charging NiMH batteries in a NiCd charger?

[[email protected]]

It's that time again... my $15 cordless phone's NiCd batteries died,
and could not be resuscitated. A replacement NiCd battery pack costs
$12, almost as much as a new phone.

Hacked together three NiCd AA's with electrical tape and bits of metal,
but the connections keep getting loose - really annoying on those
long-distance calls.

Just bought a 3-AAA battery holder from Radio Shack that will fit in
the phone, and now have to buy rechargeable AAAs.

I'm considering NiMHs, because I really don't like NiCds. But, the
phone charger is designed for NiCds.

What will happen if I charge the phone with the NiCd phone charger?
Will the batteries' lifetime be cut short? Will they burn my house
down?

SHould I just remove the 3 AAA NiMHs when the batteries get low, and
charge them in a standard NiMH charger (which I do already have)?

 

[Baxter]

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.

 

[ehsjr]

It's that time again... my $15 cordless phone's NiCd batteries died,
and could not be resuscitated. A replacement NiCd battery pack costs
$12, almost as much as a new phone.

Hacked together three NiCd AA's with electrical tape and bits of metal,
but the connections keep getting loose - really annoying on those
long-distance calls.

Just bought a 3-AAA battery holder from Radio Shack that will fit in
the phone, and now have to buy rechargeable AAAs.

I'm considering NiMHs, because I really don't like NiCds. But, the
phone charger is designed for NiCds.

What will happen if I charge the phone with the NiCd phone charger?
Will the batteries' lifetime be cut short? Will they burn my house
down?

SHould I just remove the 3 AAA NiMHs when the batteries get low, and
charge them in a standard NiMH charger (which I do already have)?

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

 

[[email protected]]

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.

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?

 

[Bob Monsen]

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

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.

---
Regards,
Bob Monsen

A great discovery solves a great problem, but there is a grain of discovery
in the solution of any problem. Your problem may be modest, but if it
challenges your curiosity and brings into play your inventive faculties,
and if you solve it by your own means, you may experience the tension and
enjoy the triumph of discovery.
- George Polya

 

[[email protected]]

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.

 

[budgie]

Most NiCd chargers are designed for NiMH cells as well.

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.

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.

Concur.

 

[GregS]

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.

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

 

[[email protected]]

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

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.