Originally Posted by Limey Linda
Remove the old battery. If the plates are shorted internally then it will pull the voltage down. Try again with a known good battery, maybe the one from your car.
Note: Do NOT try to switch the cables back to the original battery once it starts (i.e. while the motor is running.) The alternator circuit does not like being run without a load, and you'll risk blowing the alternator's rectifier components.
Verify the battery is the problem, then replace if needed. Shut the motor down whenever disconnecting batteries.