Can I substitute a NiMH battery for NiCd in a cordless phone?

"To reduce the risk of fire, use only 3.6V 850mAh Nickel Cadmium (Ni-Cad) cordless telephone replacement battery pack."

I've heard about the dreaded "memory effect" with NiCd batteries, so I'm interested in replacing it with a NiMH one. Someone who is selling a 3.6V

1000mAH NiMH battery on Ebay claims it works with my phone, but will it really be safe? Wouldn't want to install a NiMH battery and have the house burn down.
Reply to
curious
Loading thread data ...

the

I thought most phones came with NiMH cells these days for environmental reasons.

The memory effect is much misunderstood and isn't the most likely cause of cells failing to perform as expected.

Memory effect was first noticed in early satellite batteries that were subject to many IDENTICAL charge and discharge cycles as the satellites orbited the earth. When the cells were asked to discharge a bit more than usual the voltage fell more than expected. This happened at the point at which it would normally have been recharged.

In a phone the most likely cause of failure is long term overcharge. Once the battery is fully charged most phone chargers switch over to a maintenance charge program. This is designed to keep the cells topped up because all cells "self discharge". Problems occur if the chargers over estimate the self discharge rate. In fact dumb chargers have to use a slightly high maintenance charge rate to allow for the variation between different cell batches, temperature effects and aging. NiMH have a higher self-discharge rate but not everyone sees this. They are also less tollerant to long term overcharge.

Personally I always prefer to buy products that use loose AA size cells rather than an assembled battery pack. That way if the manufacture has gone bust you can still replace the battery.

Reply to
CWatters

NiCd is actually better in this use - it does not have problems with sustained overcharging, which most phones do. NiMH may do. How long does it say to charge it before use? If it's under an hour, then there may be a actual risk of moderate overheating. If it's over that, then in practice, there is no fire risk.

Reply to
Ian Stirling

quoting:

Ready made NiMh cordless batteries have cells that are spec'd for this kind of service. It is in my experience that the NiCad packs are not generally spec'd in the same way, so actually the NiMh packs are far better.

Reply to
JM

It says to charge them for 24h ...

The Panasonic replacement battery costs as much as a new phone .. so i said "my ass" and found this cheap NiMH pack in a supply store for $4 :)

Matt

Reply to
L

My NiCd's suffer from all known battery abuse :(

Thanks all for the replies. I will go ahead and install the NiMH's :)

Matt

Reply to
L

Interesting, both you and CWatters mention that NiCd batteries handle overcharging better than NiMH. Maybe I'll stick with the NiCd battery.

12 hours.

Good to know.

Thanks, everyone.

Reply to
curious

Interesting, the NiCd battery that *comes with* the phone isn't designed to handle the phone's charger?

I will take this into consideration, thanks. :-)

Reply to
curious

The problem lies in the charging. NiMh and Nicads are similar, but not the same, and NiMh in particular has unfortunate side effects if not controlled properly during charge (like smoking).

Look at websites devoted to battery chargers (TI, Maxim, Linear Tech) and you'll find each has its own charge techniques. Having implemented chargers for both those (and others, such as Li-Poly and Li+), I can assure you there are differences and you have to be really careful.

Cheers

PeteS

Reply to
PeteS

Unless you have problems with it, it's generally not worth replacing. NiCd seem (from my experience) to have better longevity in low-drain applications. And most phones have a long enough life that extending the life by 50-100% isn't really useful. If you find that you are often running out of battery, then swap.

Ok, then there is certainly no fire risk.

Reply to
Ian Stirling

| In a phone the most likely cause of failure is long term overcharge. Once | the battery is fully charged most phone chargers switch over to a | maintenance charge program.

In my experience, most cordless phones don't switch over at all -- they just charge at a C/10 rate or so forever. So yes, it's long term overcharge. Maybe the better quality ones do better, but I've never seen one. Obviously, any cordless phone that used lipo batteries would have to shut off the charger when it's full.

