How do you get a replacement battery same size

If your battery pack is made up of three 12v batteries, you can buy them from Home Depot or online from many battery distributors. The basic 12v sealed AGM battery is rated for 7 amp hour. There are many applications for this battery; home alarm backup power, Verizon Fios battery backup for Verizon phone line coupled to its fiber optic system, hobby power for radio control engine starters before electric power took over, etc. Examine labeling on your battery pack for specs. AGM is more expensive than conventional sealed gel cell batteries and probably hold up better to high current demands of electric lawn mowers.