Resistor value for battery charger

Okay, I think this is a basic question...

Bought an rechargable drill at a garage sale. Of course it doesn't work - because both batteries are discharged. Turns out the problem is with the battery charger. It is composed of a transformer that plugs into the wall and a base unit that holds the 12VDC battery.

The wall unit works okay and produces 17VDC @ 400mA. The problem is with the base unit. When I took it apart I found a single ceramic resistor that was wired in series - but it was burned beyond recognition.

Okay, my question: what value of resistor do I need to buy to replace the burned one?

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Mine has a 4.7ohm 1Watt. (there's a IN4001 rectifier diode in there as well).

Great, thanks much!

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Well, its a Ryobi HP1201 drill and the charger is part 1400674 but I could find no schematics for it (the charger). I did find a parts diagram for the drill but not a breakout for the charger.

Ron

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I was also unsuccessful locating a parts diagram for the charger. Although, I did find many results of people having the same problem..

Perhaps someone that reads this group has the same drill and can give you specs by disassembling theirs..

Mike

Some of the charges are smart-chargers, in that they have a third wire connected inside the batterypack that senses the pack voltage as it is charging.

You can tell if yours has this because there will be a third contact on the pack and in the charger connection well.

It sounds like yours may be a simple constant voltage charger with a current limit resistor.

I suspect the reason the resistor is damaged is your battery pack's nicad batteries has one or more shorted cells. This causes the pack to draw more current than it would normally.

If this is the case, it's time to get another batterypack. You could open the pack and measure which of the cells have a voltage and which are at zero volts. You will likely find some are dead. Replacements are available, but, the other cells are likely to fail soon as well.

I've tried the capacitor "shock" method to revive some nicads, but with limited reliability. The "shock" is a current surge from a capacitor that removes some of the internal crystaline structures that develop in a dead battery which push through the battery separator material.

Only two contacts and the series resistor.

Thanks to all.

Ron

PS There are some smart folks out there!

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