With NiCds cells, overcharging for a long time at a low rate leads to voltage depression, which many people mistakenly call `memory'. Discharging the cell down to 1 volt/cell and then recharging will help undo this, at least until it happens again.

| NiMH have a higher self-discharge rate but not everyone sees | this.

The only way somebody wouldn't see it would be to not be looking -- the difference is huge. :)

| They are also less tollerant to long term overcharge.

... and (other forms of) abuse in general.

| Personally I always prefer to buy products that use loose AA size | cells rather than an assembled battery pack. That way if the | manufacture has gone bust you can still replace the battery.

It's very rare that you can't find a battery pack for something, even if the manufacturer has gone bust. Places like Batteries Plus can usually make something if needed.

However, I do prefer to get things that use AA or AAA cells, even if they're in an assembled battery pack. This is because it's usually quite easy to rebuld a pack -- soldering to cells isn't too bad if you can be very quick about it (but there's definately an art to it) -- and with AA or AAA cells, the cells you need are cheap and easy to find. It's very satisfying to fix your battery pack for $2.50 (4 pack AA 700 mAh NiCd cells are $2.50 at Harbor Freight Tools when on sale) rather than the $30 that the manufacturer might want. (Cordless phone packs are usually cheaper, but ham radio packs ... sheesh!)

--
Doug McLaren, dougmc@frenzy.com
Out of my way, all of you!             What a brave corporate logo!
 Click to see the full signature
Reply to
Doug McLaren

3-AA cell NiCd battery. The phone would last for about 3 days of zero to minimal use before the battery went dead. I picked up an identical shaperd 3-AA NiMH pack at Walmart and substituted that.

Now, the charger was designed to charge the 650mah NiCd pack at about C/10 so the charge rate for the 1300mah NiMH pack is only C/20 and is most unlikely to harm it. 1300mah is pretty low for NiMH AA's but the phone now runs for well over a week so we shall see.

Ted

Reply to
Ted Edwards

Once

I have a six year old DECT system and this appears to switch rates. Initially it charges at a constant rate then presumably when the cells ar full it switches on and off roughly every 15-20 mins. The NiCad batteries last about 18 months then we notice they either get hotter and hotter while on charge or their capacity seems degraded.

Reply to
CWatters

quoting:

Yes, that's generally what I see in the original NiCad batteries that come with phones AND replacement NiCad's.

Reply to
JM

quoting:

But remember the cells used to make the NiMh pack are spec'd for continuous slow charge so even slightly higher charge rates won't harm the battery.

These cells have the capability to recycle excess gassing back to the electrolyte (up to a cirtain point), rather than overcharge damage the battery. The trade you get with these cells is lower capacity.

Reply to
JM

I have been charging for 18 hours now and the phone is as dead as can be .. original battery was 3.6V 600mAh new one is 3.6V 300mAh ...

Looks like that battery was DOA :(

Matt

Reply to
L

--
----------------------------------------------------------------------
Mark W. Lund, PhD            ** Battery Chargers
 Click to see the full signature
Reply to
Mark W. Lund, PhD

it worked (sort of) withthe NiCd'a do you think the NiMh battery killed it?

Matt

Reply to
L

No not unless the battery was faulty.

Reply to
CWatters

I can't imagine that it could have, there is already a current limit, so even if the cell was shorted it probably wouldn't have damaged the charger.

The way to tell, of course, is to measure the current while the battery is plugged in, but measuring the voltage when the battery is not connected should also tell you if it is working.

If you don't have a voltmeter, get one. They are dirt cheap if you get one at Harbor Frieght, etc., and are an essential part of any man's toolbox.

Best regards mark

L wrote:

--
----------------------------------------------------------------------
Mark W. Lund, PhD            ** Battery Chargers
 Click to see the full signature
Reply to
Mark W. Lund, PhD

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.