I have a 24 vdc big UPS APC which is an inverter to 120 vac.
Its battery died and 12 vdc input is much more useful to me.

My guess is it would not be easy?

 

not unless it had been designed and built that way.

 

No, it's going to think your 24 volt battery has turned to mud, and refuse to work.

(It won't have enough gain.)

 

I know it wont work as is.

An optional idea is (consider this for a boat), I have 3 12vbatteries. One battery for starting, two for house use. All hooked up for 12v use.
Is there a way to combine two 12v batteries so as to get 12 and 24 vdc from them with full power available to each voltage, my assumption is not really realistic. Considering recharging issues.

 

Sometimes these devices have inner jumpers that allow them to be configured for 12 or 24 VDC as their input voltage...
Do you have a pic of the PCB or schematic you could share?

 

If your 12V batteries share the same ground, you can't just attach some combination of terminals to get 24V.
You can't run the starter battery in the 12 to 24 volt position or corrosion will eat your motor.
Perhaps connect the +12V of the starter battery to the grounds of the house batteries and connect to the + of a house battery to get 24 volts?

Then you would be using up your precious starter battery which wasn't designed to go into deep cycle mode, and still have to be incredibly careful of corrosion.
It seems so much easier and safer to just buy another battery.

 

It seems so much easier and safer to just buy another battery.

Quite true... but then again our friend could be looking into the possibility of plugging this thing to his car (he said 12V would be much more useful), or something similar... Can't know what his intentions are until he elaborates a bit more...

 

Post #4 says, "consider this for a boat".

 

Post #4 says, "consider this for a boat".

shame on my reading skills and powers of observation...

 

What about this http://powersupply33.com/24v-to-12v-converter.html ?