12 Volt cordless drill

I'd bet that a trickle charge current for a car battery is probably way too much to trickle charge some C or sub-C cells! Look on the internet for charge rates for cells of the same type and size as yours, and compare that to the trickle charge current rating on your car battery charger.

Regards,

Paul

John wrote:

With a car battery charger you will never know when the cordless drill battery is fully charged and will risk overcharging it and destroying it. The $60 charger has a built in circuit to stop charging when the battery voltage rises to the correct voltage for a full charge. This is very different from that for a lead-acid car battery.

MAHA sells a unit that will charge Nickel Cadmium, and Nickel Metal Hydride, batteries. It has charging pins that will adapt to almost any pack. I find it very useful as it will charge almost any pack, in any configuration, from I believe three to twelve volts. It has a corded temp sensor that will disconnect the charger if the pack gets too hot.

It also includes an adapter that will allow operation from a cigarette lighter receptacle.

The feature I really like is the ability to discharge then charge the pack. I find that this will bring back many NiCad packs.

It's not as convenient as the OEM charger, but it makes up for it by adapting to many packs.

I believe this same model is also sold at Radio Shack for $29 to $39.

Louis--

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The auto type charger is not good for this. You can build a simple charger for less than $10.00. The trick is to figure a way to conveniently connect the charger to the batteries.

The simple charger consists of a DC supply providing at least 2 volts more than the battery voltage, an LM317 chip configured as a current regulator, and a 22 ohm resistor to set the current to ~56 mA for trickle charging. The DC supply can be a wall wart rated to provide ~200 mA or more.

Generally speaking, if you set the charge rate for between

50 and 60 mA you'll be ok for recharging drill batteries at a trickle rate. If you want to be more exact, you'll need to determine the capacity, C of your batteries in order to determine charging rate. It will take about 30 hours of charge at this rate (~56 mA) to fully recharge a completely discharged drill battery. If you go to a C/10 rate (about 100 mA), you can recharge in about 14 - 16 hours. The downside of that is a greater probability that you will harm the battery by overcharge - say by leaving it on the charger too long, or perhaps by having the circuit operate at something lower than C/10. The ~C/20 rate (56 mA) gives you a much better margin for error.

A more sophisticated charger can be built or bought, but if you want to keep it cheap and simple, it's hard to beat three components.