RV Batteries

RV batteries are an essential part of making RVs convenient and comfortable.   On motorhomes there are two separate battery systems:  a starting battery for the engine, and deep cycle coach batteries.  The starting battery functions just like the battery in any other motor vehicle, providing power to start the engine and operate lights and other 12-volt vehicle features, like radios and power windows.  The battery is charged by an alternator driven by the engine.  Deep cycle coach batteries are used to supply power for lights, fans, furnaces, and some electronic devices in the RV.  There may be one or more 12-volt batteries or two or more 6-volt golf cart batteries .  12-volt batteries are connected in parallel to supply higher amperage.  6-volt batteries are connected in series to create 12 volts.  Golf cart batteries are usually stronger, more durable, and can be recharged more times than 12-volt deep cycle batteries.  The house batteries should also be charged by the vehicle alternator whenever the engine is running and the should be separated from the starting battery by a battery isolator.  A battery isolator allows the alternator to charge multiple banks of batteries but prevents them from drawing down each other so you don't drain your starting battery while running your RV lights and accessories.  It is convenient to have a switch to connect battery banks in case your starting battery runs down and you need power from the house batteries to start the engine or the house batteries run down and you need power to start the generator.  Some RVs even have a separate battery dedicated to starting the generator.

Other batteries.  The main focus of this article are the large (and expensive) 12-volt batteries that power your RV systems.  In addition to the 12-volt systems for your engine and your coach there are often other battery powered devices in your RV.  Smoke detectors usually use a 9-volt battery.  Remote controls for TVs, power awnings, etc usually use AA or AAA batteries.  You may have battery powered lights in cabinets or closets.  You may have battery powered lanterns and flashlights.  You will need to test the batteries in these devices regularly, especially before each trip, to be sure they will be functional when you need them.  For convenience, keep a battery tester in your RV or camp kit.  Carry spare batteries with you for all your devices.  And be sure to test your spare batteries before each outing.  They won't do you much good if they're dead.  One trick to keep batteries from running down in items in storage is to put one of the batteries in backwards.

Batteries are a critical component of the 12-volt electrical system that powers most RV lights and fixtures.  Not only do they provide power for the lights, but they also power control boards for furnaces, refrigerators, and hot water heaters.  Most modern furnaces also have a 12-volt fan.  Some older furnaces worked by convection only.  A convection furnace doesn't have a fan but it doesn't distribute the heat as well as a forced air model.  An anomaly of 12-volt furnaces is that when the batteries get low, the fan keeps running after the burner has shut off.  If you wake up in the middle of the night and your furnace is busily blowing cold air, your batteries may be low or you might be out of propane.  If that happens occasionally, (and you're not out of propane) you probably aren't charging your batteries enough during the day.  If it happens frequently, you probably need new batteries or a larger battery bank -- or don't run the furnace so much.

Pretty much all of these batteries are some form of lead-acid batteries.  Lead acid automotive batteries have been around since Cadillac introduced the first electric starters way back in 1912.  The basic design has not changed much in over a hundred years, but there have been some improvements.  Basic batteries use lead plates submerged in a solution containing sulfuric acid and are known as "flooded cell" batteries.   They have removable caps so you can check and replenish the water level as needed.  So-called "maintenance free" batteries are sealed and don't require the frequent addition of water in normal use.  Absorbed Gas Mat (AGM) batteries use saturated mats between the cells instead of liquid, reducing the chance of spillage.   Another option are gel cell batteries, in which the electrolyte, instead of being liquid sulfuric acid is a gel.  Gel cells are usually lighter than other batteries and very unlikely to spill.  By the way, it is best to only add distilled water when batteries need more liquid, but, in an emergency, ordinary tap water can be used.  The damage caused by contaminates in the tap water will be less than allowing the plates to be exposed.  Distilled water isn't very expensive.  It would be a good idea to carry a gallon jug of distilled water in your RV to top off your batteries as needed.  You can buy it at just about any grocery store for a dollar or so a gallon.  While you are at it, grab a cheap turkey baster to use to add water to your batteries.

Modern lithium ion technology has produced batteries that exceed the performance of lead-acid batteries but they are still rather expensive.  Keep an eye on this technology.  I have seen reconditioned medical grade lithium ion batteries offered for sale on ebay at fairly reasonable prices, so they might be worth looking into.   New lithium marine/RV batteries run $250-900 so they are quite a bit more expensive than traditional lead-acid batteries.  From time to time I have seen recycled medical lithium batteries on ebay for a couple hundred bucks, but they are not always available.  You also might need a different charger to properly charge lithium ion batteries.

