I have an electric rotary tool that's battery pack is not taking much of a charge anymore and I want to modify it to use an AC\DC power pack. The tool is a 96v rotary tool. The amperage isn't listed, but I'm guessing since the power pack is made up of 8 AA sized batteries wired in series that the amperage would be the same as one of the NiCD rechargeable batteries. This is just a guess, but because the tool is a couple years old and wasn't expensive; I'm figuring the batteries have an output of 1.2 volts & around 1 amp. So what I'm wanting to figure out is if running the tool with not enough amps is more harmful than running it with not enough volts. I have two power supplies one of them with an output of 9.0V 888 mA & the other having an output of 10.0V 700 mA. I realize both aren't quite a full amp of output, but this is the closest I can get. So would these burn up the motor to the tool or would this feasibly be all right? Which one would be better? I forgot which is better to have in surplus; volts or amperes. Any help would greatly be appreciated!

Both will work. Neither very well.

Let's go somewhere else for now. A car battery. Maybe 400 AMPs for a few seconds, so we can power a headlamp or a lic plate lamp and the starter motor. Current limiting (fuses/breakers) are important to prevent fires. It's a power source. It's rated in AMP-Hours, but it can deliver LOTS of current for a short time.

Now back to your motor. Lower the voltage. It runs slower. Lower the "AVAILABLE CURRENT" and the torque decreases.

An AA Ni-Cad might be able to put out 18 amps momentarily, so that battery will have much more power than either or your power supplies.

Your drill may use a 9.6V battery, but it might require an 11.6 Volt charger be able to put energy INTO the battery.

Exceeding the designed voltage by 20-25% will be detrimental. Not enough current creates heat. The load or motor will take what it needs unless it is stalled. This locked rotor current it cannot take for long so some sort of overcurrent device must be used.

As KISS said, neither supply will work well. You're unlikely to burn out the motor with either of those supplies, but you could easily burn out the supply if you place any significant load (torque) on the motor, since it will then try to draw a lot of current, more than the supplies are rated to deliver. If the supply has no overload protection (cheap ones don'y usually) it could burn out.

And your motor will probably stall due to insufficient current available from the power supply. A new battery would be my recommendation, or buy a cheap Chinese replacement tool (for $15 or whatever they cost where you live), and replace it each time it or its battery dies!