For those with comfort access: always keep a spare key fob battery inside the car. Comes in handy whenever the battery dies. Do mind that sliding the key fob in the key fob slot will
not recharge the battery on BMW's equipped with comfort access. To save battery life, avoid pushing the key fob - that's also the purpose why you got the comfort access option, isn't it ?
For those without comfort access: no need to keep a spare key fob battery inside the car, as the key fob battery is
recharged every time you slide the key fob in the key fob slot.