Help me understand how my battery pack is charging?

[rebelrider.mike]

I have a little 3.6V electric screwdriver that came in handy when it was new. After a while though, it started having trouble holding a charge, and is currently pretty useless. So I've opened it up and removed the battery pack, and I've been poking around with my multimeter. So here's what I know:

The battery pack is made of 3 batteries in series. From the labels (BYD D-AA800P 1.2V K38) I'm pretty sure they are NiCd 1.2V AAs, with 800 mAh each. If I understand my battery physics right, in series that's 3.6V, 800 mAh total. Well, its a 3.6V drill, so that makes sense so far...
Two of the batteries are measuring 1V, and the third is measuring 0V. I suppose a dead cell would explain why its not charging.

The wall adapter has an output of 4.2VAC, 100 mA. That seems odd to me, but the multimeter confirmed at the barrel connector, 4.6VAC, 0VDC.
So I hooked it up and measured directly at the tabs that connect the battery pack in case there's a rectifier hidden in there somewhere. Still got 4.6VAC, but also got 2.3VDC.

Well, I'm confused as to how my battery pack is actually getting charged. What does it mean that I'm getting half of the AC as DC, but still the AC? Does that mean I have a diode but no rectifier? Is 2.3VDC enough to charge these batteries? Is the 4.6VAC damaging them? Looks like there may be a diode hidden under the shrinkwrap between the barrel connector and the + battery tab?

I plan to get new batteries to replace the old ones, but I'd like to be able to understand this stuff, so I can be sure not to wreck the new batteries.

[mariush]

There's probably a diode under the heatshrink doing half wave rectification.
The power supply probably uses a linear transformer which outputs 4.2v+ at no load, less at 100mA

Since the diode will blow the flow of electricity for half the time, it's like the batteries receive pulses of energy.. even if they receive more than 1.55v per cell which is the recommended voltage or around 4.65v per pack (from memory, not 100% i'm right), they'll probably be fine as they don't charge fast enough (everything's limited by the 100mA the transformer can output)

[SeanB]

Yes, there is a diode there in the wiring loom, and this is a very simple charger, good at charging if you remember to always only charge the battery pack for 12 hours, and never leave it on charge for more than that, which kills the cells quicker than the regular 12 hour charge does.

3 new AA tagged NimH cells in place of those, and you can use higher capacity cells as well for slightly longer run time, I use 1200mAh ones, plus the addition of a series 4R7 0.5W ( what was on hand at the time) resistor in the charge wire enabled me to have a longer battery life, but they still will kill cells long term if you leave them plugged in permanently. The lower charge current is good for the NiCd cells, but absolutely needed for NimH cells, as they do not like being overcharged at any higher than C20 for even short periods, so you need to have a longer charge time at a lower current to protect them, without going to put in a proper charge controller.

[rebelrider.mike]

Thanks you guys, for the responses!

I've got some replacement batteries on order. Even though only one of the cells is bad, I'm replacing all three so they match. I'll be using 3 NiCd 1.2v AAs at 1,000 mAh each. If I understand correctly, the 100mA from the wall charger should be about right to charge them in series. Is 2.3v enough though? I guess it worked on the original battery pack, but I've been told that charger voltage should match the battery voltage. 3.6v in this case. Maybe 2.3v is close enough?

[rebelrider.mike]

New batteries arrived in the mail. I've been using Tengergy NiMH for a couple years and I love them. Hopefully the NiCds will be just as good.


The third battery is shoved over a bit to make room for the barrel connector.


Battery pack finished. 3.89v. I guess I managed not to break them while soldering.


Fits! I also put a little solder on the connectors since the new tabs aren't an exact fit.


So far it works! You can't see it spinning, but it does. Also has a nice LED guide light.


Seems the batteries are fully charged already, so I'll have to run them down first to see how the original charger works. I also plan to figure out some way to monitor their progress. Still trying to wrap my brain around the relationship between charging and volts. Maybe I should just buy a book on electrical theory and read the darn thing. Well you talked me into it; I'm off to Amazon.com.