Many rechargable batteries, especially high capacity types, accept a
very fast charge equal to or greater than 1C.  The main problem with
charging, aside from battery chemistry and design, is heat.  I'm told,
but have never verified, that the hotter the cell is, the more energy
goes into heat production rather than into charging.

This is one problem with charging at 1C for an hour - it gets warm after
the first 20 minutes, so you lose more and more of the charging current
to heat and you don't get a full charge.  You need to charge at some
larger value (1.1-1.5C) for the hour to get a full 1C into the cell.

Some charges implement a bulk charge feature to get around the heat
generation.  Since the batteries have a large thermal mass they charge
them at up to 10C for a short period of time (until they reach a certian
temperature) and then lower the charge to 1/2C or lower to complete the
charge.  Then you can complete the charge within 30-60 minutes.

I do not know whether this damages the cell.  Many people who want fast
charging also want high capacities, and while a 4.5AH 4/3A cell is not
cheap, they are often more interested in high current output and fast
charging than they are in cell health during, for instance, RC races.

-Adam

Tim Hart wrote:

>The other day I read that charging NiMH in 60 minutes might harm the batteries.  My 1800 ma/h AA batteries do in fact get quite hot during the charging.  I leave the lid off just for cooling.  The article recommended 4 hour charging when you had the time and 60 or 90 minute chargers when you didn't :)
>
>Anyway the other day I saw a charger from Energizer that claimed to work in 30 minutes!  That sounds like a ploy to destroy batteries and force the consumer to buy more NiMH batteries or maybe even give up and go back to disposables!  A 2000 ma AA would be getting around 4 amps to charge in 30 minutes....that seems quite excessive!
>
>I'm just a newbie here but I was wondering what you guys and gals had to say!?
>
>Thanks,
>Tim
>
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