Converting a Ryobi polisher from 12v battery to 12v DC transformer

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I have this little Ryobi polisher that uses a rechargeable battery. Unfortunately unlike the drill and the little 'rotary tool' the polisher sucks the battery dry in no time.

I have a 12vdc power supply (13 amps) that I want to use instead of the battery.

I have tried connecting it up and nothing happens. I have polarity right but there is a 'third wire' that connects to the battery that measures the same voltage as the negative terminal. Even if I connect negative to that the motor still will not turn..

Any ideas?

The connections inside the polisher are;
+ positive
- negative
T and whatever 't' does...


Ryobi 12v battery pack

 

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