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Old Sep 27, 2005, 2:31 PM   #1
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Hi all,

Sorry for a probably naive question. I understand themeaning of battery

capacity. But do NiMH chargers have to be matched to the battery? e.g. Do I

need a "2500 mAh charger' to go with a 2500 mAh battery? If so, could I use

a 2500 mAh charger with say a 2000 mAh battery? And what would happen if I

used a 2500 mAh charger with a 2700 mAh battery?

Thanks,

Paul.



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Old Sep 27, 2005, 6:51 PM   #2
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Hello Paul,

Some chargers are stupid and simply keep charging until a timer shuts them off, or you take the batteries out. With these chargers you need to figure out the charge rate and divide the battery capacity by that then multiply it by about 1.2. For example, a 2500 mAh battery charged with a current of 250 mA would take about 12 hours to charge up.

Other chargers can detect the end of charge signal and will charge a battery regardless of its capacity - within reason. Some of these chargers have a timer that limits the amount of charge time. If your battery has more capacity than the charger has charge time, you either have to leave it on the charger for a couple of days to trickle charge it full, or unplug the charger half way through the charge cycle to reset the timer.

To answer your questions...

The charger does not have to be matched to the cell you are charging.

If you use a 2500 mAh capacity charger to charge a 2000 mAh cell, it will finish charging quicker.

If you charge a completely discharged 2700 mAh cell on a charger that shuts off after 2500 mAh, you will end up with a partially charged cell. You can start the charge, then unplug and replug the charger to reset the timer about half way into the charge cycle and will get a fully charged cell. However, if you recharge your 2700 mAh cell before it is fully discharged, (for example after using 2200 mAh of capacity) you can just charge it normally.

Tom
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Old Sep 28, 2005, 7:20 AM   #3
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Thanks, Tom. That makes complete sense and I appreciate your reply.
The only thing I don't understand is why would a smart charger (that can
detect end of charge) also have a timer limit?

Funnily enough a separate thread just started on this topic
Sony BCG-34HRMD
that relates to my question. The person there is asking if the above
charger can charge 2500 mAh batteries. Like me, probably, he discovered
that Sony has made this nice travel charger (compact, universal voltage)
that is rated in the manual as charging batteries in the
range 1850-2300 mAh, but no longer makes 2300 mAh batteries,
only 2500 mAh.

So there's an example - it is a smart charger with auto cutoff when the cell is
full, but it is 'rated' for cells up to 2300 mAh. Any ideas why?

(BTW, I did not see the manual, but I saw the comment about the battery capacity
limits on the web),

Thanks,
Paul.


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Old Sep 28, 2005, 8:48 AM   #4
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hi paul,

yes, the sony manual for that charger provided a table for charge-time only in respect of batteries with a capacity of up to 2300 mili-amps. no mention of 2500 mAh batteries therein. indeed, if the sony charger is smart, it should be able to fully charge a 2500mAh battery but I do wonder if the timer will stop it at 2300mAh?

regards
gunn
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Old Sep 28, 2005, 9:16 AM   #5
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Hello Paul,

"Smart" chargers have a timer shut off as a safety back up in the event they miss the end of charge signal.

Smarter chargers allow you to also enter a capacity for the battery you are charging. This type of charger will shut off when that capacity is reached as a further safety back up. They assume that they can detect the end of charge signal, but use both the total capacity number as well as a timer to terminate the charge in the event the end of charge signal is missed. They also publish their charging rates so you can do the math to figure out total charge capacity limits.

As to weather the Sony charger will fully charge a fully discharged 2500 mAh cell, we are missing a couple of important pieces of the puzzle. What is the charge rate, and what is the timer limit? Once you have this information, a reasonable guess can be made...

Tom
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Old Sep 28, 2005, 11:34 AM   #6
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I'll leave that one to Gunn - I couldn't find the product
manual even though I hunted around for a while...

Paul.

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Old Sep 29, 2005, 1:57 AM   #7
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Charging formula :

Normal or basic charge is reffered to as C10. 10 means charging current is 10 % of the capacity for 12 hours. Ideally it is supposed to be 10 hours, the additional2 hours is to compensate for losses (not all charge is absorbed by the battery).

With this you can calculate the craging time ( useful if the charger has no automatic shutoff).

If the charging current is double the required, the time required is reduced by one half,example:

charger with charging capacity of 250 mA will charge a 1250 mAH battery for 6 hours only

If the Charging current is half of the normal, the time required will be double, example:

charger with capacity of 125 mA will charge a 2500 mAH battery for 24 hours instead of 12 hours.

In theory, you can charge a battery in matter of minutes if you put enough high current, in practice quick chargers limit the current such that charging time can be reduced to 2 to 4 hours only otherwise the battery will overheat and the useful life will be shortened.












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Old Sep 29, 2005, 3:00 PM   #8
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Hello Juandelacruz,

Excellent information for NiMh and NiCd batteries.

I am afraid I have to disagree with you on your last statement though. The Energizer 15 minute charger not only charges cells to over 97% of full capacityfast, it also does it without overheating them. They have come up with an advanced charging algorithm that works very well. I am very impressed with this charger.

If there are long term effects from this rapid charger, I have not noticed them over the 150 cycles that I have used it.

Technology appears to have advanced the "state of the art" in battery charging.

Tom
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Old Sep 30, 2005, 8:08 AM   #9
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Maha Chargers are great, and many are multivoltage. You buy them through Thomas Distributing. Do a search on Maha.
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Old Oct 1, 2005, 6:39 AM   #10
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Hello!

100% agree!

After been a "Collector" of batteries chargers, at least I can be safe for an a long time with my new Maha 204F charger, which also discharge NiCD/NiMH ones. Truly a lovely charger, I order mine 220 for be suitable to Israel.

Grap one of the Maha that is suitable for you...& enjoy!

Regrdsa,

Alex 007
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