It's ...
... the one that starts your bike
How old is the current battery ? If its OEM and you've had a good return out of it why bother changing to a different brand/type ? Where best to buy it is the question you need to ask, a BMW dealer will almost certainly be the most expensive
... Which is the best battery ?
... the one that starts your bike

How old is the current battery ? If its OEM and you've had a good return out of it why bother changing to a different brand/type ? Where best to buy it is the question you need to ask, a BMW dealer will almost certainly be the most expensive
