Battery Station for Pellet Stove

I "finally" found a good diagram of battery circuits that explains runtime/voltage. It came from 12 volt planet dot com.

If the battery is used in parallel "you increase the AH capacity". In series you "increase the voltage" but not AH capacity. So if your UPS only has one battery and you add another in parallel you can increase the AH capacity. Series will only increase the voltage to 24V but not increase AH.

You would need another pair of 9AH batteries in series connected parallel to the first 2 batteries to increase AH capacity. I still can't find a good diagram for this part. But basically you would connect the red + wires from each series pack to each other and then the black - wires connected to each other to increase AH capacity of a UPS. Hope this doesn't confuse?