Batteries are tricky to keep in inventory because like mentioned above, they need occasional charging to stay good.

I am the GM for an Interstate Distributor, we get panicked calls from time to time from 16V guys needing a battery immediately and that is not something we offer and something that is hard to find in a pinch.

If I had a 16V system I would buy two batteries and rotate them in and out of the car so they both stay fresh and see usage. This way you will have a ready to go spare at anytime.