Truck Camper Batteries 101

Boondocking - Solar Power - Truck Camper AdventureBoondocking in remote, hard to reach places is one of the benefits of owning a truck camper. Getting to that remote spot, however, means little if your camper’s 12 volt battery dies on you while you’re camped and you have no way to effectively charge it. One thing I have a hard time understanding is why some owners go cheap on a battery, like buying a used one on Craigslist, but are willing to fork out $30,000 for a brand new, beautiful camper. It doesn’t make much sense to me. When you camp off-the-grid, most of the systems and appliances in that bright and shiny truck camper are pretty much rendered useless by a weak and ineffective 12 volt battery. The best course is to buy a fresh, new battery for your truck camper rig, but it’s also important to buy the right one. The purpose of this article is to provide a basic understanding of the 12 volt batteries found in your truck camper without getting too technical. I’ll explain how they work, how many batteries your camper needs, what type and what sizes to buy, how to wire them up, and the methods available to recharge them.

Battery Basics

Let’s start with the basics. The 12 volt battery does one thing, it stores electrical energy. The 12 volt battery is classified by type (how its built) and application (how it’s used). Wet cell, gel cell, and AGM are the three most common and popular types, while the automotive starting, hybrid marine, and deep cycle are the three major applications. Your truck camper rig has two 12 volt battery systems, one for the truck and one for the truck camper. Each is critical to the operation of your rig and to your camping enjoyment. Buy fresh batteries for your rig. Never go cheap when buying batteries. Battery failure when you’re far from home and far from help is never a fun thing to go through.

If you’re interested in boondocking, your truck camper should have at least two 6 volt or two 12 volt deep cycle batteries. They should be true deep cycle batteries, rather than an automotive starting or a hybrid marine battery. The reason is deep cycle batteries are designed to withstand repetitive discharges up to 80 percent or more numerous times, and still provide amperage at their rated capacity. Automotive starting and marine hybrid batteries aren’t made to do that. Sure, a hybrid battery, which is rated in cold cranking amps (CCA) rather than amp hours, will work in your truck camper, but it isn’t optimal and won’t last as long as a true deep cycle battery.

At this point you may be wondering what the term “cycle” means as it relates to deep cycle batteries. A cycle is simply one complete discharge and recharge cycle. A “deep” discharge is typically lower than 20 percent (11.58 volts) of a battery, while a 100 percent recharge is achieved when the battery’s resting voltage reaches 12.7 volts. The number of times your batteries are discharged directly relates to their lifespan, so you should try to prevent your batteries from discharging more than 50 percent (12.06 volts) in order to prolong their life. Avoid running your batteries dead (10.5 volts) at all costs. Lastly, as you can see in these figures, a 12 volt battery’s usable voltage is actually quite limited and falls between 12.7 and 11.7 volts, though as a general rule of thumb you rarely want to let it fall below 12.0 volts.

Temperature has a positive and negative effect on batteries. All 12 volt batteries are rated at 77 degrees F, the optimum operating temperature. As the internal temperature of the battery increases a battery’s amp hour capacity increases, as temperature falls its capacity decreases. For instance, at 40 degrees a battery’s capacity in amp hours drops to about 80 percent, while at 0 degrees a battery’s capacity falls to about 50 percent. Conversely, a battery’s capacity rises about 20 percent at 110 degrees F. Battery voltage is impacted in the same manner as temperature. Anyone who has tried to start a car in freezing temperatures knows this from experience. This is why you should have temperature compensation on your charge controller if your battery is in an outside compartment. If you like to camp in freezing temperatures, I recommend insulating your battery compartment and buying a quality battery warming system. Doing so will not only improve the performance of your batteries, but will also improve their life.

One popular choice with truck camper owners is to install a pair of 6 volt golf cart batteries rather than a pair of 12 volt Group-27 batteries. This approach has merit. Six volt batteries offer more amp hours than two Group-27 12 volt batteries, they have heavier plates, can suffer more discharge cycles than a regular 12 volt deep cycle battery, and when you wire two in series you have, in essence, one huge 12 volt battery. Going this route, however, does carry a small risk in that if one fails you won’t be able to power your 12 volt system. Fortunately, this risk can be mitigated by installing four, six, or any other even number of 6 volt batteries (the number is limited only by your rig’s space and by your budget). This way if one 6 volt battery fails you still have another pair that you can rely upon to power your rig’s 12 volt system.

