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Old Dec 13, 2003, 12:13 AM   #1
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Default Battery charger help

I am looking for a charger that can charge as much AAA batteries as possible (12) and one that can maybe do both NiCa and NimH.
If possible in a rapid charge option with both AC and car 12 charger.If not can you suggest my next best option...
thanks all :lol:
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Old Dec 13, 2003, 5:49 AM   #2
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If you really do mean AAA and not AA you should be VERY careful choosing a fast charger. These small batts. have little surface area to dissipate heat from a fast charging process, so there is a safety risk. VOX
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Old Dec 13, 2003, 8:11 AM   #3
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Originally Posted by voxmagna
These small batts. have little surface area to dissipate heat
Actually, if you have two batteries the same shape, and one is twice as long as the other, the larger one will have four times the surface area, and eight times the volume. The larger one requires 8 times the charging current and generates 8 times the waste heat, but has only four times the surface for heat loss, so would be at *greater* risk from overheating than the small one. (cf Mice need proportionately a lot more fuel to stay alive than elephants.)

So provided the charger gives a suitably scaled down current, heat dissipation should be OK in little cells. If charging at constant current, however, in a very unintelligent charger, it would be a very bad idea to charge AAA cells at the rate for AA cells.

I note that my Uniross 600mAh AAA cells and 1700mAh AA cells are both stamped with a constant current 7h charge rate of approx 0.18C (where C is cell capacity), ie., 110mA for AAA and 320mA for AA. Other things being equal, I should expect the AAA batteries to stay *cooler* under this regime, because of their greater ratio of surface area to waste heat generation rate. This is indeed the case in my "Quest Intelligent Fast Charger"

A further confusion that I have ignored here is that heat generated in the centre of the larger cells will have more difficulty getting out. (cf a big furnace is a lot more energy efficient than a small one). If there are any heat transfer engineers round here, perhaps they could provide us with the definitive analytical or computer-modelled solution.

In conclusion, if your AAAs get very hot compared with your AAs, it's because they're being charged too fast for their capacity.
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Old Dec 14, 2003, 9:48 AM   #4
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This one holds 10 AA or AAA batteries.


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Old Dec 14, 2003, 1:25 PM   #5
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Steve, Vox,

Looking at the link to the 10-cell charger Steve showed us, and still intrigued by the overheating business that Vox (& others) have mentioned, I notice that the following charging times are quoted in the charger description......

AAA NiMH 700 mAh - 4 Hours
AA NiMH 1600 mAh - 9 Hours
AA NiMH 1800 mAh - 10 Hours
AA NiMH 2100 mAh - 12 Hours

All these indicate an average charging current of 175-180mA at 100% efficiency, or 218-225mA with a realistic 25% or so overcharge. So this 'intelligent fast charger' just whams a current through the cells regardless of their size, which is much *smaller* than an AA could stand, but quite a lot *larger* than that recommended for an AAA. It looks to me as though there's a sliding contact to accommodate either cell, as in one or two multisize chargers that I own.

Printed on the side of my best, 1800mAh Ni-MH cells from 'GP Batteries', it says...

Standard charge 14h@180mA
Fast charge 5h @ 450mA

.....and as I said in my message above, the standard charge for my 600mAh AAA cells is 7h @110mA.

So it seems to me that 'fast' chargers that are genuinely fast for AA cells will thoroughly cook AAAs and really need their accurate end-of-charge detector, and a charger that doesn't cook the AAAs won't be very fast for AAs.

Or are there chargers that can tell the difference between the two sizes of cell? :?:
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