The 750 charger will charge batteries with very little voltage, but it requires a special procedure. This charger is designed to never damage a vehicle's ECM (electronic control module). This makes charging batteries that have less than 3 volts more difficult. The 750 will charge a battery in this condition faster than a traditional charger when the proper technique is used. Usually, batteries that have low voltage become rapidly sulfated, making them more difficult to charge.
After hooking up the charger to a battery, look at the voltmeter. If the voltage is less than 3 volts or it will not accept any amperage, use the following procedure:
1) With the amp selector on minimum, hold the dead battery activator down for 20 seconds.
2) After releasing the dead battery activator switch, slowly turn the amp selector clockwise and watch the voltmeter. If the voltmeter stops rising and drops to less than 3 volts, go back to step 1 and turn the amp selector to a point slightly less than done previously. When the voltmeter stabilizes (usually between 12-14 volts) let the charger charge for 15-30 minutes.

Note: The ammeter will be on zero during this period. After 15-30 minutes, try turning the amp selector up.

If the battery still does not accept a charge; (ie, shows amperage on the ammeter) it may not have had sufficient time to break the sulfation, or the battery may be bad (open cell).