I have a pure sine wave inverter, it charges a 12V battery and converts 12V from battery to 220V during a power cut. Since it can output 12V to charge the battery at quite a high current I was wondering if I could use it as a 12V power supply. I connected the 12V output to a multimeter and it seems to be jumping from 6.xx volts to 13.xx. I reckon it automatically selects 6V or 12V battery. So is there any way I could bypass this jumping from one voltage to another to use it as a power supply?
What you have sounds like an UPS (uninterruptible power supply).
The UPS have a charge controller that will detect the state of charge of the battery, so if no battery is connected and only some circuit, it won't work as the charge controller won't understand what is going on.
Although, you can connect a battery to it and then use it as a 12V power supply with the battery connected and this should work as the battery will keep a constant 12V (about) to the charge controller.