House Batteries

House Batteries

Unlike car, truck and motorhome chassis batteries, none of the house batteries have died from natural causes.  They were all murdered!

Leaving batteries connected during RV storage is the most common cause of death.  Forgetting to disconnect batteries causes full discharge over days or even weeks.  Full discharges kill batteries dead.   When working with lead acid wet cell batteries, the type most commonly found in RVs, avoid battery discharges over 50%.  Discharging over 50% causes damage.  Discharging 100% severely cripples or kills batteries.

Standard RV batteries have water in them that needs to be replaced from time to time.  “Maintenance Free” doesn’t always mean maintenance free.  Overcharging can “boil” away battery water.  The battery fluid can be restored with distilled water.  Older RVs, like our first one, had a simplistic converter charger that routinely overcharged the batteries.  Newer RVs have smarter converters that charge batteries in multiple stages to avoid the “boil-off” problem.  Correctly maintaining battery fluid levels is not a skill I’ll ever be able to master.  Fortunately, charging technology has improved and “boiling” doesn’t seem to be a problem on newer RVs.

During his RV educational seminars, Howard Payne of RV-Dreams often ends an important point with pained smile rhetorically saying “Ask me how I know!

Howard teaches battery maintenance, care and safety in his seminars.  See 2018 RV-Dreams Spring Educational Rally for more information.

Ask Me How I Know

The fifth-wheel batteries were replaced four times over 10 years.  The motorhome batteries were replaced once over five years.  We have yet to replace our truck camper batteries.  But there is still time.  We’ve only had the camper for one year.

Every Sunday, unless we are out of town, the truck and camper are taken out of storage and driven seven to ten miles, keeping everything in good working order.  Three weekends ago, the batteries were left connected which wasn’t discovered until last weekend.  There was still enough power to dimly light the over head interior lights.  Another attempted battery murder.  Connecting to shore power at the storage site for a week will recharge the batteries.  Was there damage?  Possibly.  No way to really know.

Preemptive replacement of the batteries will likely occur next year when the camper and batteries are two years old.

Truck Camper Batteries

The camper was ordered with the extra battery option.  Lance installed two vented battery holders under the step up to the over cab bed.  At delivery, the dealer installed two 12 Volt Interstate SRM-24 batteries.

Interstate Deep Cycle Marine/RV Battery
Interstate Deep Cycle Marine/RV Battery

To increase the battery storage capacity, it may make sense to replace the house batteries with better ones.

In the meantime, since boondocking will be more frequent and for longer durations, knowing the useful capacity of these batteries in understandable and actionable terms becomes even  more important.

One useful measurement is the number of Amp Hours (AH) that the battery holds.  This is the simplest battery measurement that gets at the maxiumum amount of stored electrical energy.  One Amp Hour is one amp flowing through a circuit for one hour.  If you have an 80 Amp Hour battery, then the battery is capable of producing one amp continuously for eighty hours.  In theory.

Interstate doesn’t provide Amp Hour ratings on their batteries.  However, their support page provides a method for estimating Amp Hours.

Interstate Support

To calculate the estimated amp hour rating for your automotive or deep-cycle battery, take the reserve capacity rating and multiply it by 0.6 to get an approximate Ah rating.
The Ah rating describes the ability of the battery to provide power over a 20-hour period. If the rating is 100 Ah at the 20 hour rate, then the battery can supply 5 amps for 20 hours. (5×20=100).

From the specifications tab, the Interstate SRM-24 battery’s “Reserve Capacity (RC) @ 25 hr” is 140.  Applying the above formula  we have 140 times 0.6 or 84 Amp Hours capacity.

The  2018 RV-Dreams Spring Educational Rally  taught attendees about battery life.  Using over 50% of a batteries capacity will shorten the battery’s life.  When estimating the usable sustainable battery capacity, divide the Amp Hour rating by two.

The camper has two 12 Volt batteries.  They are wired in parallel.  Both batteries together provide 12 Volts and the combined usable sustainable battery capacity for both (wired in parallel) is 2 times the capacity for a single battery.

On this truck camper (2017 Lance 850) with the extra battery option, the usable sustainable battery capacity is 84 Amp Hours.  This number, by itself, isn’t terribly useful.

What 84 Amp Hours Can Do…

It depends.

  • Battery charge in Amps as a function of time
  • Battery discharge in Amps as a function of time

For healthy batteries, the usable charge stored in the battery needs to be above zero.  Said another way, deposits have to be greater than withdrawals.

We have no idea how much electricity we use or how much electricity we can collect from either the truck or solar.

Over time, charging needs to be more than discharging with the limitation that the batteries need to stay over 50% charged at all times to maintain battery health.  Charging can come from any of the following

  • truck’s two alternators,
  • shore power,
  • builtin solar panel (90 watts, Lance option) and
  • portable solar panels (160 watts, Zamp).

