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Back to Basics: How To Stretch Battery Power

Originally Published in MotorHome Magazine

Many motorhome owners occasionally want to boondock, or live “off the grid,” but may be apprehensive about running out of 12-volt DC power. One of the more common problems is having the auxiliary batteries (also known as “house” batteries) depleted soon after running the AC generator for several hours. This is usually a result of inadequate recharge capability – often combined with worn-out or damaged batteries. Here’s how you can make life without hookups easier.


3 Essentials for living off the grid:

  • House batteries of adequate size, in good condition.
  • Good recharge methods and adequate wire size.
  • Clean wiring connections with no corrosion.

  Types of Batteries

House batteries are known as “deep-cycle,” which means they are designed to withstand the rigors of repeated discharging and recharging. There are two primary types: conventional (flooded) and valve-regulated (gel and absorbed glass mat or AGM).

Checking Voltage

Check the voltage of the house batteries after re­charge has occurred – after driving for at least a couple of hours, or after running the AC generator for the same amount of time – or after solar panels have been in full sun for several hours. Voltage should be around 14.2, which is a generally effective recharge level for most deep-cycle batteries whether they are 12-volt or a set of 6-volt batteries wired in series for 12-volt output. (The variations: Flooded batteries are capable of handling higher voltage levels, while valve-regulated types require slightly lower levels.)

With the engine running, your voltage check may range from 13.8 to 14.5 because alternator output varies with ambient temperature. Despite the variations, alternators usually are effective battery chargers – assuming wiring is adequate.

If the voltage reading is around 14.2 at the batteries, the alternator is doing its job.

Charging Batteries

Check voltage with the motorhome connected to an AC power source (AC generator or shorepower). Again, to be effective, voltage should be about 14.2 unless the RV is equipped with a multistage inverter/converter unit (check your owner’s manual). This sophisticated unit will automatically set voltage to different levels depending on the situation. With conventional converters, output may be 13.8 or less, resulting in endless hours of AC generator operation that will not adequately recharge batteries.

If voltage output is about 14.2 and your batteries still tend to sag, the batteries probably are at fault. They might be sulfated (unable to store power) and should be replaced. Batteries that are old and/or are allowed to sit in a discharged state for more than a couple of weeks tend to become sulfated.

Using Solar Panels

After solar panels have been in full sun for several hours, the voltage should be around 14.2 unless the charge cycle has been completed and the solar panel voltage regulator has reduced voltage to a maintenance stage.

If all charging results are good and your batteries still seem to fade prematurely, have them load tested. A single Group 27 12-volt battery should be able to sustain a 5-amp load (three 12-volt bulbs) for about 15 hours before dropping to 10.5 volts. If the battery sags in 10 hours or less, it should be replaced.

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