Things I have learned (often the hard way) about batteries in recent years
Lithium batteries are killed by the cold, don't buy for your vehicles if you live in cold climates
A good make lead acid battery should last 8-10 years, gel batteries only last 2-3 years in normal use
Batteries that need to be constantly recharged (deep cycle) again only last 2-3 years in normal use, there is a limit to the amount of recharges they will take. I am currently experimenting with carbon gel batteries (not cheap) more power, more charges, and I hope for at least 5 years.
Batteries are damaged if constantly run down to less than 50% before recharging ( electric cars ???) lead acid sulphate and deep cycle lose charging capacity.
Deep cycle do not achieve full capacity until after a few charges.
Gel are better for most purposes than AGM
I have now installed voltmeters (rather than ammeters) on electric battery machine 'draws' as this gives the best indication on when you are running into trouble, provided you research your type of battery.
Batteries are a complex subject, do your research before buying, your average car battery is not good for some purposes.
MAKE SURE YOUR CHARGER IS SUITABLE FOR THE TYPE OF BATTERY BEING CHARGED - otherwise you'll kill them
Lithium batteries once fully discharged cannot be recovered (at least with present tech), they need a special charger too.
Lead acid (and to some degree AGM/Gel) if fully discharged and sulphated it may be possible with a smart charger to recover, but their life will suffer compared to a well maintained battery.
If you don't use batteries regularly (motorcycles, jet skis etc) invest in a smart charger that will maintain the battery, especially in cold climates.
Cold is the enemy of every battery currently in normal use.