Maybe we need some type of credit system that takes the health of the battery into account.

For example, if you have a new battery you also have 1000 credits. If you swap it for an older battery (which is worth, say, 500 credits), you'll have a battery and 500 credits in your account. Next time if you get a newer battery which is worth 800 credits, you'll have only 200 credits left in your account.

Also those credits could decrease as you use batteries. You could replenish them with payments.