers.

[Andy Dingley]

Trouble with that is that by the time you can feel it outside the cell (or even worse, outside the plastic battery case) it's too late.

[Syke]

Thanks everybody for taking the time. My situation is that I have the Uniross CX2000 charger which I use for charging four Uniross D type batteries for a torch. Incidentally these are now about ten years old! I'm getting a new radio which takes Ni-mh batteries and of course I wondered if it would be necessary to buy another charger. In the light of your contributions this looks like the way to go, and with luck, I'll be able to keep my old Ni-cads going with it as well. I did contact Uniross and they said that I could use the old charger safely, it would just take longer to charge the Ni-MHs.

Regards

Syke

[Man at B&Q]

it's "smart" it should be able to charge them faster than 6-10 hours. I wouldn't risk my cells in what is probably cheap s***e that will cook them if forgotten about.

MBQ