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Old Dec 10, 2005, 9:37 AM   #1
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Searched but did not find. After not much luck with my old Radio Shack charger (bought in 2000, worked good while MAH was 1500 or less) and a couple cheap ones I bought a Lightning Pack 4000N and a pack of Energiser 2500 mah NIMH batteries. Now what I want to know, and can't seem to find any information on, is what kind of trickle chargers are there available to keep the batteries topped off, without damaging them, even after say a power fail or something like that? I mean I just want to leave them "plugged in" until I need them. Maybe hours, maybe weeks depending on my schedule. Has any research been done from that point on rechargables? I think from the looking around and reading I have done, something in the 5 ma range (either through limited charge or duty cycle) is what the ticket would be. And it would not increase the charge rate after a power failure. Just thinking outloud. Thanks
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Old Dec 10, 2005, 10:28 AM   #2
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Hello Fofa,

NiMh cells do not like to be trickle charged. NiCd cells are more tolerant of trickle charging, but they don't like it either.

You have a couple of options...

One is to understand that there have been great improvements in limiting the self discharge rate of the current batch of batteries. With higher capacity cells available, you can "afford" to lose a fair amount and still be better off than you were a few years ago with the lower capacity cells.

For example, a 2500 mAh NiMh cell (at room temperature) will loose about half of its capacity in about 100 days. At the end of 100 days, you still have a cell with about 1250 mAh of capacity left in it.

The ideal way is to do a discharge/charge cycle on your cells once a month. Not many people do this, but if you do, you will get very good performance from your cells.

Another option is to hook your charger up to a timer. You will have to fiddle a bit to get the charge time correct, but usually 15-30 minutes a day will keep things topped up.

You can also keep a set of Lithium primarycells around for emergency needs. They have a shelf life of 10-15 years and seem to be able to handle the loads. They are a bit expensive, but over 10 years they may be cheaper than the electricity needed to keep your NiMh cells charged up.

Tom
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Old Dec 10, 2005, 10:49 AM   #3
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I think this whole battery charging thing is confusing. Example, 3 different charger comparisons I read all liked the Maha charges the best (including Steve here). Yet one thing I have seen all agree on is that high heat is bad and shortens the life of NIMH batteries. Yet the Maha chargers (fast charge) all take the batteries up to or over the mfg's limit of temp. during charging (as one example). Hence my purchase of the Lightning Pack 4000N which also gets really good reviews, but keeps the batteries cooler. Also all of them say to run them through the rapid charger, then switch to a trickle charger to keep them "topped off", but to be careful of power failuers etc. as the trickle charges will go into charge mode instead of trickle mode and over charge them etc. But I don't recall (and this is not a debate with your response, nor a disagreement, or has any negative conotation) that you should not, is bad for, don't trickle charge them. But I did see that same thing on some "general information" as opposed to comparisons pages about NIMH batteries. So I was left with the quandary of anticapating needing the batteries (not always possible for me) and charging them in advance, or trickle charging them. But now that you put it this way "For example, a 2500 mAh NiMh cell (at room temperature) will loose about half of its capacity in about 100 days. At the end of 100 days, you still have a cell with about 1250 mAh of capacity left in it." It makes sense to NOT worry about it, Thanks for the post.
Leads to one question however, how does the digicam detect the battery charge, and when does it decide to shutdown?
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Old Dec 11, 2005, 4:59 AM   #4
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fofa wrote:
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Leads to one question however, how does the digicam detect the battery charge, and when does it decide to shutdown?
The camea measures battery voltage and shuts down when the battery voltage has dropped to acertain value say 1.15 V per cell (this value differes between different cams). At this point 80-90% of the battery capacity has been discharged, providing the cells are in good condiion.

I have no problems leaving my cameras with the battery in for a month and then picking it up and start taking pictures, I still have plenty of capacity left, but also keep a set of freshly charged cells just in case.
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