charging NiMH batteries in a NiCd charger?

Hmmm.... I have a Panasonic that is supposed to use NiCads - battery life is abysmal. I found a NiMH replacement - it works just fine -- very good battery life now. No changes required to base/charger.

--

--------------------------------------------------------------------- DataGet® & PocketLog®

--------------------------------------------------------------------

Oh, I'm not *that* good at soldering, and the idea of soldering onto batteries just makes me nervous. I was able, though, to solder the leads from my Radio Shack AAA battery holder to the little plug that plugs into the phone. :)

Plus, I want a removable solution, in case the new batteries go bad again. This is the second time this has happened to me. The first time, I gave the phone to my dad, and just bought a phone Consumer Reports recommends. (Alas, the batteries on the CR recommended phone just died!)

My dad salvaged the old phone by attaching a 3-AA holder, but this was too big to fit in the battery compartment, and it sticks out like a sore thumb. But, it uses off-the-shelf NiCd AAs (which he has a lot of).

My solution is to use AAAs... the battery holder fits neatly into the battery compartment. Just bought some 900 mAh NiMH AAAs today. They're about double the mAh rating of the cheap old NiCds that just died...

I'll charge the NiMHs overnight in my NiMH battery charger, then put 'em in the phone.

So, is it ok to use the NiCd charger for the NiMHs? Will the NiCd charger overcharge the NiMHs?

Here is a trick that makes soldering "unsolderable" things easier.

Take a small flat metal file, and "load" it with solder by filing it. You can wad up a bunch of solder in a pair of pliers and rub it into the file, or use bar solder if you have any. A lead sinker works too but it's worth the extra effort to have the same alloy you're going to solder with. Getting some flux in there helps too. I've had the best results using silver solder.

You may want to dedicate a file for this purpose.

Next, lightly file the surface you're going to solder with the loaded up file. You will be depositing bits of solder into the grooves the file cuts, and there will be no time for the newly exposed metal to oxidize.

You should now be able to almost effortlessly solder a wire to the filed-down spot. The resulting joint will be nice and strong. If you use too much heat, you will remove all of the solder and have to start over, so work fairly quickly.

To dramatically accelerate this process, you can use a dremel with a stone wheel. This works even on non-metallic surfaces like glass and ceramics.

Most NiCd chargers are designed for NiMH cells as well. They are quite similar, but the "end of charge" indication is slightly different with NiMH cells. Because of this, if your phone isn't built for NiMH cells, it may not detect end of charge, leading to overcharge, and overheating. Cells that are radically overcharged can get VERY HOT.

I would not risk charging the NiMH cells in the unit; $12 is cheaper than a fire, or even buying a new phone. You might get away with it, but then again... If you go with a homebuilt pack, I'd advise you stick with NiCad cells.

If you are careful, you can solder the tabs from the old pack onto new, tabless cells. You need a high wattage soldering gun and sandpaper, or a file. First, roughen the flat surface of the tabs on the old pack, then tear the tabs off the old batteries. Next, roughen the + and - terminals on the new batteries. Then quickly tin the terminals, and the tabs. Then hold the tab in position with pliers, and quickly solder it to the battery terminal. When you have a tab attached to the + terminal of two cells, line up the batteries and solder them. Tape them, solder on the two wires and you're done. The key is melting the solder quickly, which (in my case, at least) means a 140 watt soldering gun. The cells will be damaged if they get too hot.

Ed

Ok... looks like I'll just be removing the NiMH AAAs from the phone when they need charging, and charge 'em in my dedicated NiMH charger.

The phone doesn't *need* to remain in the charging cradle; I'll let everyone in the house know not to put it there. (Actually I'll probably end up hiding the charging cradle in the computer room.)

Thanks for the advice, all.

My attempts at replacing a hand vac, and a drill, NiCads with NiMH at first seemed OK, but after a short while the performance was terrible. I do realize NiCads have lower internal resistance for higher currents.

greg

A large proprtion of standalone chargers are, but IMOE budget cordless phones supplied with NiCd packs seem to be constant current chargers with no attempt at charge termination. While NiCd cells will tolerate this reasonably, it is NOT in the best interests of NiMH cells.

Concur.

On 22 Nov 2005 17:43:06 -0800, snipped-for-privacy@gmail.com wrote: why not just wire the existing NiMH charger into the phone base and be done. viola doug