A Seimens auto engineer told me that if you have a dead flat (and it really needs to be dead flat) battery you may be able to revive it to work ok. The procedure is to put it on a reversed trickle charge for ten minutes (apparently this cleans the plates). Then trickle charge in the normal manner. You may be lucky. Incidentally I've found when laying up the car that a solar charger seems to be up to the job of keeping a trickle charge to the battery and keeping it alive.