rechargeable AA compatability?

[notrefined]

I have a Welch Allyn diagnostic set (otoscope and ophthalmoscope) designed to run off either 2xAA alkaline, or a similarly sized battery pack that's labeled NiCd 2.5V. Should it be safe to run the same instrument with AA NiMH batts? And if they're safe to use, can I also use them with the built-in recharger? The transformer is rated for 10.1V DC / 100ma.

 

[eluminator]

NiMH characteristics are so close to NiCd that I would expect the instrument would do well with NiMH. About the only difference you should see is the NiMH has greater capacity.

Charging is more complicated. If you leave it on the charger all the time, I'd guess that would be rather hard on the NiMH. I would probably cause premature failure which may not be a big deal. I suppose it might even cause leakage, though I really don't know.

If you only charge it after you've used it or after a couple of months if you don't use it, and you only leave it in the charger for a day or two, it may work fine. I've replaced, perhaps foolishly, the NiCd in a toothbrush and in a cordless screwdriver, and they both work fine.

 

[rwolff]

Generally, it's safe (at the discharge side) to replace NiCd with NiMH, although the reverse is not necessarily true (my digital camera says that it's OK to use alkaline or NiMH, but not NiCD). As for charging, most (all that I've seen, but there's bound to be an oddball out there) NiMH chargers can safely charge NiCd, but the reverse is not true. NiCd can handle a constant-current charge, and a "trickle" doesn't harm them so long as it's below a certain rate (IIRC, 0.1 of the amp/hour rating). NiMH doesn't like trickle charges - from what I've heard, it likes to guzzle electrons, then get cut off when one parameter (can't recall if it's cell voltage, or rate of voltage change) hits a certain value. Since the cheap way to do a NiCd charger is constant current at a "safe" trickle rate, I'd invest in a separate charger if you go with NiMH.

As for the rechargeable pack being a single unit, that sounds like a convenience issue - only 1 piece to handle instead of 2.

You didn't ask, but high-drain equipment (typically photoflashes) designed for alkalines can be damaged if you use NiCd. This is due to the extremely low internal resistance of NiCd (a 12V string of "D" size can start a car, but this'll run it flat in a few seconds and probably damage the cells) - under heavy drain, the low internal resistance can cause them to have a higher terminal voltage than alkalines. In cheap flash units, the inverter isn't designed to handle the higher voltage, and burns out.

 

[notrefined]

thanks guys /ubbthreads/images/graemlins/thumbsup.gif

 

[mrme]

If it were me, I would just put NiCads in it and carry spare alkalines. Why bother with NiMh if they have to be charged externally? It is not as if it will perform better, so why bother?

If I remeber right, self discharger rates on NiCads are lower than NiMh. Consider also that you can drain NiCads down low without damaging them. They are cheap enough now, memeory issues are relatively mute (if memory ever really was an issue).

Oh, you may have to Jerry Rig your device to charge the NiCads. I had to wedge a tooth pick against a switch a battery pack would have pressed to charge NiCads in my discman.

 

[Phlack]

[ QUOTE ]
rwolff said:
NiMH doesn't like trickle charges - from what I've heard, it likes to guzzle electrons, then get cut off when one parameter (can't recall if it's cell voltage, or rate of voltage change) hits a certain value.

[/ QUOTE ]

The AccuManager20 trickle charges the NIMH batteries; why would they do that if it is bad?

-Mike

 

[rwolff]

I don't have the data in front of me - it's something I recall seeing when NiMH first came out, and various trade publications (corporate library, a few employers ago) talked about putting them in equipment that had built-in charging circuitry for NiCd batteries. It may be that a "safe" current for trickle charging NiCds is too high for trickle charging NiMH, but it was years ago that I read the article (to which I no longer have access).