There aren't a lot of things to go wrong with the BMV battery monitor. It consists of three parts, lets look at each in turn:
1. The meter itself.
This is the least likely item to give trouble, it rarely is the culprit. You can inspect the connection for corrosion by unplugging the data cable and examining the socket with a flashlight. Other than that the only way to test it is to connect in into a known working system.
2. The data cable
This can become kinked or damaged during installation but such damage is less likely afterwards. The connections of the plugs can become corroded, especially if there is damp in the area of the shunt. A visual inspection can determine if the ends are OK. Light corrosion can be cleaned off with a gentle application of a wire brush or a dremel tool. Other than corrosion we dont find much problems with the cable.
3. The shunt and its power supply
The most common problem is that during installation the power wire is connected to the battery before it is connected to the shunt. Then before the wire is connected to the circuit board on the shunt it accidentally touches the shunt itself. That is a direct short circuit and immediately blows the fuse. These fuses are hard to troubleshoot because they are so small and hard to see. Sometimes a fuse will test OK in a continuity test and not carry current. The best way to test if the fuse is the problem is to temporarily replace it with a screw or paper clip. I dont mean that you do this on a permanent basis but just as a test. We keep spare fuses here at PKYS. Note that you cant pull the fuse holder apart, you have to unscrew it.
The other typical problem is physical damage to the circuit board on the shunt or corrosion of its terminals, again a physical inspection with a flashlight will help to see if there is something obvious. The only other test it to hook it up into a working system.
4. Metering errors
Metering errors are almost always a result of user error on installation. The shunt only measures what goes through it, if there is an alternate path for current and only part of the current goes through the shunt then all the readings it generates will be completely meaningless.
5. Start of season errors
If your battery system has been laid up for the winter it will have self discharged to some extent. Nothing will have gone through the shunt so if the battery was full at the start of the winter the meter will still think it is full even though the battery went down internally. This situation will self correct once the system is charged up. GIve it a couple of cycles to get back on target.
6. Battery Failure
As batteries fail their capacity greatly deteriorates. After a certain point the performance becomes so poor the meter will not be able to keep track of status adequately. Its time to replace the batteries when this happens.