As the push for hybrid vehicles drives battery technology, new types of batteries are coming on the market.  Some are much lighter than lead-acid batteries and deliver longer life and much better performance, but as of now they are significantly more expensive.  Direct replacement deep cycle lithium ion 12 bolt batteries are currently nearly $700 each, making them about 4 times the cost of a pair of 6-bolt golf cart batteries.  Their longer life might make them pay off in the long run but they are certainly a lot more expensive up front.  High performance, 80 amp lithium ion battery is over $1000 and will give 3,000-5,000 charging cycles.  I've even seen 300 amp batteries but they're big and heavy and expensive :  about $3500 each!  Lithium ion batteries allow you to use close to 100% of the capacity before needing to be recharged.  Lead-acid batteries only deliver about 50% of capacity before the voltage drops below workable levels.  A typical lead-acid 12 volt battery will read 12.volts when fully charged and about 11.4 volts when fully discharged.

Many RVs, (motorhome, trailers, and campers) have a single 12-volt deep cycle battery to provide power for all coach needs.  If you do a lot of boondocking you may find a single battery doesn't have enough reserve capacity to meet your needs.  When that happens you may be able to find a location where you can install a second battery.  Replacing a single 12-volt deep cycle battery with a pair of matching 12-volt batteries in parallel will just about double your reserve capacity.  Replacing a single 12-vole deep cycle battery with a pair of 6-volt golf cart batteries in series will usually result in greater reserve capacity and longer battery life.  When installing 6-volt batteries they must be connected in series in order to produce 12 volts to run RV fixtures.  6-volt golf cart batteries are usually larger so you may have to enlarge the battery tray or find extra room someplace else if you want to convert to golf cart batteries. Make sure all connections between batteries are heavy gauge battery cables and that the 6-volt batteries are wired in series to create 12 volts. 6 volt golf cart batteries are usually larger than most 12 volt deep cycle batteries so you may have to modify the battery box and/or hold downs to accommodate them.  Motorhomes will have a separate automotive starting battery.  This should not be a deep cycle battery but in an emergency you may be able to borrow one of your coach batteries to replace a dead starting battery or use it to jump start your engine.  Some motorhomes have a switch on the dashboard that activates relays to connect the coach batteries with the starting batteries when you need extra starting power, precluding the need for jumper cables.   The jump start button can be used to start the vehicle engine using the coach batteries or to start the generator using the vehicle starting battery.   If you install extra batteries it is best to install them as close to the existing battery as possible and run heavy gauge battery cables between the batteries.  Batteries give off volatile gases (hydrogen) when they are being charged so they must be installed in a well ventilated space and should NOT be installed within the living area of an RV.  Suitable locations include outside cabinets and in the engine compartment of a motorhome or on the tongue of a trailer.

There is a growing trend to large battery banks and inverters in large luxury rigs in order to handle a demand for quiet, portable 120-volt power anywhere.  An inverter is a device that changes 12-volt DC power into 120-volt AC power.    This is convenient for running entertainment systems and microwave ovens.  Some large luxury motorhomes even have large enough battery banks to run residential style 120-volt refrigerators full time, but that takes a lot of batteries and frequent recharging.  Keeping the batteries charged becomes a primary concern.  They can be charged from shore power, an on board generator, or from solar panels.  Solar systems can be expensive but once they are installed you get free power from the sun.  Some RVs have inverters built it.  If yours does not, they can usually be added (if you have sufficient battery reserves).  For small 120-volt devices you can often use a fairly inexpensive inverter that plugs into a cigarette lighter style 12-volt outlet.  Large inverters, say 1500-2000 watts, need to be hardwired with large gauge wire and the output can be routed directly to dedicated outlets.  Some RVs have an option to switch selected outlets from shore/generator power to inverter for added convenience.  But unless you know for sure you have plenty of reserve battery power, running 120-volt appliances on the inverter can draw your batteries down rather quickly.  120-volt appliances will consume power at 10 times the rate of 12-volt appliances of the same amps rating.  It is unlikely that units not designed for large battery banks will have anyplace they can be installed because they batteries take up a lot of room and add a lot of weight.  If you plan to install a large battery bank be sure the compartment floor is adequately supported to support the weight and that it is properly ventilated.  Never install lead acid batteries near any kind of device that may provide a source of ignition for the off-gassing that occurs during charging.  Lead acid batteries give off hydrogen gas while charging and it is VERY volatile!  The infamous Hindenberg airship that exploded in New Jersey in 1937 was filled with hydrogen, which is why it produced such a spectacular fire and explosion!  BTW, the flames from hydrogen are generally invisible.  Flames from gasoline are usually yellow or orange and natural gas and propane typically produce blue flames.

