Once you recharge rechargeable batteries fully the rechargeable batteries usually stay charged up for around 3 years without using them.
Some rechargeable batteries stay charged for only 2 years but some rechargeable batteries stay charged for up to 3 years before they start losing their charge.
Then they can be charged again and reused.
Rechargeable batteries do cost more than non rechargeable batteries do but they can be reused for a long time and they actually save you money and are cheaper to buy than disposable batteries.
With regular disposable batteries you use them one time and have to buy more of them but with rechargeable batteries you just put them in a charger to charge them again.