Only the solar charging systems (builtin and portable) provide the Amps being provided into the system.  However, this information is of little value because what we really need to know is home much how much energy the battery is storing at any given time – that is, Amp Hours.  We need a battery “fuel gauge.”

Discharging can come from standby loads and active loads.  Standby power is the rate at which electricity is used when devices and appliances are inactive.   Common examples of standby loads are

  • Entertainment systems (12 V TV and Radio/DVD Player in standby mode)
  • LED lights on USB and 12 V outlets
  • Refrigerator (standby mode)
  • AC/Heat thermostat

Active power is the rate at which electricity is used when devices or appliances are active.  Active load examples are:

  • Using the entertainment systems (watching TV and listening to the radio)
  • Refrigerator (active mode, that is cooling)
  • Heater running
  • Fans, Lights
  • Charging phones

Battery Meter

Battery “fuel  gauges” are sometimes called battery monitors and sometimes battery meters.  Battery meters – An RV must-have – Now with More Meters!  is a useful source providing descriptions and reviews of various battery meters.  A good quality battery meter provides a good estimate of the amount of energy currently stored in a battery.

For boondockers, the primary advantages of a battery meter are

  • Having a battery fuel gauge – always knowing the State of Charge (SoC) of their batteries
  • Knowing the charge/discharge rate of the batteries – Amp meter showing amps in/out of the batteries

From the above capabilities, boondockers can, over time, learn how behavior affects electrical consumption.  Using this knowledge enables effective battery management and can significantly improve boondocking enjoyment.

Someday our truck camper will be outfitted with a battery meter.  Until then, we won’t know What 84 Amp Hours Can Do.

Replacement Batteries

As a rule of thumb, batteries can be replaced (without rewiring) when the voltage, battery posts and physical battery size stay the same.  A battery’s group size is an industry standard identifier for a specific physical battery size.  The 12 Volt Interstate SRM-24 battery group size is 24DC.    Performing a Google search using terms “deep cycle group 24 battery,” returns a number of pages showing group 24 batteries.  Using the links from the search, a table of compatible batteries can be constructed similar to the one below.

Battery Comparison Chart
Battery Comparison Chart

Using the table as a basis for comparison, a few observations can be made.

  • The dealer provided Interstate batteries are good batteries for both storage capacity and cost.
  • The cost difference between AGM batteries and normal batteries (probably) doesn’t justify the increased cost.

We probably will continue using Interstate Batteries and replace them every two years or whenever we murder the batteries.  Whichever comes first!

Additional Batteries

Many boondockers and full-timers outfit their RVs with additional battery storage.  Increasing available battery capacity generally makes for longer more enjoyable off-grid camping.   The Lance camper already has the maximum factory supported number of batteries installed.  The camper is already short on storage space and claiming an outside accessible storage compartment for extra batteries is not ideal.  Another location for additional batteries needs to be found.

One potential solution is to use external batteries.  TorkLift provides two commercial solutions:  HiddenPower and PowerArmor.   The recommended installation method for HiddenPower wires the batteries into the truck’s electrical system which lacks the ability to use the truck camper’s solar power systems to charge the batteries.  Also, HiddenPower systems are designed to be attached to the truck frame exposing them to greater risk of physical damage from off-road driving, something that many boondockers do to reach remote campsites.  PowerArmor battery boxes are strong and resistant to thieves.   Extremely strong, they make an excellent solution for holding external batteries.  Since they tend to be large, the questions comes down to where to put them.

A better solution might be to attach batteries to the truck bed ahead of the rear wheel wells so that the batteries would be closer to the camper.

Top View: Proposed Battery Box Placement
Top View: Proposed Battery Box Placement

Battery box placement in the above drawing shows the battery boxes tucked into the side of the bed (both sides).  That is, the truck box sides aren’t manufactured straight up and down from top to bottom.  Rather, they are sculpted to create a pocket space below the top rails of the bed.  The intent is to utilize the pocket space so that a wider battery can be fit into the space between the camper and the truck bed.

Side View: Proposed Battery Box Placement
Side View: Proposed Battery Box Placement

Ford has designed the BoxLink cargo management system.  In the drawing above, the “Battery Box Anchor Point” points to a BoxLink attachment site.  These anchor points are strong enough to hold a group 24 battery, especially if the bottom of the battery box rests on the truck bed floor.

In addition to placing battery boxes in the truck bed, the batteries will need to be wired into the camper power system so that the camper can be unloaded from the truck without removing the batteries.  This will require some sort of quick connect wiring system to allow the camper to detach from the external batteries.

2017 Ford Truck Bed Dimensions can be found in their BODY BUILDER LAYOUT BOOK.


The first step toward improving battery capacity is to install a battery meter.  Battery meters help RVers observe and learn about battery consumption in real time.  That knowledge then enables balancing battery conservation against adding additional storage.

If additional battery storage is needed, it will have to be outside of the camper.  The best location appears to be ahead of the wheel wells in the truck bed.

Hope to see you on the road ahead!

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