Is My Battery Charged?

“What is wrong? My battery was low, so I turned on my generator, and 20 minutes later the battery was full, so I turned off the generator. But 20 minutes later my battery is low again!”  We have seen questions like this for years from new RV owners. Most likely there is nothing wrong with with their battery, their generator, or their charging system. They simply did not understand what was happening, and didn't understand the information their monitor panel provided. We will try to shed  light on the mysteries of RV battery systems.

This apples mostly to those with older Roadtrek models.

Our Roadtrek has a KIB monitor panel – which has been used by many RV manufacturers for years. Besides the battery information, there is also water tank level info, sometimes propane tank level and often other switches. Our water pump switch is on this panel, and some Roadtreks have the battery cut-off switch on the panel as well. There is a momentary test switch that will light up LEDs to display tank and battery levels.

Newer Roadtrek models with the Voltstart engine generator and Ecotrek lithium power modules have different monitor panels and operate differently. This applies mostly to those with AGM and lead acid house batteries and the Onan generator found on older models.

The reason these LEDs are often referred to as “idiot lights” is that the information they provide is limited and in the case of the holding tank sensors, often wrong. For the battery information there are 4 LED lights. Each light comes on when the voltage reaches a certain level. And it will stay on until the voltage drops below that level. The problem is that voltage is actually a very poor measure of state of charge of a battery. To be accurate the battery must be disconnected from all charging sources AND all loads for many hours before a voltage reading will be reasonably accurate. That is difficult or impossible to do if you are using your RV.  Any load on your battery will produce some voltage drop – the larger the load, the greater the voltage drop.

There are four panel lights, C, G, F, L on the panel.  These mean Charging, Good, Fair, and Low.  The voltage point these LEDs turn on and off are:
C – CHARGING   12.7 volts
G – GOOD    11.9 volts
F – FAIR   11.2 volts
L – LOW   6.0 volts

The typical voltage chart for a resting lead acid battery (AGM or wet cell) looks like the chart to the right.  When your compare the two, it indicates that when your “Good” LED goes out, you are down to 11.9 volts, which is about 40%.  However, that assumes no load and a resting battery, so the actually state of charge could be better than 40%.  Either way, if you want your battery to last, you need to charge it.  If you reach the point the “Fair” LED goes out, your battery is nearly dead.

So what about the person who ran their generator for 20 minutes and their battery was full?  Actually it was not anywhere close to full.  A nearly dead battery will take hours and hours on the very best of chargers to recover.  The generator via the charger was putting out a charging voltage of over 13 volts, so all the lights on the monitor will come on.  And when the generator is turned off, there is something called “surface charge” that takes awhile to dissipate.  So the “Charging” LED stayed on for awhile (how long depends on the load on the battery).  The chargers in many RVs are not much better than trickle chargers (RV manufacturers are more concerned about overcharging than undercharging), so that generator might require many days to charge that battery full.  And as batteries get full, a higher voltage is required at lower currently to reach capacity – running a generator to put 2 amps into your battery for hours, is a waste of generator capacity.  Solar is perfect for finishing a charge.  Driving is almost always the fastest way to charge your battery.  The average RV owner has the alternator charging the battery while driving, then arrives at a campground and plugs into shore power.  Now the converter/charger or inverter/charging is charging the battery (that was probably already full from the drive).  No wonder RV manufacturers were more worried about overcharging than undercharging!  Neither over- or under-charging is good for your battery and both will shorten its life.  Few RV owners get 8 to 10 years from their batteries like a well-designed off grid solar system might get.

Original Magnetek Converter

Often the converter/charger doesn't put out a high enough voltage to completely charge a battery.  Trojan says 14.8 volts (temperature controlled), and Interstate says 15 volts.  Our original Magnetek converter puts out 13.8 to 14.2 volts max.  Our PD4600 3-stage charger upgrade is still only puts out 14.4 volts for bulk charge.  Neither are temperature controlled.   Result: Our battery only gets full from driving (higher voltage, but no smarts to stop charging when full) or from our solar panels (smarts vary with your solar controller).

So how can you really know where your battery stands?  Do you really need to know?  If you drive daily or plug into shore power when camping, you probably do not need to know.  Sure, maybe your batteries only last 3 or 4 years, but that that might be affordable if you don't have expensive batteries.

If you boondock a lot without daily driving, or you have significant important loads (e.g. CPAP machine), a shunt-based battery monitor will keep track of the level of charge of your battery.  Such battery monitors do assume your battery is in good condition and it will track current into and out of your battery (or battery bank).  You will also be able to tell how much power your electrical appliances are using.  The best brand is the Trimetric battery monitor from Bogart Engineering.  An article is here on how to install one.

But without a sophisticated battery monitor, make a practice of taking steps to charge your house battery when you see that “G” LED go out.  If you are running a heavy load you can turn it off to see if the “G” light comes back on, if not you should turn on your generator or go for a drive or plug into shore power to charge the battery back up.