Replace NiCD with NiMH Batteries?

[AdrianH]

Hi

I have a LED night/emergency light that plugs into a wall plug socket in the kitchen. It switches on when dark to act as a night light, but it also has 4 x AA NiCD rechargeable batteries (built in charger) that power the light if the electricity goes off.

I am struggling to find stockists of NiCD batteries as most are saying they have been phasing them out for years now. Some of them say though I can merely replace them with NiMH batteries without any problems.

Any battery expects here that can shed some light on replacing the NiCD with NiMH?

 

[FNfal]

Should be no problems , but it depends on the charging circuit normally Nicad were charges between 50ma and 200ma .

 

[Rickster]

I wouldn't recommend it.

 

[MickZA]

W5ONV:
Can you still use the same charging supplies for both types of these batteries,like wall warts, etc and charge them inside of the radio ?
Also,Energizer and others make 15 minute chargers for the NiMH type of batteries,are these type of chargers hard on the batteries as compared to the standerd 8 hour type.I noticed that the batteries are the same for both chargers. Thanks again,73, Jim

Mark Brueggemann:
There are two common ways to charge NiMH and NiCd batteries, trickle and rapid. A trickle charge is normally accomplished with a low current wall wart. This is safe to use with most chemistries, so you can go from NiCd to NiMH directly. All you have to ensure is that it stays connected long enough to provide a full charge. NiMH cells today can have 2-3X or more capacity of the NiCd's they replace, so the charge time must be lengthened to accomodate that when using a trickle charger. The rapid chargers you see today are very effective and usually accomodate multiple chemistries. The only caviat is that you make sure it is capable of both NiMH or NiCd. Chargers made for NiCd only could potentially overcharge NiMH cells due to the different reverse voltage knee they have. Peak detecting chargers can bring a battery up to full charge very quickly with minimal heating. This is in contrast to the thermal-sensing NiCd chargers of olde, which charged batteries until they got hot before shutting off. I have several multi-chemistry rapid chargers that automatically sense the number of cells. Just connect up any pack, NiMH or NiCd and forget it. It's just as safe as a trickle charge and you don't have to wait 24+ hours for a full charge.

One more comment, I haven't seen memory effect on a NiCd in over 20 years. It's amazing how that issue keeps getting propagated.

Mark K5LXP
Albuquerque, NM

http://www.eham.net/ehamforum/smf/index.php?topic=30261.0;wap2

... I'm not an expert but these radio hams usually know their stuff.

 

[Stokstert]

Here is a write-up of charging NiMH batteries in NiCd chargers by Batteryuniversity.com.

NiMH dislike overcharge and the trickle charge is set to around 0.05C. In comparison, NiCd is better able to absorbed overcharge and can take 0.1C. Differences in trickle charge and the need for a more sensitive full-charge detection render the original NiCd charger unsuitable for NiMH batteries. A NiMH in a NiCd charger would overheat, but a NiCd in a NiMH charger functions well. The lower trickle charge for NiMH is sufficient for NiCd.