[Ecky]

As for the hybrid battery, sitting alone shouldn't hurt it. There are batteries which, once grid charged, have come back to life in perfect working order after decades of sitting.

The cells will very slowly self-discharge when the car is unused. If the battery is "weak", that usually means not all of the cells are roughly equal anymore, and some will discharge faster than others. You end up with imbalanced cells. When you go to use the car after it's been sitting, you just need to attach a grid charger and slowly and gently top them all off together, to bring them back into balance.

I can assemble a grid charger for about $40 in parts and an hour of soldering. These normally sell for $3-500 on eBay.

What usually "kills" a battery is that one or more of the cells gradually end up with more and more internal resistance, so they get out of balance from the others more easily. Once they're out of balance, normal cycling of the battery takes these higher or lower than the other cells (normally 40-80% charge range), accelerating wear on them, until they out-gas or fail in some other way. One can in theory fix the pack by simply replacing the one bad cell, but this is a time consuming process and you'll pay out the nose for someone else to do it.

Many packs which are really, truly dead (and not just unbalanced) are dead because the owner was not aware of the above and did not maintain the battery "properly". Toyota's battery management does a better job of keeping cells in balance and thus Toyota hybrids see far fewer battery failures and require basically zero maintenance.

Even packs with a bad cell or two can be limped along for years with regular balance charging.