Proper maintenance is essential for good performance and long life for all batteries.  Some components of proper maintenance include avoiding excessive discharge, correct charging, maintaining tight, clean connections, and maintaining proper electrolyte levels as necessary.  Try not to let your batteries be drawn down until they are "dead" before recharging them, then use the right charging system to restore them to full charge as soon as possible.  Frequently check all battery connections to make sure they are tight and are not becoming corroded.  The terminals that connect the large battery cables to the battery posts are especially susceptible to corrosion and when they get corroded should be removed and cleaned.   The electrolyte levels in all but maintenance free batteries should be checked frequently and kept at about 1/2" above the plates.  A battery filler comes in handy for topping off your batteries.  You can buy them at auto  parts stores.   Or you can use a turkey baster.   Always try to use distilled water to fill batteries so you don't introduce mineral or chemical contaminants that often occur in normal tap water.  However, if you're boondocking when you discover your batteries are low using ordinary tap water would be better than leaving them low.  Filtered water would be preferable to unfiltered water.  You want it to be as contaminant-free as possible.

One sure sign that your battery capacity is insufficient, is when the furnace fan continues to run after the burner has shut off.   That can also happen if you run out or propane, so be sure to check both battery and propane gauges before deciding what to do.  You may wake up cold in the middle of the night and find the furnace blowing cold air.  It is ironic that when the batteries get too low, the circuitry that shuts off the fan fails and the fan continues to run, further depleting the batteries.  If this happens with an old battery it may just mean it is time to replace it.  But if it happens with fairly new batteries or happens often, you probably need to increase your reserve capacity by installing a larger battery, multiple batteries, or converting to 6-volt golf cart batteries.  Of course, make sure you have been keeping your batteries properly charged before running off and blowing a lot of money on new batteries.  If you try to run your furnace for several days and nights without charging your batteries, you can pretty much count on it blowing cold air sooner or later.  If you don't run your generator enough each day or your solar system doesn't get enough exposure (shade or clouds for instance), your batteries are going to suffer.  You can check the state of charge using a voltmeter if your RV doesn't have a battery meter.   A fully charged 12-volt battery should normally read about 12.7 volts.  Be sure to test it without any charging voltage.  Charging voltage is often around 14.2 volts; "float" or maintenance charge should be about 13.8 volts.  Higher voltages will overheat the battery.  I once had a charger board in a converter that went bad and was putting out 18 volts.  It "fried" batteries like crazy!  I had to add water to my coach batteries a couple of times a week until I found out what was causing the electrolyte to boil away.

Here is a table of voltages and what they mean:

     12.7 = fully charged
     12.5 = 85% charged
     12.4 = 65%
     12.3 = 50%
     12.2 = 35%
     12.1 = drained


Battery indicators on the monitor panel don't usually give actual volts, but are calibrated to reflect approximate percentages, usually indicating 1/4, 1/2, 3/4, and full.  Checking the actual voltage at the batteries will give you a more accurate reading.  For best performance and longer battery life, avoid letting batteries get below 50% charge (12.3 volts) before recharging.  Note that even "dead" batteries should still show about 12 volts.  While that isn't enough to run lights, motors, or appliances, it may be enough to make a big enough spark or heating 0000 steel wool for lighting a fire in survival situation.

Charging your batteries.   The alternator on motorhomes is usually wired to both the starting and coach batteries so it charges them all whenever the engine is running.  It should be connected through a battery isolator that prevents drawing down the starting battery while using lights and appliances in the coach.  A charging circuit can be wired from the alternator through the trailer connector to charge trailer batteries.  If your vehicle and/or trailer doesn't have this circuit it can usually be added at a nominal cost.  Be sure to include a battery isolator so using power in your trailer in camp doesn't run down your starting battery and leave you stranded or disconnect the trailer from the vehicle in camp.  There are two types of battery isolators:  solenoids and diode-based models.  Soleniods are usually less expensive and may have a shorter lifespan than diode models.  In either case they allow current to flow to charge both battery banks but keep a draw on the coach battery from running down the starting battery.  There is often much discussion among RVers about whether charging batteries with the vehicle alternator or the on board generator is better.  If you need a quick charge, using the vehicle alternator is probably your best bet and the engine at idle will most likely not consume much more fuel than the generator.  In camp, coach batteries are usually charged by the the converter whenever you run the generator.   If you use the generator enough, like to run the A/C for several hours on hot days, it will usually be enough, but in cooler weather you may have to schedule some generator time just to keep your batteries charged.   But be aware that the battery charging capabilities of most converters is limited.  Newer "smart" multi-stage converters like Progressive Dynamics "Intelli-power",  have more efficient battery charging systems.  Multi-stage chargers usually sense the battery condition and select one of four modes as necessary to maintain batteries in optimal condition.  The four modes are and their functions are:

