Extend the life of your motorhome batteries by learning how they function and what steps to take to keep them in optimal working condition.
By Mark Quasius, F333630
August 2014
Batteries are critical to any motorhome. They are used to power a motorhome’s lights and to operate the water pump, fans, and other 12-volt-DC devices. They can supply power to an inverter, which then converts it to 120-volt-AC power that can be utilized when the motorhome is being driven or when shore power is not available. Batteries are housed in a tough environment and can be a source of aggravation and require maintenance. By gaining a better understanding about how batteries work and how to maintain them, you can glean the most benefit from your battery bank.
A battery is nothing more than a storage vessel. You fill it up, just like a fuel tank, and gradually use its contents. The difference is that you are storing electricity rather than gasoline or diesel fuel. Batteries operate by way of a chemical reaction. The electrolyte inside the battery, typically sulfuric acid, reacts with the lead plates inside the battery to produce power by transferring electrons from the electrolyte to the battery plates. During the course of giving up these electrons, the electrolyte undergoes a chemical process that transfers sulfur to the battery plates and turns the electrolyte into water. When a battery is recharged, the sulfur recombines with the electrolyte, changing it from its water state back to acid.
There are two basic needs for batteries in a motorhome. Chassis (starting) batteries are used to crank the motorhome’s engine and to power any of the automotive energy needs. A chassis battery is designed to pass a large current capable of cranking a starter motor for a short period. The battery then slowly recharges as the vehicle is driven. Deep-cycle batteries are designed to provide smaller current discharge rates but for a much longer period of time and are what you will find in the house side of your motorhome. Do not buy a deep-cycle battery to use for chassis purposes. While they may be less expensive, they are not designed to provide the quick burst of power that’s required to start a motorhome engine.
Batteries are rated in life cycles. You can fully discharge a battery a given amount of times and then fully recharge it. Once the life-cycle rating is met, the battery will no longer perform the work it was intended to do. If you do not fully discharge it during each use, the battery will last much longer. This is graded on a curve, so discharging a battery to 50 percent charge level will more than double its life. If you use even less of the battery before fully recharging it, the battery can last even longer, because the life chart is exponential as the amount of discharge is reduced.
For this reason, it’s best that a flooded battery never be discharged below 50 percent of its state of charge. If this rate of discharge is exceeded, the voltage will be much lower and the battery won’t provide adequate power to run your DC-powered equipment. Discharging a battery to less than 50 percent state of charge also puts undue stress on the battery and shortens its life. To determine the charge status of your battery, refer to the voltage drop chart included in this article. These voltages are what you will read when the batteries are connected in your motorhome. A battery that is disconnected from the system and has been at rest for a few hours will have slightly higher readings.
Flooded batteries, commonly used in motorhomes, require maintenance. When the batteries are recharged, the sulfate recombines with the electrolyte. If the battery continues to be charged after it is fully charged, outgassing will occur when the electrolyte boils. This produces explosive hydrogen gas and acidic vapors that will exit via the vent caps on the batteries. These acidic gases can corrode the battery terminal connections, hold-down bolts, and other metallic items in the battery compartment. For these reasons, it’s important that flooded batteries always be placed in a vented compartment to prevent the buildup of explosive hydrogen gases.
When outgassing occurs, the electrolyte level decreases. At that point, it will be necessary to add water to the battery to top off the electrolyte level. Be sure to use distilled water. Regular tap water includes minerals that can attach to the lead plates and reduce the battery’s ability to conduct electricity. If the electrolyte level drops beneath the top of the battery plates, a battery’s life will be reduced. Do not add electrolyte to the battery. Add only distilled water.
As noted, outgassing occurs whenever the battery receives more voltage than it needs. Battery charging typically is handled by a converter or inverter/charger. Modern chargers utilize a three-stage battery charging system. The initial charge mode is known as the “bulk charge.” During this stage, the charger restores power to the battery by producing constant current until the battery reaches a preset voltage. Once the preset voltage is achieved, the charger goes into “absorption mode.” During this stage, the voltage is held while the current flow gradually tapers. Finally, the charger will kick into “float mode” in order to maintain the charge level without overvolting the battery. If a battery charger is properly programmed, little, if any, water will need to be added during normal use. If you find that you are continually adding water, be sure to have your charger tested to determine whether it is supplying excessive voltage and damaging the batteries.
One of the more popular advancements in battery technology is the development of absorbed glass mat (AGM) batteries. A typical flooded battery consists of lead battery plates submerged in an electrolyte solution. In contrast, an AGM battery places the electrolyte in fiberglass mats that surround the battery plates. The battery chambers are not truly vented, although they are valve-regulated to allow for thermal expansion. Because of this, outgassing is not a concern, and AGM batteries do not need to be placed in a vented compartment. The battery does not require the owner to check or add water to it. Corroded battery terminals and mounting hardware caused by the acidic outgas vapors also are not an issue with an AGM battery. In addition, AGM batteries have less internal resistance, so they recharge faster than flooded batteries. An AGM battery can run longer, because the voltage falloff curve is flatter than on a flooded battery. The result is that an AGM battery will provide more runtime hours than an equally rated flooded battery. They also can get up to three times the cycles of a flooded battery. The only real drawback to AGM batteries is that they cost more than flooded batteries.