Wolf Creek 850 Truck Camper - Truck Camper Adventure
Lifeline 6 volt AGM batteries mounted on their side.

Deep cycle batteries can purchased in numerous sizes with Group-24, Group-27, and 6 volt golf cart batteries being the most popular for today’s RVs. Simply put, larger batteries provide more amp hours than smaller ones (the smaller Group-24 battery provides about 85 amp hours while the larger Group-27 provides around 105 amp hours). Unfortunately, truck camper battery compartments are limited in size with the largest battery box being able to hold only two batteries (if you’re camper’s compartment can hold only one, you should give the Torklift Hidden Power a serious look as it provides your camper with another 12 volt battery that’s mounted underneath your truck bed). When it comes to battery ratings, amp hours are the key and you want more of them. That means buying the largest deep cycle battery/batteries that will fit in your truck camper’s battery compartment. The following chart provides you with a good idea of how many amp hours you can expect to get for each battery size.

Battery SizeAmp hoursVoltage
Group-2470-85 amp hours12 volts
Group-2785-105 amp hours12 volts
Group-3195-125 amp hours12 volts
4-D180-215 amp hours12 volts
8-D225-255 amp hours12 volts
Golf Cart180 to 225 amp hours6 volts
L-16, L16HC etc.340 to 415 amp hours6 volts

Types of Batteries

In RV applications there are basically two types of deep cycle batteries from which to choose: Flooded, lead-acid (or wet cell for short) and Absorbed Glass Mat (AGM). Yes, Gel-Cell batteries are technically a third alternative, but in my opinion they shouldn’t be considered as they don’t last as long and have too many disadvantages associated with them for RV use. Let’s take a closer look at each:

Wet Cell Battery: These are the cheapest and most common batteries found in the market today. You can’t go wrong with wet cell batteries, but they do require periodic maintenance and proper charging. Overcharging can boil out the electrolyte in them and warp the plates, while under charging will leave sulfate on the plates which reduces storage capacity. The maintenance consists of periodic gravity checks using an hydrometer, periodic equalization charges to remove sulfate from the individual battery plates, and careful monitoring of electrolyte levels to make sure the plates are covered (make sure to use distilled water only when maintaining electrolyte levels). All things considered, wet cell batteries work great and are well worth the cost. With proper care and maintenance, they will provide many years of reliable service.

AGM Battery: In sharp contrast with wet cells, AGMs require no maintenance or watering as they are sealed. AGM batteries are still technically lead-acid batteries, but the electrolyte in them is encased in a fibrous glass mat that can’t be spilled. Since AGMs contain no liquid they are practically impervious to freeze damage and can be mounted on their side, an important benefit for some battery and storage compartments. Most AGMs, like those made by Lifeline, also have very thick positive plates and can suffer more discharge cycles. More importantly, they charge up to five times faster and have a slower self-discharge rate, about 1 to 3 percent a month. Unfortunately, AGM batteries cost two to three times more than a regular wet cell, but the benefits to your solar power system make them well worth the cost.

Battery Wiring

If your camper has the room and you have a desire to switch to 6 volt golf cart batteries, you’ll need to make an important wiring change. The 12 volt batteries found in your truck camper are wired in parallel. When switching to 6 volt batteries, you must wire them in series to produce the needed 12 volts. See the diagrams below.

12 volt battery connection diagram - Truck Camper Adventure

6 volt battery connection diagram - Truck Camper Adventue

Battery Charging

Properly recharging the batteries in your truck camper is vitally important to your boondocking enjoyment and to the life of your batteries. Three stage battery charging is accomplished by using either the converter/charger in your camper or by using a solar power charge controller. Battery charging takes place in three basic stages: bulk, absorption, and float, though the important process of battery equalization for wet cell batteries is sometimes considered another stage as well. The smart chargers found in today’s RVs will detect and control when these stages begin and end by sensing the battery state. Before we go into the details of three stage battery charging, it’s important to understand that not all batteries are the same. Some require higher voltages and current levels than others for a proper charge. Consult the user instructions that came with your batteries for specific charging numbers and make adjustments to your charger devices accordingly.

