RV Batteries and Chargers

RV Batteries and Chargers

Batteries are critical to an RV. They start the engine and provide power to run accessories when driving. When parked they provide power to run lighting, water pumps, and various other 12 volt accessories. They can even be used to power an inverter to provide 120 volt AC power. Because of their heavy use they can sometimes be a source of aggravation and maintenance. Let’s take a closer look at batteries, how they work, and how to properly care for them. We’ll also delve into battery chargers and charging methods.

The battery compartment from an Allegro Bus

The battery compartment from an Allegro Bus

RV Battery vs Car Battery

A single battery may be fine to operate your passenger car but it’s just not enough power to handle the more advanced requirement of an RV. To do this, multiple batteries are arranged together into a battery bank. There are two types of batteries and each type has a different intended use. Automotive batteries are what you have in your car. They are designed to output a large burst of amperage to start the vehicle, then slowly recharge from the vehicle’s alternator. The key here is that they dump a lot of amperage in a short time, then rest for a while slowly getting recharged. Most diesel motorhomes will have two engine-starting or chassis batteries in order to crank these larger engines.

When camping, your coach electrical systems operate quite differently. They will draw fewer amps than a starter motor but they will do that over a longer period of time. For this application deep cycle batteries are used. Deep cycle batteries are designed for maximum performance by slowly draining amps from them and sustaining this draw over a longer period of time. Most coaches will have four deep cycle batteries in a battery bank to ensure that there is enough capacity to supply adequate power over a longer length of time. This can vary and some smaller coaches may only have two, while larger coaches may have as many as eight batteries in a bank.

How RV Batteries Work

RV electrical systems are 12 volt systems. Whenever you connect multiple batteries together you have to make the proper connections in order to maintain 12 volts. If batteries are connected in parallel they will retain their voltage, but if they are connected in series the voltages will add up. So, if you have a bunch of 12 volt batteries that you want to connect into a battery bank, you simply connect all of the positive and negative posts together to give you increased load capacity or amp-hours. 

6 Volt Batteries vs 12 Volt Batteries 

But many RVs are using 6 volt batteries for their deep cycler applications, which are commonly used in golf cart applications. They have a heavier plate design and are more durable, plus they output more power than a 12 volt battery of similar dimensions. The only downside is that you need to connect them properly to get a 12 volt output. Basically, you connect a pair of 6 volt batteries together in series to give you a single 12 volt battery. Then you connect the positive posts of this pair together with all of the other pairs and then do the same with the negatives. The following diagrams show a four battery bank of both 12 volt and 6 volt batteries and will help explain this better.

diagram of series versus parallel battery connections

Diagram of series versus parallel battery connections

Types of Batteries used for RVs

Batteries come in different types. For RV applications the most common are flooded batteries or AGM batteries. 

Flooded Batteries 

Flooded batteries are filled with electrolyte. This electrolyte is a diluted form of sulfuric acid. When a load is placed on the battery, the acid puts a charge on the lead plates and creates electricity. At this time some of the sulfur and oxygen leaves the acid and forms a sulfate on the lead battery plates, leaving water as the remainder. If a battery is totally discharged, the battery plates will be coated with sulfate and the electrolyte will be mostly water, which is why dead batteries can freeze in cold temperatures but fully charged batteries will not freeze. When the battery is recharged, the sulfate falls of the battery plates and recombines with the water to change it back to acid again. If the battery is overcharged (in other words excess voltage is applied to it when it already is charged up), the water can boil out of the battery and acidic vapors can leave via the vented battery caps. This reduces the water level and if it gets below the battery plates it can harm the battery. It also tends to corrode the battery connections because of the acidic vapors. Maintaining the proper water level is critical to good battery performance.

AGM Batteries 

AGM batteries use electrolyte but not in a liquid form like flooded batteries. The acidic electrolyte is absorbed into glass fiber mats that are wrapped around the lead battery plates. With AGM batteries there is no need to ever worry about adding water because there is no water in them so maintenance is greatly reduced. The battery terminals also are not as susceptible to corrosion as flooded batteries because there is only microscopic outgassing, or venting, of electrolyte vapors. The outgassing of flooded batteries means they need to be in a vented compartment and clear of anything that could create a spark. AGM batteries can be located anywhere because they truly are sealed. AGM batteries are typically used in aircraft and they can be used in any orientation, even upside down, with no negative consequences. AGM batteries also offer greater performance over a comparable flooded battery because the voltage drop curve is flatter.

Voltage State of Charge
12.6+ 100%
12.5 90%
12.42 80%
12.32 70%
12.20 60%
12.06 50%
11.9 40%
11.75 30%
11.58 20%
11.31 10%
10.5 0%

Battery Charge Voltages

Battery Chargers

We all know how easy it is to take power out of a battery. What about recharging them when they are low? Batteries in a motorhome can be charged in a number of ways depending on the current mode of operation and whether they are chassis batteries or coach batteries. 

Chassis Batteries and Coach Batteries 

First off, we need to understand that there are two separate battery banks on a motorhome – the chassis batteries and the coach batteries. 

The chassis batteries are for starting the motorhome’s engine and powering the headlights, wipers, and other chassis related accessories. When driving down the road the vehicles alternator will charge the chassis batteries. 

The “house” portion of the motorhome has a separate bank of batteries called the coach batteries. These are deep cycle batteries that will power the lights, domestic water pump, fans, and any other accessories that are related to living in the coach while parked. These batteries are kept separate from the chassis batteries so, in the event that you run them down too low, you will still be able to start the motorhome’s engine. 

