I did a little reading on the internet and WOW there is a lot of bad info. out there. This topic gets beat to death all over the place and by guys who don't know what they are talking about.
I found a pretty good article
HERE, that I believe provides accurate information.
Summary of info. as it applies to trailer batteries, as I understand it:
There are four standards by which batteries are rated by manufactures;
- CCA Cold Cranking Amps - Number of amps delivered at zero degrees F for 30 seconds (not very important for most of our trailer needs)
- CA Cranking Amps - Same measurement but at 32 degrees (not used very often now days)
- AH Amp Hours - An amp rating taken for 20 hours. How many amps a battery will provide over a 20 hour period. This is something to consider when selecting trailer batteries.
- RC Reserve Capacity - the number of minutes a fully charged battery at 80 Degrees F will discharge 25 amps until the battery drops below 10.5 volts. This is very important when selecting trailer batteries.
One plate in a battery produces approx. 2.1 volts. A 12 volt battery has 6 plates. A 6 volt battery has 3 plates. 2 6 volt batteries that occupy only slightly more space than one 12 volt battery will provide similar daily performance in amp hours and reserve capacity. The 12 volt battery will provide much more cold cranking amps (we don't care about CCA's for use in trailers). The thicker plates of the 6 volt battery will increase longevity. Two six volt batteries should have a longer life span than one 12 volt battery of the same capacity and that is the major benefit of 6 volts over a 12 volt.
Select a Gel Cell or an Absorbed Glass Mat (AGM) rather than a Wet Cell if the application is in a harsher environment or the battery is not going to receive regular maintenance and charging.
Another consideration when choosing batteries is form factor. Sometimes you can get greater capacity out of one 12 volt battery, rather than two 6 volt batteries and vice versa. From what I read, some 6 volt batteries are available in a "taller" form factor.
The following is pretty good info. on battery life and performance. It's a direct quote form the article linked above:
"
8. Battery life and performance - Average battery life has become shorter as energy requirements have increased. Two phrases I hear most often are"my battery won't take a charge, and my battery won't hold a charge". Only 30% of batteries sold today reach the 48-month mark. In fact 80% of all battery failure is related to sulfation build-up. This build up occurs when the sulfur molecules in the electrolyte (battery acid) become so deeply discharged that they begin to coat the battery's lead plates. Before long the plates become so coated that the battery dies. The causes of sulfation are numerous. Let me list some for you.
- Batteries sit too long between charges. As little as 24 hours in hot weather and several days in cooler weather.
- Battery is stored without some type of energy input.
- "Deep cycling" an engine starting battery. Remember these batteries can't stand deep discharge.
- Undercharging of a battery to only 90% of capacity will allow sulfation of the battery using the 10% of battery chemistry not reactivated by the incompleted charging cycle.
- Heat of 100 plus F., increases internal discharge. As temperatures increase so does internal discharge. A new fully charged battery left sitting 24 hours a day at 110 degrees F for 30 days would most likely not start an engine.
- Low electrolyte level - battery plates exposed to air will immediately sulfate.
- Incorrect charging levels and settings. Most cheap battery chargers can do more harm than good. See the section on battery charging.
- Cold weather is also hard on the battery. The chemistry does not make the same amount of energy as a warm battery. A deeply discharged battery can freeze solid in sub zero weather.
- Parasitic drain is a load put on a battery with the key off. More info on parasitic drain will follow in this document."