Ask PYS: Power

John, on Aug 21, 2013 asked:

My sailboat has 2 new 4D batteries, a 70ah starting battery and a Xantrex battery monitor. After I replaced the batteries I reset the Xantrex. Everything is working fine but over the last few months the battery monitor has slowly gone from the normal 100% to 99%. Jeff suggested I check the link shunt for proper installation and it appears to be OK (all connections are downstream of the shunt). Any suggestions as to why the battery monitor is indicating less than fully charged batteries?

The other problem could be the type of cable used to measure the voltage across the shunt. Since the voltage differential is so small, it is important to have a twisted, shielded cable between the battery and the monitor. Make sure none of the twisted, shielded cable is not running along AC lines or large DC lines. You could test this by running a good shielded twisted pair cable temporarily and see if you still have the problem. Otherwise, you might be looking at a bad battery monitor.