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I'm building a new battery pack for an electric drill. One of the cells went dead, but I'm replacing all three to make sure they match.

The new battery pack will have 3 AA NiCd cells in series. Each one will have 1000 mAh and 1.2V. The original charger has an output of 4.2V AC ~ 100mA. Will this be sufficient to charge the new pack? Also, how long should I charge?

I've read tons of info on charging single batteries, but I can't seem to find any info on charging them in series.

update:
I appreciate the responses and thought I'd let you all know how it turned out. I ended up leaving out the diode from the old battery pack, and swapping out the proprietary charger connector with a more standard one. Rather than reusing the original charger, I have it on a smart hobby charger. This new charger can be set for multiple configurations, including 3s NiCd, and charges them up accordingly. So the cells charged up happy at 100mA, and the drill now works perfectly again!

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    \$\begingroup\$ You can charge nickel chemistries in series OK, also lead, as with precautions they will both tolerate overcharge. It's during the controlled overcharge that cell balancing occurs. All the hoohah you read about the problems with series cells are for lithium chemistries. They will not accept overcharge under any circumstances, so cell balancing must be done in a more complicated way. \$\endgroup\$ – Neil_UK Apr 13 '17 at 6:27
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It seems good to me,

A C/10 charge for NiCd batteries is recommended at a rate equal to one tenth the mAh capacity (100mA in your case) for 14 hours. so charge from empty for 14 hours.

The battery requires a DC supply to charge, you'll need to convert the AC somehow. I guess you can re-use whatever setup the old battery used.

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  • \$\begingroup\$ Thanks for your answer! I thought it was odd that the charger outputs AC and not DC, but that's what the tool came with. So having 3 in series doesn't affect the C/10 value? \$\endgroup\$ – Mike Gray Apr 13 '17 at 6:43
  • \$\begingroup\$ So just for fun, I broke out my multimeter and measured the voltage at the barrel connector of my power adapter. It read 4.6VAC, 0VDC. I then hooked it up and read the voltage at the tabs that connect to the battery pack. Just in case there's a rectifier hidden in there somewhere. I still read 4.6VAC, but I got 2.3VDC. I'm not sure how to interpret those results. Suggestions? \$\endgroup\$ – Mike Gray Apr 14 '17 at 10:19
  • \$\begingroup\$ In series they all see the whole current flowing in one end and out the other. 2.3VDC could be a half-wave rectifier on a 4.6VAC supply. \$\endgroup\$ – Jasen Apr 17 '17 at 4:56

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