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Camper Batteries 101: The Basics

With his deep knowledge of both electronics and truck camping, we talked to Jim Tomblin about camper batteries and truck camper electrical systems.

Jim has been involved with electronics since he was a kid.  At age nine, Jim built a tube radio that ran on a 90-volt battery.  He told us that he loved to take things apart and put them back together.  He went on to pursue electronics in high school and was involved in what he calls, “unusual electronic technology” while serving in the Navy.  From 1969 to 1989 he was involved in every aspect of the IBM mainframe computer industry from tubes to ICs, bytes to terabytes.  He also ran a successful electronics business for fifteen years, which he sold and then retired.

Jim is also a truck camper.  He is our go to guy with any camper electrical question.

Camper Batteries with Jim Tomblin.jpg

TCM: Truck campers have 12-volt and 120-volt systems.  What’s the difference?

Jim: Your truck camper’s 12-volt DC electric power system includes a 12-volt battery and 12-volt converter and allows a truck camper to run its 12-volt components free from shore or household power.  Shore or household power is 120-volts and is what we use to charge our truck camper batteries through a built-in AC to DC power converter.  Plugging in your truck camper to shore or household AC power also allows you to power household items within your camper including hairdryers, coffee makers, microwaves, and air conditioners.

TCM: In a truck camper, what can you run off your batteries?

Jim: You can run your lights, furnace heater fan, water pump, and DC-powered refrigerator off your camper batteries.  However, it’s not recommended that you power your refrigerator from your batteries as it draws over 10-amps an hour (a very high rate).

Other things can run off your 12-volt DC battery power including 12-volt televisions, 12-volt entertainment systems, 12-volt air compressors, etc.  You can also buy a 12-volt inverter which will allow you to use your battery’s 12-volt power to run and charge your 120-volt laptop, electric razors, and cell phones.  Some individuals actually run their coffee pot’s off their inverter.  That takes lots of battery energy.

TCM: How do you charge your batteries?

Jim: You can charge your batteries by plugging your camper into a household 120-volt electric outlet at home or a campground.  A household 120-volt outlet is 15 or 20-amps and can charge your batteries easily. While your camper is plugged in to a household outlet you can run appliances while you charge your batteries, but caution must be used because you can overload the house outlet breaker.

If your batteries are low and need charging, you may need about seven amps to begin charging just your batteries. You could charge, run your refrigerator on 120 and your air conditioner if it has a low BTU rating.  Educate yourself by finding out the total amp usage of each of your appliances.

Example: A 7500 BTU air conditioner is nine amps, refrigerator on 120 volt three to five amps, microwave fifteen amps.  As you can see you only have 20 to play with and you shouldn’t push that limit.

Thirty-amp outlets are commonly found at campgrounds.  That allows multiple high amp devices to run without tripping a breaker.

Honda 2000 generator for truck campers

You can also charge your batteries using an internal or portable generator, or with a solar panel system.  Again your generator will be your limiting factor. The most popular truck camper generator, the Honda eu2000i, provides about 13 amps continuous so you have to be aware of what you can and can’t run.

TCM: How do you know how many amps your batteries are using?

Jim: If your camper is just sitting and there are no lights on and your refrigerator is on LP gas, your camper will pull about one-half amp an hour.  If you have two Group 27 batteries, you have about 80-90 amps available to use.  That’s thirty-six amps that you would use in three days if your camper is just sitting.  One camper light uses about one and a half amps an hour.  So for twelve hours, one camper light uses eighteen amps plus the six amps you need to add for the camper just sitting.

TCM: If you boondock for a few days, how long will your batteries last?

Jim: It depends on the weather you are camping in.  In warm weather, when you don’t need anything but a small fan once in a while to move air around, two Group 24 or Group 27 batteries will last about three days.  Just being in weather below 40 degrees will reduce your battery capacity about 20% without discharging them.  So if you had 80 amps to play with at 70 degrees you would only have about 64 amps available at 40 degrees.  You can go through the capacity of your batteries in one to two days.

In cold weather, a standard Atwood heater, in heat mode, fan motor running uses seven amps.  My camper heater goes for about five minutes every fifteen minutes.  So, there’s about twenty minutes of run time every hour, which means that every three hours your heater runs for an hour.  That’s seven amps used up ever three hours.

For every hour of heater usage, three actual hours, seven amps of power is pulled out of the battery.  Plus, there are another one and a half that’s being used for your camper just sitting without any lights on.  So, twelve hours of running your furnace to warm yourself results in about thirty-two amps drawn out of your batteries and you haven’t turned on any lights.

TCM: What brand of batteries do you suggest for truck campers?

Jim: In my opinion, the AGM batteries are the best.  They don’t require any maintenance and they are a lot more forgiving.  AGM batteries are also good because they are packaged differently.  You can put them in a cabinet right in the camper and you don’t have to worry about leakage of sulfuric acid or outgas like a regular wet cell battery.