A Charge Solenoid is installed to connect both the chassis and battery banks together. This solenoid is engaged whenever the ignition key switch is in the “on” position. The engine can then recharge both battery banks when driving down the road. As soon as you are parked the solenoid opens and the two battery banks are divorced once again.

A convertor
A Xantrex inverter/charger


Xantrex inverter/charger

Charging Batteries at a Campsite

 We don’t want to have to run the engine all the time to recharge these batteries, so we need other options. The first option is a battery charger that is dedicated to the coach’s electrical system. This charger is a 120 volt device that will be powered whenever we plug into shore power or run our generator set. In addition to charging the batteries we also need to provide clean power with consistent voltage to power the 12 volt electrical components while we are parked rather than use the batteries to power them. This device is called a converter. 

The converter is hard-wired into the 120 volt electrical system as well as the 12 volt system. It “converts” 120 VAC power to 12 VDC power to run the 12 volt accessories as well as charge the coach battery bank. So, now we have the ability to recharge the coach batteries via the engine alternator when driving or via the converter when parked with shore power or when running the generator. When boondocking we can use the batteries to provide power. When the voltage gets to that magic 50% mark, we can fire up the generator set for an hour or two to power the converter and recharge the batteries. The actual charge time will vary according to the size of the converter as well as the size of the battery bank. If the battery bank is 440 amp-hrs and it is at 50% you’ll need 220 amp-hrs to bring it to its fully charged state. If you have a 60 amp converter you’ll be looking at close to 4 hours to output that much power. Larger converters take less time but cost more.

Another popular device is the inverter. An inverter does just the opposite of a converter. It creates 120 VAC power from 12 VDC. It is used to power 120 volt devices via battery power so that you don’t have to run the generator all the time. Small inverters run from 250 watts up to 1,000 watts and are installed into an electrical circuit. No battery charging capabilities exist on a basic inverter so most diesel pushers use an inverter/charger unit. These units combine the features from both an inverter and a converter. They are generally found in larger sizes, such as 2,000 watt and 3,000 watt. These inverters are connected to a dedicated 120 volt circuit breaker in the main breaker panel and also connect to the coach batteries via large diameter battery cables. They feature an automatic transfer switch that will pass shore power through when present, or switch over to battery fed inverter power whenever shore power is not present. Note that the running of the generator is the same thing as shore power in this instance. Whenever 120 VAC power is present the inverter not only passes that power through to the electrical devices fed by it, but it also acts as a converter and provides 12 volt current to power the house accessories and recharge the coach battery bank. Typical inverter/chargers have battery charging capacity in the 100 to 140 amp-hr range so they are larger and faster than a converter. When an inverter/charger is present there is no need for a converter.

Both converters and inverter/chargers feature three battery charging modes. When a battery is low on charge the first mode is the bulk mode. This mode provides full charging output to the batteries. As the battery approaches the full mark the charger will kick into absorption mode. In this mode the voltage is regulated to not be excessive so that the batteries can absorb this charge without boiling. Finally, in order to maintain a charge in a battery that is very close to full the float mode will be engaged. In this mode charger output voltage will be limited to no more than 13.2 volts so that excess outgassing does not occur. You can safely leave your batteries connected to the charger indefinitely if it’s in float mode without fear of boiling out water. Of course, this assumes that your charger’s float mode is functioning properly. If you are adding water frequently it’s time to have your charger’s float voltage checked.


Low Voltage Circuits

Motorhomes are meant to be used when 120 volt shore power is not always available. It would be a real pain if every time you walked into your motorhome you had to fumble around in the dark to start the generator so that you could turn some lights on, which is why RVs have 12 volt battery powered electrical systems as well as 120 volt AC electrical systems. 

The 12 volt system provides power to operate the interior lights and the many other 12 volt electric systems within the coach, such as fans, water pumps and furnaces. These systems will be connected to the coach battery bank via a 12 volt fuse panel that is located somewhere inside the coach. A converter or inverter/charger will also be connected so that these batteries can be recharged from shore power or when running the generator.

Powered Roof Vent

Powered vent fans are commonly used in most RVs

Powered Roof Vent Fans

Powered roof vent fans, such as the one pictured above, are found in most every RV. In addition, you’ll undoubtedly find that you have a tank monitoring system to check the levels of your fresh water, propane tank, and holding tanks. This information may be displayed on a small remote panel or incorporated into a networked touch screen control panel. Your refrigerator and hot water heater are designed to run on either 120 volt AC current or propane. But, you will need 12 volt power to operate the igniter and the control circuitry for any propane fired refrigerator. Even the light bulb inside that fridge is powered by 12 volt power. Basement compartment lighting and power awnings are other examples as are the power steps that allow you to enter your motorhome.

battery disconnect solenoid

A typical battery disconnects solenoid

Battery Disconnect Switch

If you were to leave your motorhome parked for a while and shore power was not available, the various parasitic loads on the 12 volt circuits would eventually drain the batteries. RVs have a battery disconnect switch, which is generally located close to the entrance door. This switch controls a battery disconnect solenoid. This solenoid is a latching solenoid which means you send it power to move it but you do not have to continue feeding it power in order to hold it in that position. This makes it perfect for an RV application because the solenoid won’t consume any battery power when parked. Pressing the rocker switch in one direction will shuttle the solenoid to the open position while pressing the rocker switch in the opposite direction will send the shuttle in the other direction and close the circuit. This allows for an easy way to shut down the 12 volt power to the coach while you are away. If you are plugged into shore power, this is not necessary. Keep in mind that not everything will be disconnected when the solenoid is in the off position. Some loads, such as inverters, keyless entry systems and propane leak detectors bypass the solenoid and will remain active.