They are just a lead acid battery.
I've used batteries from lawn mowers, motorcycles, gas golf carts, etc. in Mules and Rangers as long as they fit in the battery box.
I am using a battery from a Harley dresser in mine right now and it has excellent cranking amps on cold mornings.
There are new Lithium-Ion batteries available that are supposed to last 7-9 years and are around 100 bux.
I think Battery Tender has one available under their brand.
Texans since The Old 300 in 1824
NRA Lifetime Member