Bulk Charge: The first stage of battery charging. Current is sent to batteries at the maximum safe rate they will accept until voltage rises to near (80 to 90 percent) of a full charge. During this stage, voltages typically reach range between 14.0 to 14.7 volts (my WFCO Electronics 45 amp converter/charger generates 14.4 volts). The amount of current generated depends on the charger that you’re using and any settings that were made. There is little risk of overcharging during this stage because the battery isn’t fully charged yet.

Absorption Charge: The second stage. Once the battery has reached 80 to 90 percentage state of charge, the smart charger will start the absorption stage. The voltages generated during this stage vary by controller and setting and can be more or less than the voltage generated during the bulk charge. For instance, the voltage generated by my WFCO 8945 converter/charger is 13.6 volts while my Zamp solar charge controller generates a voltage of 14.4 volts for AGM batteries. Note, that this stage takes more time than the bulk stage. While most chargers maintain a steady voltage during this stage, the current generated declines gradually until the battery reaches its full capacity.

Float Charge: The third and final stage of battery charging. After batteries reach a full charge, the charging voltage is reduced to a lower level, typically between 13.2 to 13.6 volts. This is done to reduce gassing and prolong battery life. The float charge is often referred to as a maintenance or “trickle charge,” since its main purpose is to keep an already charged battery from discharging.

Equalization: Equalization is performed on wet cell and calcium batteries primarily to restore battery capacity, revive battery efficiency, and extend battery life. It removes the lead sulfate that accumulates on battery plates when batteries are undercharged. Equalization should be performed once a month using a higher voltage usually between 15.5 volts and 16.0 volts. Note that some AGM battery manufacturers, like Lifeline, now recommend that equalization be done as a recovery tool for their AGM batteries as well. Click here for specific instructions on how to charge and equalize a Lifeline AGM battery.

Battery Charging Methods

The 45 or 55 amp converter/charger that came with your rig is the most common and well-known way to charge your batteries when you’re connected to 110 volt AC shore power, but there are ways that you can use to charge your batteries while you’re boondocking, too. These off-the-grid charging methods include using a generator, solar power, wind power, and your truck’s alternator. Each method has its pros and cons. Let’s take a look at each:


WFCO Electronics RV Converter/Charger - Truck Camper Adventure

The most common and easiest way to recharge your camper’s batteries. Can only be used when connected to 110 volt AC shore power (or when using a portable generator). The old single stage converter/ chargers installed in our parent’s and grandparent’s RV’s got a bad rap, and rightfully so, but today’s 45 and 55 amp converter/chargers are so much better. The converter/charger works in place of your batteries (I typically disconnect my batteries when using mine unless they need charged) by providing a constant 13.6 volts DC output. They also distribute 110 volt AC power as well as provide automatic three-stage charging for your batteries when needed. Many units, like the one picture here, also provide AC and DC branch distribution. The DC circuits even illuminate when fuses are blown to quickly identify them.

Solar Power:

RV Solar Power 101 - Truck Camper AdventureMy personal favorite, this renewable source of energy is becoming increasingly popular and for good reason–not only is it efficient and reliable, but it’s also clean and quiet and doesn’t require fuel like a generator (there’s plenty of free sunshine out there). It also mounts in an out-of-the-way area on the roof of your RV and doesn’t require regular maintenance as there are no moving parts. Used with a charge controller. Unfortunately, overcast skies and shading can greatly reduce its effectiveness and there is a high initial start-up cost, but this cost can be greatly reduced by buying a Renogy 100 watt solar power starter kit and by doing the installation yourself. It’s not that difficult. Check out my Solar 101 article for more information on solar power.

Wind Power:

IMG_2723Another clean and renewable source of energy, wind power has also seen a recent surge in popularity though wind power has been employed by man since the 11th Century. Fortunately for RV and boat owners, miniaturized wind turbines can now be purchased without breaking the bank. For example, a wind turbine that can generate 255 watts at 35 knots can be purchased for only $400. The benefits having such a system are obvious and can be used as part of an overall strategy that employs both solar and wind; solar power during sunny days, wind power during overcast days and nights. The big negatives with wind power are the initial costs, the noise that the turbines create, and the setup and take down time. To learn more about wind power, click here to take a look at my review of the Windwalker 250 RV wind turbine.


