Why is my Amp Hour Meter giving strange readings after its winter break?

Amp hour meters do a great job at assisting the boat owner with battery management.  They provide an accurate reading of voltage and of amps going in and out of the batteries and by keeping track of these readings over time they can provide a reading of "amp hours" or the state of charge of the battery as a percentage.  They also often maintain a record of historical data and can use that to figure out battery efficiency.

Amp hour meters do however have their limitations.  The amount of power that can be drawn from a battery varies according to the circumstances and is particularly affected by temperature and by the rate at which power is taken from the battery.  In addition it is not always clear when a battery is full because it is always possible to put more charge in, but that charge may not always be able to come back out again.  Batteries may also self discharge over time meaning that they become empty without anything leaving the battery.  The meter thinks the battery is full when it is in fact empty. Discrepancies between the amount going in and the amount coming out are because some of the energy turns to heat.

So how do Amp Hour meters manage to keep an accurate reading of all this vague and moveable data?  The answer is they don't, they cheat!  They rely on getting a fairly frequent reality check.  This happens every time the battery is discharged at least 20% and then charged back to full.  When this happens the meters reset themselves to say they are full and then the meter can resume its measurements with some semblance of accuracy.  What happens when there isn't a periodic reality check?  The readings become more and more inaccurate over time, the longer the time the more the inaccuracy.  Readings after a winter break are often totally inaccurate.  The readings for start batteries which almost never cycle tend to be permanently inaccurate which is why I never recommend using an amp hour meter for a start battery. 

So how do we correct the problem of an inaccurate reading after a winter lay - up?  Normally you don't have to do anything because when the battery gets fully charged and then starts discharging the meter will figure it out for itself.  Sometimes it may take an entire cycle to do this.  If you are in a hurry you can charge the battery up to where you think it is full and then reset the amp hours on the meter.


This article was originally written a long time ago and since then a new development has come along that improves the performance of some meters in these circumstances.  It started with the Balmar Smartgauge which doesn't count amp hours at all but comes up with a measure of state of charge by measuring the impedance of the battery and relating it to a table based on battery type and battery history.  This kind of meter isn't fooled after a winter layup.  It does however have the disadvantage of not measuring current.  In the second version of this idea Balmar came up with the SG200 Meter which both counts amp hours and measures impedance.  It too wont be fooled by a winter layup but will still give full reporting of amps going in our out of the battery, amps consumed, and state of charge.

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