Your motor in your starter box needs 12 volts, so that is standard. Ah (Amper per hour) is the capacity of the battery, or just in simple words, is the capacity of how many times you can use the starer box. Simply, a 17 Ah is better than a 12 Ah, and a 50 Ah is better than the 17 Ah. The problem of course is the mass the battery is taking. A battery of 50 Ah is very bigger than that of 17 Ah. In conclusion, you must buy a battery of 12 Vlots (standard) and see what size you prefer. There are chargers in the market so you can charge the Lithioum batteries, don't worry, you don't gonna throw the battery to the rubbish once it finishes.