NiMH so useful, so confusing

June 28, 2006 8:15 AM

Another question about NiMH rechargeable batteries: Can I use brand A batteries in a brand B charger? Also, how can I tell a 'smart' charger from the timed version I hear tell of?

After reading this previous question about mix[ing] rechargeable batteries with different mAh ratings, I still have the following question.

I have two sets of NiMH batteries, assuming I don't charge the 2000mAh batteries with the 2350mAh batteries can I use both sets in the same charger? The specs and power requirments on the back of both chargers seems to be the same with two key differences...

The charger that was originally for the 2350mAh batteries was bought in the UK, and usually only takes 2-3 hours.

The charger that was originally for the 2000mAh batteries was bought in North America, and usually takes 4-5 hours.

I've been operating under the impression that the charger speed is based on the base voltage they get in the countries they expect to be in - I have not tried the UK charger in North America, although it is speced for 100-240 volts I just haven't bought a adapter for the plug yet. However, it's possible one charger is 'smart' and one isn't - but how would I tell?

Can I use the newer 2350mAh batteries in the charger for the 2000mAh batteries, assuming I don't do them at the same time? Or should I go and buy an adapter for the UK charger - (and if I do that, can I use that charger with the older 2000mAh batteries?)

posted by tiamat to technology (10 answers total)

AFAIK, you can use any NiMH charger on any NiMH battery, although the Mfr's usually say not to. I have had no problems using a variety of chargers.

Smart chargers come with advertising and literature bragging about their being smart. If they aren't bragging, they aren't smart.
posted by Kirth Gerson at 8:19 AM on June 28, 2006

Someone recommended this Battery University site. Note that it belongs to a charger manufacturer.
posted by Kirth Gerson at 8:23 AM on June 28, 2006

A smart charger will shut itself off when the battery is fully charged. They measure a voltage drop through the battery. Some chargers will run at full strength for a fixed time, and then switch to trickle.

I believe you are safe swapping batteries and chargers, assuming the chargers will handle your local mains electricity. And you can probably even mix batteries in the charger, although I had a charger once that insisted batteries needed to be inserted in pairs, so check that.

If you're in a money-spending mood, you might want to get one of the newfangled chargers that can charge batteries in 15-30 minutes.

What you should not do is put NiCads in a NiMH charger or vice-versa.

Here's a good article on NiMH battery chargers by someone who rolls her own.
posted by adamrice at 8:28 AM on June 28, 2006

I've used one charger for multiple brands for years. Just stick to good-quality/brand batteries and keep the sets together.

You can charge 2500-rated nimhs in a 2000-rated charger...it'll just take longer.
posted by selfnoise at 8:31 AM on June 28, 2006

If you run too much current through an NiMH batter in the late stages of charging, it can melt or explode. That's because the resistance doesn't increase as it approaches full charge the way it does in LiIon.

I would be very leery of charging an NiMH battery with anything except its designated charger. It's possible that it might work, but it's also possible that you could burn your house down.
posted by Steven C. Den Beste at 8:32 AM on June 28, 2006

I use a RadioShack smart charger that was reasonably priced, and has switches for NiCd and NiMH, as well as standard and high capacity charges. It also has a discharge feature, for draining NiCd batteries before charging to improve memory. It holds four double A or triple A batteries. Cat. No. 23-335. It obviously was not intended for a specific brand of battery. Works great.
posted by weapons-grade pandemonium at 8:47 AM on June 28, 2006

With a timed charger you are almost always either over charging or under charging. For example, a cell's capacity goes down with age/use, but the charger still charges for the same amount of time, so eventually it will start overcharging.

If you want best long term performance, you need a good smart charger. In practice, if you can't afford the money/trouble of finding a good charger at the moment, don't worry too much about it. It will reduce the life and performance of your batteries, but they are consumables after all (even though they are rechargeable).

I really don't think you have to worry about explosions and burned down houses. Worst case, you put a 1000mAh cell in a charger designed for 2500mAh cells. It is likely to seriously overcharge the cell, and the cell will get plenty hot. It might even fail. It might even vent (which you can call an explosion if you want, but it isn't a very good choice of words). Still not likely to burn down your house..

All electronics deserve a certain degree of care. Electronics can start fires. This is much worse when you keep flammable things near the electronics, like when you have a very cluttered desk. Pay attention to what is happening, use common sense, and you should be okay. For example, burying something under a pile of paper is bad, running it on a clear and clean countertop is good..

In this case, check the temperature of the batteries with your hand a few times. If they get too hot to touch there is a problem (do those 15 minute chargers heat batteries above 50degC? It wouldn't surprise me if they do, but it can't be good).
posted by Chuckles at 9:04 AM on June 28, 2006

weapons-grade pandemonium mention of the 23-335 led me to the wired review of same.

So once I'm at home again I'm going to double and triple check whether either of my chargers is 'smart', and if neither is I think I'll be making a trip to radio shack (or ebay).

Thanks all!
posted by tiamat at 10:55 AM on June 28, 2006

Yeah, brand-matching isn't important, just make sure that if it's a fast charger (ie 3 hours or less) that you don't use it with cells of significantly lower mAh than either the cells that the charger was designed to charge, or the cells that are the norm at time of purchase.

Generally this won't be a problem since you buy batteries more frequently than chargers, and the mAh of batteries creeps upwards over time.
posted by -harlequin- at 11:46 AM on June 28, 2006

"the mAh of batteries creeps upwards over time."

Meaning, the mAh of batteries you can buy increases year by year as manufacture becomes more sophisticated. I don't mean it increases on batteries you've already bought, quite the opposite :-)

This clarification was probably completely unnecessary :)
posted by -harlequin- at 11:49 AM on June 28, 2006