P9250594Easily the most popular method to recharge your camper’s batteries. Working with your rig’s converter/charger unit, generators work great at recharging your batteries, but not all are good for camping. Sure, industrial generators are relatively cheap and provide lots of wattage, but they are also big, heavy, and extremely noisy. Few things irk campers more than a neighbor’s noisy generator. Fortunately, there are several low noise, camping friendly generators on the market today–like the Honda EU2000i and Yamaha EF2400iS–which are not only light (the Honda EU2000i weighs only 46 pounds) but are also extremely quiet. The big negatives with generators are that they must be hauled around, generate deadly carbon monoxide, and can easily be stolen. Make sure you keep them chained up at all times when camping within sight of strangers.

Vehicle Alternator:

IMG_3754Using your truck or tow vehicle’s alternator is the cheapest and easiest method to recharge your camper’s batteries. Assuming your RV is already plugged in, all you need to do is run your engine. The speed and the efficiency of this charge, however, is limited by the quality of your vehicle’s alternator and the gauge of the charge wire in your RV. The best alternators are those that produce a lot of amperage like those found in heavy-duty trucks (160 amps is the norm). The quality of the charge can be improved by upgrading (read enlarging) the existing charge wire from the 12 gauge wire you typically find in your RV to an 8 gauge or larger wire. You’ll also want to make sure a battery isolator is wired into the circuit to prevent the batteries in your RV from draining. Click here if you’re interested in a modification that upgrades the wiring and other components of your vehicle’s alternator charge system.

Battery Usage Tips

If you’re interested in buying a TV, radio, or DVD player for your camper, I recommend getting those that run on 12 volt DC power. An inverter, a device that converts DC power to AC power, is very handy, but also uses power when it is in use or is in standby. If you need an inverter, get one that produces a pure sine wave of at least 300 watts, like the Morningstar Suresine 300, so you can run your satellite receiver and other sensitive electronics. Yes, modified sine wave inverters work fine in most applications, but if you have the space and you’re going to fork out the big bucks for a new inverter, you might as well buy one that produces pure AC current like you find in your home. The wattage produced by your inverter should be determined by your needs (i.e. loads).

When looking for an inverter, get one that wires directly to your batteries. Most of the models which plug into 12 volt outlets don’t have very good filters and won’t run sensitive electronics like laptop computers. Some of the best, most expensive inverters are those which not only connect directly to your batteries but also route AC power to all of your 110 volt AC outlets. Some high-end inverters have two 110 volt outlets built right into the device or give you options on how to distribute the AC output. These options include wiring through a transfer relay, through a manual switch, or hard-wiring dedicated inverter-only outlets. I’ve used transfer relays and dedicated outlets in the three inverter installations I’ve done, both methods work well.

If you have both an inverter and a generator here are two simple rules you should memorize on when to use them. Use your generator to run high wattage AC items like your air conditioner, microwave, hair dryer, and coffee maker, while your inverter should be used to run low-wattage items like your laptops, TV, and DVD player. It all depends on how many batteries you have connected to your inverter, of course, but these two basic rules will prevent your batteries from being discharged too quickly and will serve you well while you’re camping off-the-grid.

A boondocker’s best friend is a high quality, digital battery monitor like a Xantrex Linklite or Trimetric TM-2030. These meters take the guesswork out of the state of your batteries and offer vital information like voltages, amps, and total amps consumed. Another option if you’re going solar, is to buy a charge controller with a battery meter built-in or one that can support a remote meter. This will allow you to monitor the state or condition of your batteries, but won’t provide all of the historical information that a Trimetric can provide. Don’t bother buying the $15 voltage meters that plug into 12 volt electrical outlets–they’re inaccurate and a total waste of money.

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About Mello Mike 295 Articles
Mello Mike is an Arizona native, author, and the founder of Truck Camper Adventure Magazine. He's been RV'ing since 2002, is a truck camper and Jeep enthusiast, and has owned and restored several Airstream travel trailers. He enjoys college football, hiking, travel, off-roading, photography, and fishing. He retired from the U.S. Navy in 2004 as a CWO3 after 24 years and now works as a project manager for a major banking and security firm. He also does some RV consulting and RV inspections on the side.
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