Building a circuit to modify a UPS

A friend of mine has a 750 Watt UPS it works on 6v batteries but the batteries got bad but we cannot get 6v batteries. He has some 12v batteries that he could use on the UPS he was telling me that he saw some circuits online to convert 12v to 6v and he built them and they all work but it kept burning out when I checked what he built they are some simple 1amp dc to dc converters.

I think he might need a circuit probably with probably about 100amps output even though it should be more to match the spec of the UPS (125amp roughly).

I was wondering if I had to build a Inverter then add a power supply stage to it to accomplish that (we arent working based on price we are working from availability also he has alot of electronic parts we could use).

If I take that approach what I realize is that the Dc wouldn't be clean going to the UPS because I wouldn't be able to use a Regulator since I need high current. Culd I get some advice on this.

It does not seem likely that the unit would have a single 6V battery, delivering 750W. That's over 125 Amperes.

When I opened up my UPS, I saw four 6V gel cells, in series. 24V total.

Perhaps yours is similar? Can you open the cabinet and see whether you could hook up your 12V battery? Or perhaps it needs two. You may need to attach custom terminals.

It is unlikely that the 750W UPS (it is more likely 750VA with a PF of 0.7 making it 500W) uses 6V input. My UPS is an APC made 1000VA and it uses 12V batteries.

Will it be cost effective to convert 12V to 6V just to run the UPS? Perhaps you can modify within the UPS circuit to use 12V rather than 6V. It will perhaps be possible because the drive transistors will be required to handle lower currents.

Even if you have lots of electronic components to spare for a 12V to 6V converter with 100A capacity, the point you will be making will be trivial.

What is the AH capacity of the 6V battery that needs replacement?