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Gyrobob 04-05-2010 05:02 PM

New batteries in cordless drill won't charge
I just replaced the original nicad batteries in a Black and Decker 9099 7.2v drill with new nicads. it took about an hour to assemble a new battery pack from 6 sub C batteries. Now they won't charge up. The wiring is exactly the same as with the old pack, and the charger is putting out about 12-13 vac.

I notice the charger's specs are 120VAC input and 12VAC output. How is it possible for the batteries to get recharged with AC current?

I realize this is a cheapo drill, not worth spending the time fixing, but it is a matter of principle now. I can't let this thing defeat me!!

Thanks for any info.


brric 04-05-2010 06:50 PM

Sounds like a fabrication or compatibility issue to me.

AllanJ 04-05-2010 09:11 PM

So the old battery pack was one sealed piece and you fabricated a new pack using six separate nicad batteries (cells)?

The old battery pack might have had some additional components (such as diodes) inside that you did not include in your new pack. A diode will convert AC to DC. Resistors would also be needed to keep the recharge current from getting too high.

AC will not recharge a battery and simply connecting it up that way could damage the battery or components inside the charger.

MattOverMinder 04-07-2010 03:18 PM

There are a few things. First, when you say that the charger's specs are 120VAC in / 12VDC out, are you referring to the power brick that plugs into the wall? If you, that simply means that it is taking 120AC from the wall and converting it to 12VDC for the drill. This all happens inside the charger... the drill just sees the 12VDC for charging the batteries. As for why the new ones won't charge, there could be several reasons. A few that come to mind are...

- Make sure the polarity is correct for the charge plug to the batteries. Charging batteries is backwards from using them. "+" from the charger goes to "+" on the batteries, and "-" to "-", of course.

- Another thing is that I'm assuming you soldered these batteries in place. Heat from soldering can damage batteries. You have to work quick... not too quick, but you can't leave the iron on the battery for several minutes. Heat and batteries do not get along.

- Have you tried measuring the voltage from the pack, itself? Are you getting anything. If it's giving around the 7.2VDC that's expected, you could have a loose wire in the drill, somewhere.

Gyrobob 04-08-2010 10:16 AM

AllanJ: I took the old battery pack apart so I could use its clear plastic container for the 6 subC cells. There are no diodes. The original battery pack was simply 6 cells in series. It is a 2-speed drill so there are 3 wires going to it, to allow operation on all 6 cells, or just three cells.

MattOverMinder: I said the charger’s specs were 120VAC in/12VAC out.,… and, yes, I am referring to the black lump that plugs into the wall, and that has a skinny black 2-conductor wire that plugs into the drill. I thought about polarity, but with AC, it is not an issue. The charger is putting out 12.2VAC as measured by a VOM. The batteries I used to rebuild the battery pack had tabs on them – they were designed for battery packs, so the soldering was on the tabs, not the body of the batteries. I did check the battery pack after I put it together and it is showing about 4 volts, which doesn’t surprise me since the cells were sitting around for a year or so before I finally got to this project.

It still puzzles me about 12VAC being applied to a battery pack.

MattOverMinder 04-08-2010 10:46 AM

OK, sorry, I mistook the 12VAC out part. Yeah, that means that somewhere within the drill, itself, is a rectifier of some sort. One thing to try would be to reopen the drill, disconnect the new battery pack, plug the charger into the drill and then check the leads that connect to the battery with your meter and make sure that they are showing DC voltage. You absolutely cannot charge a battery with AC, so something has to be rectifying the current somewhere. Are you absolutely sure that you're using the charger that came with the drill? It seems odd to me that the charger wouldn't be doing the rectifying. Just had to ask... ;)

Gyrobob 04-08-2010 11:41 AM

It's always good to check for the really obvious stuff.

Yes, I am sure it is the original charger. The printing on the side of the drill specifies which model number charger to use, and the charger has that model number on it.

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