NiMH in HP battery packs


Hi all,
This subject is not new but perhaps someone would repeat the conclusions for me.
A classic HP battery pack with 450mAh NiCd cells needs 14 hours to fully charge using the standard charger. Theoretically, a pack with 1800mAh NiMH needs 56 hours using the same charger. My question: is the current from a classic charger sufficient to fully charge 1800 NiMH cells or will they never reach full capacity?
Do NiMH cells self-discharge just as fast as NiCd cells, i.e. 1% per day?



NiMH self discharge a little faster than NiCd's. The standard charger probably would charge the NiMH eventually but I would try and find a higher current charger or get a universal charger that can charge at various currents and various voltages. I have one that will charge from 1 to 30 cells at from .05 to 6 amps. These are common in the RC plane and car hobby and aren't too expensive. Then you could put the right connector together and charge it that way. If you want to go with the standard charger I would opt for a little higher capacity NiCd, you should be able to get 800 or 1000 mah cells that size now days.

Chris W


don't don't don't try this - your NiMH cells might explode, these things need special chargers, preventing overcharging
better stick to standard NiCd cells (they are available in much higher capacity than the old ones)


your NiMH cells might explode

You are thinking of lithium polymer cells. NiMH only need a special charger if you are going to fast (1 to 3 hours) charge them. And even then it's not much different than a NiCd fast charger, but it is different. For slow charging NiCd and NiMH are the same, you charge them at 1/10c for 14 hours. For example a 450 mAh pack needs to be charged at 45 mA to do a slow charge. When fast charging, NiCd's can be charged as fast as 3C (not recommended if you want the most life out of them though). Where NiMH can only be fast charged up to 1C. The peak detection of a fast charger is also different for NiCd's and NiMH's but most modern chargers have a setting for both.

When several hundred dollars and hours upon hours of work goes into an RC airplane, and the life of that plane is dependent on the batteries, you tend to learn a few things about batteries.

Chris W


Chris (and others),
AFAIK they differ about trickle charge tolerance, better not leave NiMH into a NiCD charger.
NiMH are not forgiving about leaving them charging compared to NiCd.

I have a NiCd field charger that does not handle NiHM batteries very well, sometimes it just desn't stop! It does not seem to me they are SO compatible.

Giuseppe Marullo


Well, I disagree on this point. The end of the charging cycle is determined differently on NiMH / NiCd cells (I think it's determined by the charging current, if the cell is fully charged; the charger then has to start trickle load. The difference in current empty / charged is not huge, so this requires good electronics).

An interesting read:

After all, it is unlikely to damage the cell (or your health) when charged for only a couple of hourse and due to the low charging current. But: if left being charged for a longer period, damages are quite possible, since the charger won't stop and the cell overheats.


The NiMH cells will charge just fine in these machines or any other NiCad device that uses slow trickle charging. No worries about small thermonuclear explosions, reaching full charge, etc. Don't substitute NiMH cells in systems use fast NiCad chargers. Also NiMH cells are not nearly as bad as NiCads about having to fully charge/discharge them. I have been using NiMH cells in my HP's since the batteries first came on the market. I have also confirmed the charger compatibility issues with the battery manufacturer's field apps engineer.


The time to full charge is no big deal either. You get most of the charge in the first half of the charge time. If you put your NiMH cells on a NiCd trickle charger for 16 hours you will have a 95% charge which is still more than a NiCd at 100% charge. I don't worry about getting a full 100% charge anyway. That last 5% of the charge accounts for a much bigger percentage of total wear the battery gets while charging.

Biggest wear factor on either type is usually overcharging.


David is right: most hp chargers(*) work perfectly together with NiMH, and yes, Bo did correctly calculate the charging time. to my experience, NiMHs are a bid unforgiving concerning 'deep discharge', i.e., running the calculator until the voltage of the individual NiMH cells drops below 1 Volt. That's easy to handle: at the first sign of low bat indication (be it the red LED on a '67 or the BAT annunciator on the '41), give them a full charge, and that's it.

(*) tested with '67 adaptor and '41 batt pack. on '97 and models with similar charger hardware (cassette drives, printers...) i recommend disconnecting the charger after raeching the calculated charging time. these models really "eat" accumulators because of their crude charging "strategy" (i.e.,there is no such thing like current regulation or voltage limiting to protect the cells).


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