A not-too-technical discussion of solar & RV batteries. Part 2

6 volt batteries are taller.
Batteries like to be discharged slowly and charged slowly as well. The size of your battery bank in relation to the charge/discharge rates can be an important factor. As a general rule, more batteries are better than less, but many RV’s don’t have room for more. When changing from 12 volt group 24 or group 27 batteries to heavier duty 6 volt golf-cart batteries, you have to consider that the 6 volt ones are usually an inch taller. Often there is not enough room in the compartment for the switchover.

Keep ‘em together.
When expanding to more batteries, you need to keep them close together. It is not a good idea to have three batteries in one compartment and another battery in an area 5 feet away. There are losses in the longer cable and it is not a good mix to have batteries separated.

Connection point for positive (+) and negative (-) cables.
This brings up another problem we often see in wiring the battery bank. Take a look at the diagrams to the right and you will see that each bank of batteries has the negative (-) charge/load connection on one end and the positive (+) on the other end. This is important for the life of your batteries and many, many dealers, even coach manufacturers don’t follow this important wiring rule.

You see, if both the negative and positive connections are made on one 12 volt battery at the end of a line, then the closest battery will work hardest and the others will work less. This is due to the resistance in the big cables and electricity takes the avenue of least resistance, just like running water flowing down a hill. The end result is that the harder working battery will have a shorter life and the other lazy batteries will have a shorter life too, because they are all connected together. It is best to charge and discharge all of the batteries in the bank equally by wiring them correctly.

How many batteries are best for my solar system?
When sizing a typical RV solar system I generally tell people to try to have one battery for every 40 to 60 watts of solar panels. This is a good rule of thumb. It is better to go a little more rather than less if there is a choice. Sometimes there is no choice due to space limitations so you just go with what you can. I have seen too many batteries for existing solar panels however. One fellow had 8 large batteries hooked up to one 60 watt solar panel. He had no daily power to use because some batteries loose up to 1% a day just sitting there. His panel barely kept up with the natural daily loss of his battery bank.

How many solar modules should I buy?
When trying to determine how many solar panels (modules) a customer should buy I try to evaluate their needs by adding their expected loads but the fact of the matter is everybody’s needs are different and often can’t be exactly determined. Sure you can use a load evaluation form and make an educated guess, but most of my customers start smaller with the anticipation of expanding their system later. Often the higher cost of a completely independent solar system is an important factor. Well, you can start smaller and expand later but I would install a larger controller and larger wires than what is currently needed so that expanding is much easier and less costly later.

Compartments:
Just about any type of battery compartment will do, but the compartment should be sealed from the inside living quarters and the battery compartment should be vented to the outside to let explosive hydrogen gases escape. We’ve seen batteries installed in the cabinet under the sink. Not a good idea. We have several choices of battery storage devices, but when considering location please keep in mind that extreme heat or cold can severely limit the life of a battery so a little insulation can go a long way.

Charging:
Typically RV batteries are charged in many different ways, but primarily through the converter when plugged into shore power or a gas generator. There are good converters and then there are bad ones, unfortunately most of the older converters are bad ones. Often older converters will over charge batteries when the rig is plugged into 110vac power for longer than a couple of days. This situation can be remedied by changing the converter to a completely automatic newer one.

Inverter/Charger:
If you are in the market for an inverter that supplies 110vac from your 12 volt batteries then you may want to get rid of the old converter altogether and use a good inverter/ charger like the Xantrex Freedom 458 series inverter. This type of inverter completely replaces the converter because when powered by 110vac it supplies enough 12 volts dc to run the entire coach as well as charge the batteries with the good 3-way charging method.

Bad Converter/Chargers
I know this is a lot to absorb, but you’ll get it. Older RV converter/chargers provide enough dc power to run your home-on-wheels but usually only provide a 5 amp charge to the batteries. A charge rate of 5 amps per hour takes a long time to get the batteries full if they are low and this constant 5 amp charge is bad for the batteries if they are full. Hey, if your two 100 amp batteries are 50% discharged it will take you 20 hours of running the generator to get them full again! If kept connected to the batteries after they are full you run the risk of drying up the electrolyte in the batteries. Almost every day I hear of batteries that went dry after several weeks of being plugged into shore power because the batteries were charging too long at an improper rate. I recommend to my customers that they disconnect their batteries after a couple of days of being plugged into shore power. Five amps per hour is too low for a “bulk” rate when batteries are low, and too high for a “trickle” charge rate. A 3-way charge system is the best way to go.

I hope these posts help you better understand your RV and small solar battery needs! Until next time. Mike