Solar Voltage Variance

I rencently installed a 80 watt panel and a controller and have become OCD about watching the battery voltage and charge rate. Is it normal for the battery voltage to vary up and down when the camper is not in use and the power is shut off? My dual AGM setup varys from 13.9 to 12.8 when it is just sitting in the diveway.
Thoughts?

Thanks- Gary Mc

I rencently installed a 80 watt panel and a controller and have become OCD about watching the battery voltage and charge rate. Is it normal for the battery voltage to vary up and down when the camper is not in use and the power is shut off? My dual AGM setup varys from 13.9 to 12.8 when it is just sitting in the diveway.
Thoughts?

Thanks- Gary Mc


Depending on your batteries they are likely fully charged in the 12.8-13.1 ball park. However when a battery is fully charged and then held at a higher voltage for a bit it picks up an excess voltage (I forget the technical name for it) which will settle out with time once the charge source is removed. You are likely seeing some of that phenomenon.

Quick 101 on charging with a stage charger: batteries first get a "bulk" of constant current until the battery voltage rises and current input tapers off. Then they are "over charged" in which the voltage is held constant (usually in the mid 14's but varies on the charger profile for the battery type) at a variable current which is whatever the battery will still take in at that voltage. I think most chargers calculate this period based on time and current flow but its limited so the battery charges fully but doesn't begin to boil out. Then the charger drops to a "float" charge which is a maintenance voltage in the mid 13's. At this voltage a battery sits happy and finds its natural fully charge state and doesn't naturally dissipate.

So with all that in mind I'm assuming you just have a basic solar controller that holds your voltage at 14.1V or such? If so there is technically some concern that once your battery is fully charged and as it sits and keeps getting that 14.1V from the solar panel day after day that it may start to boil out. With a flooded cell battery you'd just top off the cells so that isn't a worry but with an AGM you're screwed and battery life is going to be shortened if this happens. Now I'm not saying this is happening but its something than can happen and to be aware of. Might want to chat with a solar shop and the battery manufacturer to make sure you don't prematurely kill your batteries (with aren't cheap usually!).

it is normal to see an intelligent charger vary the voltage.

the cycle usually goes:

the battery voltage is allowed to float, and it begins to drop to the lower threshold of appx 13.2
which when reached will trigger the charger to bring the voltage up to its upper threshold of 14.3
then its allowed to float again.
the cycle is slow and takes a few minutes.

is this what your seeing?

here is a good example of a typical charge controller http://www.solorb.com/elect/solarcirc/scc3/