Dead power supply on a charger - can I replace it?

July 17, 2008 1:21 AM

Can I replace a power supply on a cordless drill battery charger with a different voltage power supply?

I have a black and decker cordless drill, and three batteries. The power pack for the charger has died, and it seems the charger is the older model, thus rather hard to get.

The existing power supply that plugs into the wall is 14.5V, 200ma.

I have a bunch of 12V power supplies laying about the house, unused.

If I replace the dead power supply with the 12V unit, will I cause any problems, or will it just charge slower?

posted by tomble to home & garden (3 comments total)

The 12V rating is what the charge will produce under load. Generally these chargers will output in the 14 V range. Which means that they will charge the batteries to a degree, just not fully.
posted by ptm at 2:02 AM on July 17

Probably not a problem, but you'll kill the battery pretty quickly.

In general, rechargeable batteries like to be fully charged and fully discharged. Under charging them (as will result from applying a lower voltage) and then draining them a few times will permanently decrease the storage capacity of the battery. In NiCad batteries, then this "memory" effect is particuarly pronounced.
posted by three blind mice at 4:19 AM on July 17

Open up the charger. You might find that the 14.5V is dropped across a diode, removing 0.7V and thereby being a good match for a 12V (lead acid) battery (one wants to maintain about 1.5V higher charging voltage than the stated 12V-cell capacity). You could remove/bypass this diode and use one of your 12V supplies. It is still a compromise, but it is loads better than having an 11.3V charging voltage on the battery.

Or, go looking for 15V supplies, which aren't too hard to find, and stick another diode in series with the output inside the charger. This seems like a much better solution.

If you end up using a supply with a much larger current rating than the original, you will want to current limit the charger by adding series resistance (10 Ohms or so, 1/2 or 1W resistors) for safety. This way, if the charger leads are shorted out, very little current will be drawn.
posted by fatllama at 5:19 AM on July 17