A not-too-technical discussion of solar & RV batteries

It's almost 5 years since this blog started and we have discussed a lot of different topics. I have run into some rather critical health problems lately so my evaluation of wattage draws of home appliances came to a stand-still.

Also we have sent two of our children off to distant lands for schooling. In the last two weeks our youngest went to Hawaii to continue her education and our oldest moved to snowy Reno to do her graduate studies.

Recently I put together a 4-page discussion of batteries used in small solar systems and Recreational Vehicles. I repeatedly have seen that people have many issues when it comes to caring and using their wet-cell batteries correctly. I decided to include this discussion in this blog over a few posts. If you are in the Vegas area stop by Vegas Trailer Supply and pick up your printed copy of the entire discussion. It should be a good addition to your RV or solar library!

PART ONE: A not-too-technical discussion of solar & RV batteries.

Types of batteries:
The most common type of battery used in small solar systems and RV’s is a wet-cell 12 volt “deep cycle” or 6 volt “golf-cart” battery. Typical car or truck engine starting batteries are never used because they are not designed to take the constant charging and discharging cycles. AGM (absorbed glass mat) and gel batteries are also used for special applications and when less maintenance is wanted, but are more expensive and less used than typical deep cycle or golf-cart batteries. Many high end rugged batteries are available that we are not including in this discussion, but their cost is also high end.

6 volt can be better choice.
6 volt golf cart batteries usually outlast deep cycle marine batteries if they are maintained properly. Take a look at the three cells in a six volt battery, and then look at the size of each of the six cells in a 12 volt battery. Which one is larger? The individual cell on the six volt battery is larger, and thus has heavier duty lead plates in it.

Deep-cycle: bad name if you ask me.
Deep cycle batteries should not have been given the name “deep cycle” because many people mistakenly think that they can regularly “deep discharge” their batteries until the lights go dim. Yes, deep-cycle batteries will take a deeper depth of discharge than a typical car battery, but they will undergo damage or dramatically shorter life if constantly discharged too much. Usually the battery bank is 90% discharged when the incandescent lights start to dim and that’s way to low. If you regularly discharge your battery that low then you are doing irreparable harm to your batteries.

What state of charge are my batteries?
It is my experience in the solar and RV business for 25 years that most RV owners have no idea how far they can discharge a battery (bank) safely. When someone comes into the store and says to me “Mike, I am having battery problems.” I give them my little one-hour battery condition chart then ask “What is your battery voltage?” Often I get answers such as “12 volts, it’s a 12 volt battery isn’t it?” I then show them that 12 volts on a 12 volt battery is nearly at 100% discharge. Or, I get an answer to my question like, “13.2 volts”. I say “do you see anywhere on that chart where 13.2 volts is listed?” No, you will not find it listed. The higher reading was a result of the customer not waiting an hour for the surface charge to dissipate.

It is quite possible for a battery that is completely discharged to receive a 10 minute charge and then read 13.2 volts immediately after stopping the charger. When an hour has passed from any charging cycle, you will get a more accurate idea of the state of charge in your battery bank.

Use a digital volt meter.
When taking readings, please use a digital volt meter instead of an old type analog meter. It is okay to have an inexpensive digital meter purchased from your local discount store, but I advise a $20 - $40 dollar hand held volt meter for longer life and a bit more accurate readings.

Never discharge more than 50%
I tell my customers to never let their battery bank get below 50% discharged on a regular basis. Of course, in an emergency, you can use that remaining 50%, as long as it is not done frequently. Most people say they don’t have any idea when their batteries are 50% discharged. I then give them my chart. Batteries that are watched and not “deeply discharged” will last twice as long as other batteries. Those that remain consistently in the 70 to 80% full zone will have even longer life.

TO BE CONTINUED

All the best, Mike