Generally, lawn mower batteries are not universal. That is to say, they may be designed for a specific brand or model of lawnmower and may not work in another machine. If you have an unknown battery, it's best to take it to a hardware store or professional repair shop to test if it will charge and operate in your mower. Most lawn mower batteries are 12 V Group U1, but not all of them are the same. You must match the terminal positions and cranking amps to ensure a stable charge. If the battery is standard, it will be lead acid or nickel-cadmium. If the battery is AGM, it will be a gel-cell or swollen lead-acid battery.