Fully charged new Ni-MH battery gets used up in seconds

[james_s]

That's effectively just a power supply with one half of a transformer, the other half is the coil in the base of the toothbrush. The charger is part of the toothbrush itself.

[mikerj]

so have you checked the spring is not shorting the battery +ve tab and its metal body?

I am not getting a short between the spring and the motor's body.

That's not what people are saying.  That spring looks like it could very easily short the outside can of the cell (the -ve connection) to the +ve battery terminal.  Is there something else you've left out from this assembly, e.g. some kind of insulating disk to ensure that spring cannot contact the outer body of the cell?  Relying on the green shrinkwrap on the cell to do this will result in disappointment and maybe smoke.

[niemand]

I am not getting a short between the spring and the motor's body.

That's not what people are saying.  That spring looks like it could very easily short the outside can of the cell (the -ve connection) to the +ve battery terminal.  Is there something else you've left out from this assembly, e.g. some kind of insulating disk to ensure that spring cannot contact the outer body of the cell?  Relying on the green shrinkwrap on the cell to do this will result in disappointment and maybe smoke.

Now I understand. The photo shows the full assembly, nothing left out, except for the housing. I will find a way to tape up that exposed spot. "Smoke" is right. I caused that rip when I was trying to insert the spring, which scraped the shrink wrap while another part of the spring came into contact with the solder on the motor (as can be seen in the photo), and the spring heated up fast, and there was smoke.

[niemand]

You'll need to use the A range to measure Amps, and then you open the circuit and wire the meter in series with the battery. DO NOT attempt to measure current across the battery, that is a classic rookie mistake that blows meter fuses.

I've made an attempt to read current draw, but nothing registered (DMM displayed "0.000"). The battery is 3/4 charged, reading 1.44v. I think the current is not showing in the meter is because of the same reason that the battery needs to be soldered in order for the toothbrush to function (as Fraser suggested above). I don't have wiring or test leads supplies that can simulate a more solid (soldered-like) connection.

Interesting observation: Before I took out the battery from the handle, as soon as I lifted one of the tabs and put it back in place to once again make contact with the battery tab, the "3/4 charge" display immediately changed to "empty battery" icon. It never changes back to 3/4 charge. I would have to recharge the battery again (put everything back in place and put the handle back in the charger). Even after I've rigged it up to measure current draw, the display would only show the  "empty battery" icon.
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[niemand]

Thanks to Fraser's sharing of his experience, the toothbrush is now working excellently after I soldered the connections to the battery (the new battery's tabs onto the tabs left behind from the previous battery). The toothbrush can now, with one full charge, deliver 22 sessions of brushing with an average brushing duration of 2 minutes and 45 seconds per session. Even after the 22nd session, the toothbrush didn't stop dead, but I decided to recharge it at that point because the motor became noticeably less vigorous. Prior to the new battery, the motor would just stop dead in the midst of the 5th session.