I have no instructions, it had gas in it alredy,oil is fine no leak. If i take it back i only get stor credit it started at the store but now it want stay on what could it be. It is a used lawn mower no warrenty.

I bought an electric cordless lawn mower in a garage sale, but it didnt have the charger for its battery, I opened the lawn mower and I saw the battery is a BB 17-12 (model BP 17-12) and it says 12V 17Ah / C20 / 1.75 VPC 25* rechargable valve regulated lead acid . So I do not know what to use if an AC adapter or a charger and what kind please, help!! thanks and regards

a used inexpensive walk behind mower in great working condition