NiMH Takes Forever To Charge - Causes?
- Thread starter dlong
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WalkIntoTheLight
Flashlight Enthusiast
Old NiMH cells often do this. It's because the smart charger is looking for a -dV/dt signal when charging. That is, when the cell is finished charging, its voltage will dip slightly. This tells the charger to stop charging.
Old cells don't have a big a dip as healthly cells. The charger might miss the signal, and continue charging until some other condition is met (such as a timer).
You can increase the size of the -dV signal by using a higher charging current. This works great for healthly cells, but unfortunately old cells have high internal resistance and don't like high charge currents.
Or get a better charger, that can detect the smaller -dV signal.
Or, the easiest solution, is to just live with it. They're old cells, so who cares if you abuse them a bit when charging? It's not a safety concern with NiMH, so overcharge them if you want. If you want to guess when they're fully charged, feel them to see if they're getting overly warm.
Old cells don't have a big a dip as healthly cells. The charger might miss the signal, and continue charging until some other condition is met (such as a timer).
You can increase the size of the -dV signal by using a higher charging current. This works great for healthly cells, but unfortunately old cells have high internal resistance and don't like high charge currents.
Or get a better charger, that can detect the smaller -dV signal.
Or, the easiest solution, is to just live with it. They're old cells, so who cares if you abuse them a bit when charging? It's not a safety concern with NiMH, so overcharge them if you want. If you want to guess when they're fully charged, feel them to see if they're getting overly warm.
archimedes
Flashaholic
Yes, older cells tend towards higher internal resistance.
SweD
Newly Enlightened
- Joined
- Oct 14, 2008
- Messages
- 62
Hi,
I'm slightly curious as to some numbers on this. For sure, I've had, and have older cells, being a NiMH only kind of guy, but can't say that I've noticed this to any great extent, if at all. Older cells hold their charge less than new ones for sure, but charging times have pretty much been the same for my cells, all of which being charged on a C9000, pretty much linear upto the 1.47 cutoff point with a 2 hour top up charge. Just curious, have never seen it.
When you say that they take longer to charge, ... , how much longer?
Termination method seems very much to make a difference.
/Dennis
I'm slightly curious as to some numbers on this. For sure, I've had, and have older cells, being a NiMH only kind of guy, but can't say that I've noticed this to any great extent, if at all. Older cells hold their charge less than new ones for sure, but charging times have pretty much been the same for my cells, all of which being charged on a C9000, pretty much linear upto the 1.47 cutoff point with a 2 hour top up charge. Just curious, have never seen it.
When you say that they take longer to charge, ... , how much longer?
Termination method seems very much to make a difference.
/Dennis
archimedes
Flashaholic
I think the question is ... are they really taking longer to fully charge and/or are they missing termination ?