The capacity of the AGMs also provides more available power as they can be discharged to 50% repeatedly.  If you keep the AGM battery in your camper, they stay warmer and you will get more battery capacity.  They also are capable of charging faster than wet cell batteries.  The downside to the AGM battery is that they cost over $200 for the standard sizes.

TCM: What does AGM stand for?

Jim: AGM stands for Absorbed Glass Mat.  The electrolyte is held in the glass mat.  AGM batteries are sealed, so you don’t need to maintain them.  If you were to turn them upside down they wouldn’t leak.  That’s the advantage to having them.  If you get an AGM battery make sure your converter is compatible with AGMs.  They are different in how they work.  Battery companies will be able to tell you if your converter is compatible.

TCM: What if you don’t want to spend that much on AGM batteries.  Are there other batteries you recommend?

Jim: You could get flooded wet cell deep-cycle, trolling motor, RV-type marine batteries.  Trojan batteries are very popular.  Also, some people get the generic WalMart batteries or Costco deep-cycle batteries because they have a good warranty.  They are about  $60-$80 each.

When replacing your batteries, first check your battery compartment and document the depth, height and width.  Look for Group 24, 27, or 31 deep cycle batteries.  Each group is a different size physically and some batteries are slightly bigger in some dimensions so you will know if the battery(s) your purchasing will fit.  Make sure you get deep cycle batteries and not a starting deep cycle or cranking battery.

Also, you can’t mix AGM with flooded wet cells.  The size of your battery compartment will dictate the size of battery(s) you will be able to use so if you had two group 24s and have the room for 27s you can do that and get another 30 amps of capacity.  Normally campers don’t come with 31’s.

Some individuals switch to six-volt batteries.  It takes two of them to get 12 volts.  The six-volt, “Golf Cart” batteries are the ones of choice.  They can be discharged deeper than the above mentioned and have a longer life.  Again the physical size will sometimes be a problem so check carefully.

TCM: When do you know if you need new batteries?

Jim: There are two reasons why you would want to replace your batteries.  First, you may decide that you need more capacity.  And second, if your batteries have aged or they haven’t been taken care of, you may want to replace your batteries.  Batteries over six-months old can begin to fade.  You’ll know if your batteries are too weak because they will only last a couple of hours when you are out camping.  Always replace your batteries as a pair.  It is not good to have one weaker and one stronger.

Hydrometer to test camper batteries

TCM: How do you properly maintain your batteries?

Jim: When you get fresh batteries, check the fluid/electrolyte levels every month or so with a hydrometer.  When electrolyte is low use distilled water to refill.  If everything is okay for a few months and there has been no need for distilled water, then checking every six months should be okay.  Different brands of converter/chargers treat the batteries differently.  How often you discharge and charge makes a difference as to the fluid/ electrolyte levels.

You want to make sure that the electrolyte is covering the battery plates.  There’s a fill-ring in the batteries.  Fill your batteries to that level. If my fluid levels are slightly down from the ring I usually will not fill them.  My batteries only need filling about once a year.

For safety, always use rubber gloves and wear safety glasses when dealing with your batteries.  Also for safety, you can get a rubber bulb from auto parts stores.  The rubber bulb looks like a turkey baster.  Use the rubber bulb to put distilled water in the batteries if the distilled water is not up to the water fill-rings.   Be sure to check and fill each of the cells in your battery.

If you have a good converter and you are plugged in at home, the distilled water levels should be okay.  Typically, you should only need to put distilled water in your batteries once a year.  If you are putting it in once a month or more, something is wrong.  Some older campers converters may require battery maintenance more often.  With the AGM batteries there are no water levels to worry about.

battery tender charger

TCM: What do you do with your batteries when you winterize your camper?

Jim: If you are in an area where it freezes in the winter, I would take your battery or batteries out of your camper.  Then purchase a battery minder, a battery maintaining device that you can find at a Batteries Plus store or online.  A battery minder plugs into a household outlet in your garage and will keep the batteries conditioned through the winter.  While the batteries are connected to the battery minder, check the batteries periodically to make sure that the minder/charger is working properly.  I would get the kind that is a three stage charger with the desulfating technology.

If your camper sits longer than a couple of weeks and your not able to keep it plugged in to shore power, it would be good to remove them and hook them up to a battery minder/small multi-stage charger.  When your batteries are not in use they will deteriorate 10% capacity per month, which diminishes the life of your batteries.  Sulfation will occur and crystals will form inside the battery thus reducing the capacity of the battery, which is not a good thing.  If you have AGM batteries, they will only deteriorate 1% of their capacity per month.  They do not sulfate as rapidly as a flooded wet cell battery.

If you have to leave your camper sit for a couple of days or weeks, disconnect your batteries.  Some campers have a “battery cut-off” switch that performs that function.  By disconnecting your batteries you won’t drain them because of your LP and CO sensors and an auto type radio if you have one.

Click here to go to Part 2, Camper Batteries 102: Digital Multimeters and Trimetric Meters


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