Join Date: May 2005
Location: Mayenne France
My Motorhome: Frankia T640 FF2
Thanked 228 Times in 205 Posts
15.6 volts if accurate is high for a flooded cell battery, I'd check with another meter and if the figure is correct I'd be looking to find out why so high.
You need to let it rest and check the voltage of the battery with it disconnected from the vehicle, if the result of that check is a figure above 12.6 volts, reconnect the battery and carry out the test again using same procedure as you show in your post.
It is not uncommon for a system to show above 14 volts if the battery is in a very low state of charge.
Long term 15.6 volts that will damage the battery.