If you have two 6.25 volt 215 amp-hour batteries in series, you have 12.5 volts at 215 amp-hours. That is 12.5 x 215 = 2,687.5 watt-hours.
You'll get the rated capacity if you don't draw power at too high a rate. Usually, batteries will have different capacity ratings for different draw rates. Faster draw = smaller capacity.
Putting batteries in parallel is generally not a good idea without external control electronics designed for that situation.
I was thinking of a big RV 12.5 volt battery, then thinking of two golf cart batteries, and then concluded that it would only run the stationary concentrator for a few hours. You really need a very large battery, like a forklift battery, to run most stationary concentrators for any length of time. I gave up on that angle, and figured that we could move to a motel that had power cheaper than trying to build a back-up supply.
Inogen does make a stationary concentrator that consumes a LOT less power. It requires only 100 watts, and that makes battery back-up feasible. But insurance isn't going to give us peons a concentrator like that. Your 215 amp-hour rig would run that for about 24 hours with no CPAP.
Best solution I could find was a couple of E size tanks of oxygen, a POC with 8 hours of battery, and a plan to bug out in case of prolonged power failure.
Why run an inverter to change 12VDC to 120VAC when you are going to use the AC to run a power supply to convert it back to 12VDC? Seems inefficient.
That would be very inefficient, as you say. But with the stationary concentrator, there is no conversion back to 12V. The need there is for 120 VAC to replace the power mains. Our CPAP runs on a 24V 90W supply, so that could be powered by a couple of 12V batteries in series, and that would be an efficient arrangement.
Last edited by denton; 07/24/17.
