The function of this device is to display the vehicle's energy reserves. Unfortunately this is not quite as simple as with the auto, where one only has to measure the level of gasoline in the tank. For lead-acid batteries, the safest method would be to be measure the acid specific gravity. In practice this is too complex, therefore the capacity meter keeps track of the current input and output. The also monitors temperature and battery voltage. Unfortunately there have been some failures, fixed by exchange of the EPROM (a programmable circuit). The capacity meter gets confused if you charge while driving, e.g. by solar panels connected over the load shunt. If a solar module is attached, then this should not be led across the load shunt, but directly to the batteries, with it's own protection and discharge protection diodes, and automatic controller.
The capacity meter is sometimes a pain in the ass. It mostly switches off if one is in the middle of a bridge. Here the trick for defeating the meter: Connect Pin 13 (orange) and pin 14 (yellow-red) with a switch, when it's closed, the capacity meter is bypassed. In the above picture the lines are already tapped (the blue lines) and led to a switch. By the evasion of this protection organ however, you put yourself in the position to discharge the batteries deeply, which for a lead battery can be deadly.
Caution - with this mod, the way is smoothed for battery death, flip the switch only in emergencies - assume that the warranty for the batteries just expired. I absolutely recommend a voltmeter in this connection. The block voltage should never, never fall under 10,5V or the driving voltage under 31 V.
After the capacity meter is disconnected from the current supply (batteries removed or the like), it will show no points " 0 points ". Attach fully charged batteries again, and the battery charger, and the gauge will very rapidly reach "10 points" (82%).