The heart of a solar system is the storage batteries. However, the type of battery needs to fit the needs of the particular RV’ing style. Some one who always stays in an RV park with a shore power connection can use any type of battery and need not worry about sizing other than whether it will fit the available space.
At the other extreme is someone who boondocks almost 100% of the time and full times.
The current battery types available to us fit three basic categories:
A. Starter batteries. These are designed to provide a great deal of current for a brief period of time. They do that well, but are not created to allow much depth of discharge.
B. Marine batteries. These may offer the ability to start engines just as starter batteries, but can be cycled using 20% of their total capacity.
C. Deep Cycle batteries. These may be discharged up to 50% of their total capacity.
Within each type there are different chemistries. What is available to us are various lead-acid types such as flooded, gel, maintenance free, and AGM. These chemistries all share one essential requirement. For best lifespan, they must be recharged to 100% after each discharge cycle. If not, then they may sulfate. That leads to progressive capacity loss. That, unfortunately, happens quite quickly.
Recharging may take a great deal of time. The reason is the charge acceptance rate. Typically, a flooded battery may accept only about 12.5 amps per 100 amp-hours when it reaches 85% of fully charged. According to Battery University, it may take up to 168 hours to get to 100% state of charge. Clearly that is something to avoid when running a generator, but remember–solar is a “free lunch”!
Gel cells are good at low and slow discharge rates–but require that same slow rate for recharging. If their low rates are ignored, there will be permanent irreversible damaged and capacity will be lost. They may not be a good choice for an RV.
Sealed maintenance free batteries are quite similar to flooded–but require slower rates of recharging. The recharge rate ought not to reach the gassing voltage, because there is no easy way to replace the electrolyte. This design would not be my personal choice for an RV.
AGM’s do a little better, accepting 12.5 amps per 100 amp-hours at 90% state of charge. They charge faster and support high loads well. They require almost no maintenance.
If money is no object–then the “new kid on the block” is lithium ion. They do not share the requirement for 100% recharge, have a large cycle life, and tolerate 80% discharge. They do have special requirements and they are not the least bit forgiving. If they are discharged too deeply they are destroyed. Their other “weakness” is low temperature charging. They must be warm enough, or they will be destroyed. They are essentially zero maintenance.
Yet another battery format is “hot off the press”. This one is back to the “tried and true” and is an AGM based on a Carbon Foam design. 1000 cycles to 20% state of charge. As with LI, be prepared for sticker shock.
Outback now has a Nano-Carbon battery that appears to be a competitor for the Firefly. It has several form factors, so may be better for RV use than the Firefly.
The best way to choose a battery type may be to look at the over all cost per cycle. For example, AGM and LI cost per cycle LI wins. But flooded vs LI, flooded wins.
How to decide how many amp-hours of battery bank are needed? (If you guessed “energy audit”, give yourself a star!)