Model 28 has a 36 volt system. Staab Battery offers several options that will fit this unit including some of the longest lasting deep cycle batteries on the market. Be aware some machines came from the factory with optional batteries so please confirm the dimensions and capacities of the batteries you are considering before placing your order. If you need any assistance with selecting the proper battery for your unit please contact us at 1-877-897-1226 and we will be happy to assist you.