Ask PYS: Power

Odd Battery Monitor Behaviour – Brian, on Feb 22, 2015 asked:

A quick regarding my battery monitor showing negative 60+ amps for the start battery, a group 31. I was concerned so put a meter on and it's showing 13.4 Volts which the float voltage. The two things don't seem to make sense? I am away for a month so am not too concerned as I think if I was truly losing amps the voltage would be down. The start battery is a new group 31 which I believe is a 250 amp battery.

You bring an interesting point. Over the course of Fall and now early part of Winter your battery monitor is noticing a small float current going over into the battery to keep at the float voltage. Although the float voltage and current are very small, over time even a small amount of current will add up to a large number. For instance, let's say you have 0.1 amps of current going in your start battery, that would translate to 2.4 Ahrs of capacity a day. Over time, for instance 10 days, that would total 24 Ahrs.

In short, there is nothing wrong with a battery monitor to go in the positive range. As soon as you start depleting the battery, the monitor will reset the capacity to 100% and will start counting the Ahrs either going in or out. Hope this helps explains the behaviour you noticed.