     Boost Mode - (14.4 volts) to rapidly bring the battery up to 90% of full Charge.
     Normal Mode - (13.6 volts) to safely complete or maintain the charge.
     Storage Mode - (13.2 volts) to maintain charge with minimal gassing or water loss during                                    periods of  non-use.
     Desulfation Mode - (13.2 volts with 15 minute 14.4 volt burst every 21 hours)

Compare that with the charging circuits on older converters that typically supply a constant voltage of 13.6 volts.  Some may sense when the battery is fully charged and reduce the amperage to maintain a "trickle charge".  The different voltages are required to provide proper charging and maintenance.  Boost mode helps recharge batteries quickly; Normal Mode tops off  and maintains the charge; Storage mode provides a "trickle charge" to compensate for normal voltage drop of batteries that are not in use; Desulfation mode provides a high boost during storage to "burn off" sulfation of the lead plates to maintain electro-chemical efficiency. 

 Lead-acid batteries give off hydrogen gas while charging.  Hydrogen gas is highly volatile.  It was the gas that exploded in the famous Hindenberg disaster.  Batteries should always be installed outside of the living area in a well ventilated compartment.  Trailer batteries are often installed on the tongue, for excellent ventilation and easy access for maintenance.

An easy and fairly inexpensive alternative to smart converters is to install an automatic automotive battery charger connected to the coach batteries and plugged in so that it charges them whenever there is 120-volt power -- from shore power or from the generator.  When using an external battery charger it is best to disconnect or disable the charger in the converter.  Another way to "quick charge" your RV batteries is to run the vehicle engine.  Estimates show it will use only slightly more gas than running the generator since the alternator is far more efficient at charging the batteries than converters and auxiliary battery chargers.  Of course, solar panels are also a good way to keep your batteries charged -- if you have a large enough array and sufficient sunlight.  You can buy little inexpensive panels that are plug into your cigarette lighter socket and are intended to be placed on the dashboard to provide a small "trickle charge" for helping keep batteries charged in storage.  These will not recharge batteries enough when they are being used when off grid.  Large solar panels are mounted on the RV roof and require a controller to provide proper voltage.  Of course solar panels work best in direct sunlight, so don't expect them to quick charge your batteries if you're parked in the shade.

Maintenance chargers can be used to maintain batteries while in storage.   Often referred to as "trickle chargers" because of the low (1-2 amp) charging current, they are pretty good for keeping batteries charged but not for recharging depleted batteries. If you have a good multi-stage charger connected to your house batteries, you shouldn't need a maintenance charger there, but you may still need one to maintain your starting battery.  The default charging voltage from the converter will usually override charging from automatic or maintenance converters so you may have to disable the converter in order to get the most from your chargers.  Those little solar battery charges that plug into the cigarette lighter are "trickle chargers".  By the way, make sure the lighter is live when the key is off or they won't do anything!

Converters and Inverters.  Converters convert 120 volt AC power to 12 volt DC power to run lights and 12 volt accessories from shore or generator power.  Converters usually include a charging circuit to charge your batteries whenever you have 120 volt power available to the converter.  Inverters change 12 volt DC power into 120 volt AC power so you can run normal household appliances using your RV batteries.  If you want to run any large appliances you will need a very powerful inverter and a large battery bank to support it.

Batteries in storage will lose about 1% of their charge per month, even if they are disconnected.  Because of the parasitic draw of some RV appliances and accessories,  they can be be drawn down a lot faster if not disconnected.  Some RVs come with battery disconnects.  They can be fairly easily and inexpensively added if necessary.  If you don't have a battery tender connected to maintain your batteries while your RV is in storage it is a good idea to disconnect them to avoid any parasitic draw down.  A solar charger that plugs into the cigarette lighter can help keep batteries charged in storage.

Charge!