Charging Batteries

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BeligerAnt
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Charging Batteries

Post by BeligerAnt »

Here's the lowdown on charging NiMH/NiCd batteries...

NiMH and NiCd batteries must be charged at a constant current. The current is calculated from the battery capacity (C).

Charging at a rate of C means the charge current (in mA) is the same as the battery capacity (in mAh). So a 300mAh pack would be charged at 300mA.

Charging at C/10 or up to C/5 is fairly safe with little danger of over-charging. Nickel-based batteries can be charged indefinitely at C/10 without damage. Charging at C or higher is possible, but needs to be carefully controlled to prevent damage to the battery. A specialist charger is required for this!

Assuming 100% efficiency, charging at a rate of C for 1 hour would fully charge a battery. At fast charge rates this is roughly true, but detecting when to end the charging is quite complicated and beyond the scope of this article - use a specialist charger!

At lower charge rates, you generally need to charge for 1.4x the expected time. So, for a charge rate of C/10 you would charge for 14 hours instead of the 10 hours you might expect.

So from the example above, charging a 300mAh pack at 30mA for 14 hours would be the right thing to do. Leaving it on charge for longer would not damage the battery.

The charger voltage needs to be rated at least as high as the battery pack voltage. A 4.8V charger will not properly charge a 6V or 7.2V battery pack. However, because the charger output is constant current, it can be used to charge a lower voltage pack. Hence a 9.6V charger could be used to charge a 7.2V pack.

The Futaba/Ripmax charger supplied with the Skysport 4 is rated at 4.8V @ 70mA and 9.6V @ 70mA.

It will charge a 700mAh transmitter pack in 14 hours. Charging at C/10 it could be left on for longer without damaging the battery pack.
It can be used to charge a 4.8V 300mAh antweight battery pack in about 6 hours. Because it is charging at over C/10 it is best to make sure it is switched off after 6 hours and not left on.

Warning:
Do not overcharge battery packs, especially at high charge rates. They get very hot and destroy themselves. The heat may well destroy the equipment they are fitted in if they are charged in-situ. (I know this from bitter experience! :( )
Gary, Team BeligerAnt
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