One question that always seems to spring up(at least for me): Can you charge a NiMH battery in a "Ni-Cad only" dumb charger? I figure the voltage is the same, and the current level should be quite low. You should be able to use it, just increasing charge time in proportion to battery capacity.
Obviously this isn't ideal, and I use a MHC401fs personally, but you always see "Must be charged in NiMH charger" in advertising for NiMH batteries, is this strictly true?
The problem comes from determining when the batteries are fully charged. Your system might be acceptable for a completely dead cell, but how would you work partially discharged cells?
The dumb chargers rely on a timer to terminate the charge. You would end up having to keep track of how much you used the cells, what temperature they have been exposed to, and what their rate of self discharge is. If you put all of this information together along with the efficiency of the charger, it would probably work.
It seems easier to me to just go with a smart charger.
Well, the main difference between the NiCd and NiMH cell
charger is that the NiMH cell charger has a way to detect
when the cell is completely charged (or near so) and
shut off. This feature is important because the NiMH cell
is damaged more easily from overcharging then the NiCd cell.
The NiCd cell can take low current overcharging for
hours without too much damage while the NiMH cell isnt
This means you would need to find a way to detect when
the NiMH cell was fully charged or else risk shortening
it's life by a lot of cycles.
Since you need charge rates around C/2 or better
(that's 1 amp for a 2000mah cell) to use the minus
delta V method (talked about in the other thread)
you cant use a C/10 NiCd charger for that either.
Also, in most cheapy NiCd chargers they dont regulate
the current level, which is another requirement for
using the minus delta V method.
While the cheapy NiCd charger will charge the NiMH cells
there is always the danger of overcharging and ruining
the cells so that's the main drawback.
You posted exactly 50 seconds before mine :-)
Also SilverFox got me thinking:
It's possible to build your own constant current charger
if you want to do that we could put together a circuit
that would run off of a dc wall wart which are fairly
cheap, but you'll have to watch for the minus delta V
with a meter yourself.
Thanks guys! Accurate, concise and comprehensive! I wouldn't use a dumb charger myself, I have a Maha charger, which is great. But I have a friend who I discovered tonight is still using some aged 600mAh nicads in his digital camera, needless to say he's not getting many photos per charge. I have quite a few AA NiMH around as I sell a few on the side, and I gave him some...he just wanted to know if he could get by with his old charger. I might just tell him to err on the side of caution: Discharge the batteries completely (which he does at the moment anyway- Nicads) and I'll calculate a charge time to 90% charge. The batteries are cheap anyway so frying a couple wouldn't be a disaster.
Strictly speaking, both nimh and nicad will take constant current trickling at low rates. Manufacturers spec 0.05 to 0.1C for trickle rate but I'd feel much better with a lower rate. 0.1C seems to be a standard for nicads (both the MagCharger and Streamlight constant current trickle chargers push at this rate) but for nimh, 0.05C or lower is safer. I believe the charger in the TigerLight trickles its nimh pack at a constant 0.1C.
In any case, it's always wise to A) not leave the cells on charger for extended amounts of time (a month or more) and B) check for heating or bulging of the cells.
Don't forget to add in an efficiency factor when calculating a charge time. 15-25% are about where I'd start. It'll take longer than the numbers may indicate.
Thanks, I will warn him.
From memory, this charger charges 2 sets of 2 batteries in series/parallel, and it quoted a 7 hour charge time for 2 batteries, 14 hours for 4. Given that it was supplied with 600mAh nicads, even with 2 batteries I guesstimate (I'll check, the charger did give mA ratings) that the charger is only providing around 80-90mA. Given that the Maha's have a trickle current of 50mA, it should have a little leeway before it melts/explodes (especially if he charges 4 batteries at a time!).
Actually, now I come to think of it,he does have a 7 day timer, I'll tell him to use that!
[ QUOTE ] branny said:
I might just tell him to err on the side of caution: Discharge the batteries completely (which he does at the moment anyway- Nicads) and I'll calculate a charge time to 90% charge. The batteries are cheap anyway so frying a couple wouldn't be a disaster.
[/ QUOTE ]
Ironically, this is also a lifetime limiting solution, perhaps severely so. NiMH don't take well to being discharged to low voltages, 'grinding out the last few shots' can easily reverse charge the weakest of the lot, causing premature (fairly random) failure. Because NiMH don't take 'trickle charges' like NiCd or Lead/Acid cells do, you're often best advised to get a charger that can treat each cell as an individual, this reason being often cited. Part of the American Spirit no doubt, we're all special you know and should get individual treatment.....
OTOH, you're spot on, it's a cheap experiment, I'd have a lash on that basis alone.
Regarding the original charger question: Since you can get a smart charger with 4 independent channels for as little as $10, why would you want to mess around with a dumb charger and risk the batteries?
Regarding the cordless phone question: I have been using a very old NiMH pack in a Panasonic that came with Ni-CD batteries. It has been going strong for about a year and the phone works much better now than on the original batteries. I believe this is a safe mod. The only way this might cause a problem is if left on charge continuously without any use. Then the batteries would probably not last as long. My NiMH pack is over 6 years old now.
Inside my portable electric shaver was 2 AA's that died about 6 months ago. I've been using it as a corded shaver. Recently, I replaced those 2 Nicad AA's with Rayovacs IC3 15 minute 2Ah AA system. I decided to do this since each battery has an auto shutoff.
It actually works quite well, 30 minutes will charge it right up and
then the batteries cut themselves off. The only problem is the shaver has a 6 LED showing how much charge is left - it always stays at the bottom - which leads me to believe that the nimh's V is lower than nicads. Actually when it's almost completely full, it reaches about half way up the LED bar.
As a side note, I now charge up this shaver between 1 and 2months.
Whereas it used to last about 2 weeks. awesome.
The Rayovac PS1 charger is often only $10, unless packaged with batteries which makes it a couple of dollars more. It is carried at Wal-Mart as well as many other places. There are better (i.e. faster) smart chargers, but not for anything close to this price. Mine has served well for years. I think it is a great value.