Modern recreational vehicles represent a significant evolution in design and comfort. The electrical demands of systems and appliances contained within them presents formidable challenges to electrical storage batteries. The availability of multiple types and constructions of batteries can leave the RV owner questioning which battery is best for which application. Hopefully, this article will leave you with a better understanding and ultimately aid in an economical and appropriate selection of replacement batteries for your RV.
Batteries provide low-cost, rechargeable storage of electrical energy. Most batteries commonly used in deep-cycle applications are lead-acid. The acid is typically 30% sulfuric acid and 70% water at full charge. Lead-acid batteries are 85% -90% efficient. If you consume 1000 watts from a battery, it might take 1200 watts to fully recharge it.
Batteries are divided into types by application and construction. The major applications are automotive, marine and deep-cycle. Deep cycle includes solar electric (PV), recreational vehicles (RV) and industrial backup power. The major construction types are flooded (wet), gelled and AGM (absorbed glass mat).
Automotive Batteries are used to start and run engines and are designed to provide a large current over a short time period. Automotive batteries have a large number of thin plates for maximum surface area. The plates are composed of a lead ?sponge,? similar in appearance to a very fine foam sponge. If deep-cycled, this sponge can be quickly consumed. These batteries can last 30-150 cycles.
Deep cycle batteries are designed to be repeatedly discharged down to 20% of the rated capacity (80% discharged). This cycle is called the depth of discharge or DOD. A true deep cycle battery will have plates constructed of solid lead and not sponge. Generally the thicker the plates the more cycles the battery will be capable of providing. Deep cycle batteries can serve as a starting battery in RV?s. Allowance must be made for the lower cranking amps compared to a similar size starting battery. It should be oversized 20% to the recommended starting battery group. Industrial rated deep cycle batteries can be too heavy or cumbersome for some RV applications but when properly sized they will give extended life and performance far beyond that available from regular RV type batteries.
Marine batteries are usually a ?hybrid,? falling between the starting and deep cycle batteries. The plates may be composed of lead sponge, but it is coarser and heavier than that used in starting batteries. Hybrid types should not be discharged more than 50% and are normally not recommended for RV use. Marine batteries are usually rated at ?MCA? (marine cranking amps @ 32 F) while starting batteries are rated at ?CCA? (cold cranking amps @ 0 F).
The thickness of the positive plate determines a factor called ?positive grid corrosion?. The positive (+) plate is gradually eaten away over time as the battery is cycled. This plate material falls to the bottom of the battery as sediment reducing the output of the battery and eventually the battery?s failure.
Automotive batteries typically have plates about .040? (40/1000?) thick, while industrial deep cycle batteries (forklift type) may have plates over ?? thick. True deep cycle RV or PV batteries will have plates that are around .09 to .15? thick.
Most industrial deep-cycle batteries use lead-antimony plates rather than the lead-calcium used in AGM or gelled deep-cycle batteries. The antimony increases plate life and strength, but increases gassing and water loss. Batteries must be checked periodically for water level or be equipped with special caps that recombine hydrogen and oxygen gasses and vapors created during charging. These caps can reduce water loss by 95%. The self-discharge of batteries with lead-antimony plates can be high, as much as 1% per day on an older battery.
Sealed batteries are made with vents that normally cannot be removed. Maintenance Free batteries are sealed, but are not usually leak proof, as they must allow gas to vent during charging. If overcharged too often, they may lose enough water to make them fail prematurely. Their service life is generally limited to the specified warranty period.
Battery Size Codes
Batteries have ?group? sizes, based upon the physical size and terminal placement. It is not a measure of battery capacity.
Gelled batteries contain acid that has been ?gelled? by the addition of silica gel. It is impossible to spill acid even if they are broken. However, they have several disadvantages. They must be charged at a slower rate to prevent excess gas from damaging the cells. They cannot be fast charged on a conventional automotive charger. This is not a problem with solar electric systems, but if a generator or inverter charger is used, current must be limited to the manufacturer?s specifications. Gel cells must be charged at 2/10 th less voltage than flooded batteries. If overcharged, voids can develop in the gel causing a loss in battery capacity. In hot climates, water loss can cause premature battery failure. This type of battery should generally be avoided in RV applications.
Absorbed Glass Mat
Absorbed Glass Mat (AGM) is a sealed battery that uses a very fine fiber boron-silicate glass mat between the plates. They are also known as ?starved electrolyte? batteries as the mat is 95% saturated rather than being fully soaked with acid. This facilitates true zero maintenance and prevents acid leakage if broken or during transit.
Most AGM batteries are ?recombinant? allowing the oxygen and hydrogen to recombine inside the battery. This gas phase transfer of oxygen to the negative plates effectively prevents over 99% of potential water loss during charging. The charging voltages are the same as with standard batteries. Most AGM batteries have no charge or discharge limits.
AGM?s have a very low self-discharge (1% to 3% monthly). They can remain in storage for much longer periods and are virtually freeze proof. Even under severe overcharge conditions hydrogen emission is far below the 4% maximum specified for enclosed spaces. The plates in AGM?s will withstand shock and vibration better than any standard battery.
Even with all the advantages listed above, there remains a place for standard flooded deep-cycle batteries. AGM?s cost twice as much as good quality flooded batteries of the same capacity. In many installations where the batteries are set in an area where fumes or leakage are not a concern, flooded batteries remain a good economic choice.
Effects of Temperature
Battery capacity (ampere hours it can store) is reduced as temperature decreases and increased as temperature goes up. Batteries are rated at 25?C or 77?F. At -22 F (-27?C) battery AH capacity drops by 50%. Capacity is reduced by 20% at freezing. Battery capacity increases up to 15% with higher temperatures.
