Maintenance free batteries are simply sealed lead acid batteries that, with a different plate composition, do not gas when charging. I prefer the ones I can add water to. These will gas mostly hydrogen when charging, especially after a deep discharge. Hydrocaps can replace the battery caps for about $7 a cap which will convert the hydrogen/oxygen back into water. Trickle charging (lower amps) will gas less if at all, as compared to fast charging (high amps) which may gas a lot and cause more water to be converted to gas.
Another problem to watch for is corroded terminals from sulfuric acid fumes. Keep them clean and use anticorrosion rings, or battery terminal grease or spray. A light acid such as a soft drink, baking soda and water, vinegar, citrus fruit juice etc. can be used. On car batteries, I used to tape a penny onto the terminal. This would take the brunt of the corrosion instead of the terminal. Seagoing ships use this principle and it is called a sacrificial anode.
Maintenance wise, keep the water level above the plates and use only distilled water. As batteries age they will lose capacity. Once they only charge up to 80%, the capacity will drop rapidly. Running a battery down 80% and charging only up to say 60% is harder on a battery than running it down 50% and then back to a full charge. Also for longer life, the fewer really deep discharges like 70% or more, the better.
Ideal conditions are at 77F. At higher temperatures, the batteries will outperform their ratings, but will have a shorter life. When at lower temperatures, they will in effect become smaller. At 40F they will lose 10-15% of their capacity to store electrical energy until they warm up. That's not a problem until you deeply discharge a battery at or below 32F, in which case the water can freeze and burst the casing and break the plates.
A really good discussion on batteries can be found in the book Battery Book for Your PV Home by Fowler Solar Electric for about $8.00. It's short, concise and well worth the money.
Offered by Steve.