Interesting question, just been through a lot of stuff on batteries and charging AUX batteries for offroad and camping use on another forum where a couple of auto electricians chimed in. As a result of that, I just purchased a 16 amp battery charger and will revisit the charging system I use with something like this which increases the voltage to a higher level together with associated electronics to achieve 100% SOC (State of Charge):
The sparkies maintained that the voltages were regulated low in modern vehicles (particularly the turbo diesels like mine) and as a result the battery is usually only charged to about 65% of capacity.
The battery charger I purchased was this one:
and it includes settings to rejuvenate (eg de-sulphate) and to equalise the battery (basically stir up the elctrolyte so stuff settled to the bottom goes back to the top.
Sorry both links are to Australian Suppliers, but Ctek is another (more expensive) global brand of battery charger like the one I purchased (eg my 16amp one was same cost as their 7 amp one).
I also know you can get gadgets that you can hook to your battery in the car that do the desulphation thing.
Anyway, I think there is something in what the sparkies were saying as despite driving 300 odd km on the day I purchased the battery charger, it took 5 hours to fully charge it that night.
I'll know more next weekend going camping for 5 days on Moreton Island and will be depending on the battery to keep the beer in the fridge cold as you have to be self sufficient on the island and I don't like to go thirsty. I am hoping a good boost before we go will be enough to get through with cold beer all the way.
having said that, 3 years is not too bad.