Even though battery capacity at higher temperatures increases, battery life is shortened. Battery capacity is reduced by 50% at -22?F but battery life increases by about 60%. For every 15?F over 77, battery life is cut in half. This is true for any type of lead-acid battery. Hot and cold intervals tend to average the life expectancy of most battery applications.
Cycles versus Life
A battery ?cycle? represents one complete discharge and recharge cycle. A cycle is usually considered discharging from 100% to 20% of rated electrical storage and recharging back to 100%. Common ratings for depth of discharge cycles include 10%, 20% and 50%. It is important that you evaluate battery cycle ratings based on the depth of discharge, not just the number of cycles.
Battery life is directly related to how deep the battery is cycled each time. If a battery is discharged to 50%, it will last about twice as long as if it is cycled to 80% DOD. If cycled to only 10% DOD, it will last about 5 times as long as one cycled to 50%. Obviously, there are some practical limitations in a RV, namely weight and space. The most practical number to use is 50% DOD on a regular basis, allowing for 80% once in a while. When determining the number of batteries required for your RV you require a current load estimate. This will allow you to calculate an average DOD of around 50% for best storage versus cost factor. A battery cycle of 10% will minimize the lead dioxide that accumulates on the positive plates during continuous light cycles.
All lead-acid batteries supply about 2.14 volts per cell (12.6 to 12.8 per 12 volt battery) when fully charged. Batteries in storage eventually lose their charge. Depending on battery type, age and ambient temperatures this can amount from 1% to 15% per month. In systems that are continually connected to some type of charging source (solar, wind or AC) this is not a problem. However, one of the biggest battery killers is allowing batteries to be stored in a partly discharged state for a few months. A ?float? or maintenance charge should always be maintained. The charge level can be determined by measuring either the available battery voltage using a voltmeter or by measuring the specific gravity of the battery acid using a hydrometer.
All deep-cycle batteries are rated in ampere-hours (AH). An ampere-hour is calculated as one amp (ampere) consumed for one hour or 10 amps for 1/10 th of an hour. If you have something that consumes 20 amps and you use it for 20 minutes, then the amp-hours used would be 20 (amps) x .333 (hours), or 6.67 AH. The accepted AH rating time period for batteries used in solar electric (PV) and RV battery systems is the ?20 hour rate?. This means that the battery is discharged down to 10.5 volts over a 20-hour period while the total amp-hours it supplies is measured. Sometimes ratings at the 6 and 100-hour rate are given for comparison or for different applications. The faster a battery is discharged, the lower the AH capacity. Conversely, if it is drained slower the AH capacity will be rated higher.
State of Charge
Voltage readings for a 12-volt battery system at 77 F effectively measure the state of charge. These voltages are for batteries that have been at rest for 3 hours minimum after charging. Use 0.2 volts less for gelled batteries. Voltage should stay above 12.1 for maximum life. Occasional dips below are acceptable but continual discharges will drastically shorten battery life.
State of Charge
12 Volt Battery Reading
Specific Gravity Reading
A battery can meet all the tests for being at full charge, yet lack its original capacity. If plates are damaged, sulfated or partially consumed from use, the battery may appear fully charged but lack its original output. This can occur in gelled cells if they are overcharged or in older flooded batteries that are failing. If your batteries test OK but go dead quickly under load they are probably due for replacement.
Batteries do not reach full capacity until cycled 10 to 30 times. A new battery will have an output 5% to 10% under its rated capacity.
Batteries should be watered after charging unless the plates are initially exposed. Add just enough distilled or de-ionized water to cover the plates. After a full charge, the water level should be even in all cells (usually ? to ? inch below the bottom of the fill well).
When multiple batteries are connected in series or parallel, replacement batteries should be of the same size, type and manufacturer. Age and usage level should be similar within companion batteries. Do not put a new battery in a pack that is more than three months old or has more than 75 cycles. For long life batteries you can have up to 1-year age difference.
Leave vent caps on while charging flooded lead-acid batteries.
Taking a hydrometer reading when you purchase a new set of flooded batteries provides a baseline for future reference.
Use a charge controller when using a solar panel to keep a maintenance charge on a battery. If using an uncontrolled panel select one that has a maximum output of about 1/300 th of the ampere-hour capacity of the battery. That would be the equivalent to a 1 to 5 watt panel for a pair of batteries.
Lead-acid batteries do not have a memory. Fully discharging them to avoid this ?memory? will lead to early battery failure.
Inactivity is harmful to a battery, especially if it is not fully charged.
Use clean water for cleaning the outside of batteries. Solvents or spray cleaners should not be used. It is usually not a good idea to use baking soda to neutralize acid on the outside of a battery as even a tiny amount getting inside will neutralize acid in the battery. If you do use it make sure that all the caps are on the battery. Mixing up a tablespoon in a small glass of water is sufficient to neutralize and clean any acid around the battery posts.
As batteries age they require longer charging times and higher amperage to complete the charging cycle. Flooded batteries will generally require more water as they age. Output capacity gradually decreases with age.
General care and attention will greatly extend the useful service life of your RV battery bank. Proper sizing of your battery bank to your service requirements will go a long way in providing long-term satisfaction and maximize economy. Monitoring battery cycles and charge levels will extend deep-cycle battery life. Purchase high quality batteries warranted by a reputable manufacturer. AGM batteries may represent the ultimate maintenance free battery purchase depending on your yearly operation and service demands.