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Using two 12V batteries in series with both a 12V and 24V output Answered

I have a tornado shelter

that I have wired for both 120V AC and 12V DC. All of the lighting is via 12V DC while the AC is used for recharging the batteries and running a 1500 VA UPS system that powers a small TV and radio. The UPS system normally uses two small 12V SLA batteries in series. To increase the run time of the UPS I want to use auto batteries instead of the small SLA batteries normally used. In order to cut down on the cost of batteries I want to use two auto batteries, one 100AH battery used for the main lighting and a 50AH battery in series with the 100AH battery to provide the 24 volts for the UPS system. The UPS system will only use the two batteries when the AC is out which is normally rare. Do you see any problems with this setup providing I install a diode in the UPS battery feed to prevent the UPS from recharging the batteries and use separate smart chargers connected to each individual battery? I feel I have overlooked something but I can't seem to put my finger on it.

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Uncorkvet

4 weeks ago

Thanks for the suggestion. I already have a spare UPS system, a spare small LED TV and a spare radio so I was trying to reduce costs. Actually the 100AH battery is a deep cycle battery that's been used to power the tornado shelter lights for two years because I purchased it long before I ever thought about having a TV and radio in the place. The only time the UPS will be turned on is when the shelter is being occupied and the only time the UPS will be using the batteries is when there is a power failure which is rare with my electric company. I haven't purchased the second battery yet so I could switch that to a deep cycle as well but I thought that with the little use it should get the auto battery would be fine.

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Downunder35m

4 weeks ago

Maybe it is worth thinking the other direction?
A UPS is an emergency system and requires quite a lot of extra juice to covert low DC to mains power.
As you want new batteries anyway, did you consider to go with a 12V TV and radio instead?
Would mean you could use single deep cycle battery.
And those batteries won't mind the discharge while car batteries are designed to only provide high amps for a short time.