Voltage The good, The Bad, and The Ugly

By Brad Sears
This column is in reply to a question about the charging voltage being delivered to batteries while driving. The question from Bill is “I would like to know what the maximum battery charging voltage should be when the engine is running. My rig runs around 14.4 volts at anything over 1000 RPM. Is this voltage going to shorten the life of the battery? We usually drive 5 hours or so between camping stops. The coach battery is a group 24 deep cycle flooded cell type.”

Well here’s the deal as I see it. The term 12 volt battery is a bit of a misnomer. We know that a battery is fully charged when the at rest voltage of the battery is 12.65 to12.7 volts depending on who is telling you what it should be. That means that when a battery has an at rest voltage of 12 volts that it is damned near dead.

The definition of at rest is after the battery has been charged it is discharged a bit to remove what is called surface charge, and then allowed to rest, called recovery, a bit before the voltage is tested. Taking the voltage reading of the battery immediately after charging will give a false reading. In the charging process the battery will accumulate a surface charge that does not have deep reserve. The surface charge can be removed from a normal auto battery like a group 24 by turning the headlights on for three minutes. Then turn the lights off and let the battery recover for several minutes and read the voltage. That is when 12.65 to 12.7 volts indicates a full charge.

A note here; a group 24 battery usually has an amp hour rating of about 85 AH. If you have a larger battery pack then the time the headlights need to stay on or the load needs to be increased. For instance if you have 170AH then you will need 6 minutes of discharge to remove the surface charge and so on.
The charging voltage of a battery then determines the amount of amps that the battery will accept. Keep in mind that voltage is electrical pressure and amps is the amount or volume. In order to make amps flow the voltage of the source, the alternator, must be higher than the battery. Then the amps will flow into the battery. When an electrical load is turned on then the voltage in the battery being higher will push the amps to the unit asking for power.

The amp flow to the battery will be controlled by the difference in the internal battery voltage and the regulated output of the power source, the alternator in this case. The more the difference the more amp flow and the closer they are together as the battery charges the lower the flow of amps.
Now to the numbers. Most in the auto industry, as that is where most of the lead acid and the newer designs are used, has adopted a 13.6 volt to 14.4 volt charging range as normal. Several auto makers such as some years Ford set the top limit at 15.3 volts regulated. There is no indication that this higher voltage had any adverse effect on battery life. The move to settle the top voltage in the 14.4 range was more dictated by the computers and electronics used in the newer cars keeping the voltage in a closer range.

So with all this said it would indicate that 14.4 volts is an acceptable voltage for your battery.
Now, one more word on surface charge. For years we have ignored that the surface charge had any useful purpose. However I have a different take. A year ago we invested in solar for our old Foretravel. In doing the installation we installed a big bank of batteries giving over 1250 amp hours of stored electricity. We then took our “Grand Adventure” around the country doing at least 70 days of boondocking. Normally before solar and the big battery pack we would have used our generator 4 to 5 hours a day boondocking, but after the conversion we used the generator 10.7 hours in the 70 days.
During our summer home getting ready for this years “Grand Adventure” I have been replacing interior lighting with lower current draw LEDs. In doing some tests I have found that I can run the interior lights (LEDs) all night and not draw off the surface charge. In other words the batteries are still fully charged in the morning.

I have not plugged the coach in since arriving home in April and the solar has kept the rig at full charge.
If you have a question that you would like answered about the mechanics of the machine please ask, I like answering questions rather than making up a story.
Brad