Earlier it was mentioned that the battery should never be drawn down below the 50 percent charge level. If your motorhome draws that much power from the battery bank, it may be necessary to increase the size of the battery bank. Adding batteries is the easiest way to add amp-hours to a DC power system, which will keep the batteries at a higher charge level and prolong their service life.
Multiple batteries can be connected in two ways, depending on whether you are using 12-volt-DC batteries or 6-volt-DC batteries. Your RV’s electrical system needs 12 volts to operate. If you have multiple 12-volt-DC batteries, you can connect them all in a parallel configuration. That connects all positive terminals together and all negative terminals together.
Many motorhomes come with the 6-volt-DC golf-cart-style batteries. These robust batteries offer long life and plenty of power per battery. However, two 6-volt-DC batteries need to be connected in series in order to provide 12 volts DC. If you have four batteries, you’ll need to create two series paired sets and then tie those sets together to achieve a 12-volt-DC array. See the illustration on page 44for details.
When flooded batteries no longer can perform as rated, it’s time to test the battery system. First perform a voltmeter test. If your batteries are fully charged, the voltmeter should register 12.6 volts at each battery with the battery at rest (no charger and no discharge for at least one hour). Testing the voltage will not tell you whether the battery is in good condition; it only tells you whether the battery is charged up. It has no bearing on the ability of the battery to produce the rated number of amps upon demand. However, batteries need to be fully charged when tested, so this is the first step.
Once the batteries are fully charged, disconnect any loads and stop the charger. A battery that has just been charged will have a surface voltage of 13.2 volts or more. This is a false reading, so let the batteries rest awhile to allow the surface voltage to dissipate. The voltage should stabilize at approximately 12.6 volts. If 12.6 volts (100 percent charge) cannot be achieved, then proceed to the next step and analyze each battery cell. (The voltage reading for a 6-volt battery at 100 percent charge is 6.36 volts.)
When battery electrolyte is fully charged, its specific gravity increases. When it’s discharged, the specific gravity lowers, because the liquid inside is more water than acid. For many years, the best way to measure the specific gravity was by using a hydrometer. Today, refractometers are much more accurate and do not require temperature compensation. Plus, a refractometer can be used to test antifreeze as well as battery electrolyte, so it’s a handy tool for any motorhome owner. Every cell in a battery needs to be operating properly or the battery will not work, very much like a weak or broken link in a tow chain. Remove each battery cap and take a small sample of electrolyte with your hydrometer or refractometer. Refer to the chart included in this article to determine the state of that cell. If the battery is perfectly charged, the specific gravity should be approximately 1.265. The main thing to check is that all of the cells are fairly even. If one cell is dead, the battery won’t perform. Similarly, if you have two 6-volt-DC batteries in series, neither battery will be able to pass current with one bad cell in the chain.
Once you know that the batteries are fully charged and that each cell is within tolerance, the next step is to perform a load test. Load tests are done with a unit such as a carbon pile tester. A carbon pile tester is a dead load that can be varied by turning a knob. An ammeter and voltmeter are supplied to monitor battery performance. A typical test consists of adjusting the control until the ammeter reads three times the rated amp-hours of the battery. Hold this for a few seconds and monitor the voltmeter. If the battery voltage drops into the red zone, the battery is no longer capable of performing at its rated capacity.
At that point, the batteries probably need to be replaced. But one last thing can be done in an attempt to recover the batteries. Battery plates become sulfated over time, and the sulfur no longer recombines with the electrolyte when recharged. It may be possible to equalize the batteries to see whether they can be restored. Equalizing involves applying a higher voltage current to the batteries to boil the electrolyte and “cook” off any sulfate that has attached itself to the battery plates. Equalizing functions can be handled by most modern converters and inverter/chargers, but be sure to refer to your owner’s manual for specific instructions. When equalizing a battery, it’s necessary to disconnect any loads from the batteries to prevent damage to the electrical components from the higher voltage used during the equalization process. Many manufacturers recommend equalizing flooded batteries annually to prevent the plates from becoming sulfated. Under normal conditions, AGM batteries do not need to be equalized, although in some cases it may be necessary.
If you treat your batteries right, you should be able to get the maximum life out of them. With proper maintenance and use, you should be able to enjoy many hours of trouble-free use.
Sources And References
Lifeline Batteries Inc.
(909) 599-7816
www.lifelinebatteries.com
Interstate Batteries
(888) 772-3600
www.interstatebatteries.com
OPTIMA Batteries Inc.
(888) 867-8462
www.optimabatteries.com
Trojan Batteries
(800) 423-6569
www.trojanbattery.com