My bus takes three 27 DP series dual purpose marine batteries (800 CCA, parallel series from Autozone) or one large commercial truck battery. At a $100/ea or about $300 for the latter, I quickly realized that I needed to better understand how to buy/maintain/prolong battery life.
There are only a couple of battery manufacturers to begin with. So batteries are built to their customer's specifications. I used to work for Johnson Controls, one the few battery manufacturers out there. We had people from the battery division working in our plant.
Beside the all of the good information above, you need to look at how the battery is going to be used. In my case, The Bus is driven less than 300 miles a year. I really only drive it in the fall for tailgating. Recently I have made more of an effort to drive it more year around.
The marine grade battery serves my purpose due to how it's built versus the commercial truck battery.
The second thing is when you buy your battery look for the following:
Date of manufacture - in the case of multiple batteries the newer, the closer, the better off you will be. Batteries deteriorate with age, so the clock is ticking on the shelf.
Check the water level in the batteries BEFORE you buy them. If the cells are low or not close to the same level in each cell (near the plates or even worse - exposed plates) do not buy that battery. It should be roughly a quarter of an inch above the plates. I've gone to several locations just so I could buy batteries that were manufactured within a month of one another.
Do not just buy one battery - replace all your batteries at the same time. A weaker battery will leach voltage off the good one eventually causing premature failure of both batteries. The reason for checking the water level in batteries before you buy them. You want them as close/equal to ability as possible.
Once you get it home, check water again. Add
DISTLLED water just below the plastic inner sleeve or just up to it - about an inch to no more than 3/4's of an inch below the the top of the battery.
Do this by pouring distilled water into a clean container and use turkey baster (works perfect for this) to draw out some DISTILLED water. Slowly fill up to about an inch below the top. If you over fill it, just draw it out and discard it back into the container you just drew the DISTILLED water out of - not back into the distilled water. Do not contaminate the source of distilled water. Do this for all the batteries you are putting in service at that time.
Once a battery is put into service, there is a little more water evaporation due to initial use. Afterwards, it's more of a consistent evaporation (slight) over time.
If you don't have any new battery washers, die electric grease, and some other battery top coat, pick it up before you leave. Always clean all contacts with a grease free wire brush, coat contact surfaces with die electric grease, and assemble. Don't over tighten you connections either. The fatigue of being over tightened along with the current stress will cause premature failure at the connections.
The last thing you want to do is to check your batteries afterwards - about a week later, then a month later, and then twice a year at a minimum. Ideally once in the Fall before cold weather and once in the Spring before hot weather.
A good thing to have is an hydrometer when checking water in the fall. This will give you a good idea of the battery's individual cell condition. If they are not close in the results, chances are the battery will not make it through the first real hard cold snap.
Look for corrosion around the connections. If they show signs of corrosion, you have a bad connection somewhere, such as failed ground wire or your pushing too much voltage back into the battery. The heat generated from the stress will cause the coating and die electric grease to melt off over time and corrosion will follow.
Check the fluid level if you have been over too rough of terrain or a little sideways
.
The last thing to check is the voltage. A good battery at rest will be around 12.6 volts if I recall correctly. This forum contains a lot of good info on charging and values.
I went from changing out two sets of batteries within the first two years of driving The Bus to a battery life over 7 years for the trio! I caught the bad battery before it stranded me on the road when testing the cells in the fall.
My current trio is over 4 years old and test as strong as the day I bought them. Pretty good for a battery group that is only warranted for 1 year.