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Why are my batteries’ amp hours down?

2010 February 1
Dear Boat Doctor,
My battery monitor reports a loss of about 40 amp hours per month while my boat is on the hard. Even so, the monitor reports the voltage of the house battery bank at 12.87 volts. The main breakers for the house and starting batteries are set to OFF. The charger will not provide any more than a float charge when it can be plugged in. The batteries are apparently fully charged, so why the drop in amp hours??

Wayne Wilson
Oak Hill, Virginia

Dear Wayne,
Without knowing a lot more about your boat, I can't offer exacting answers, but I think I can help. First off, I don't think you have a problem, just very accurate measuring equipment.

I think your battery monitor is consuming a good part of the 40 amp hours per month. The monitor itself takes power to run, some for the measuring logic and more if there is a display with a backlight. The monitor measures the current flowing from the batteries via a shunt. This shunt is between the batteries and main switches, but it is very likely that the battery monitor is powered before the main switches and after the shunt, meaning that the power to run the monitor would be measured by the monitor.

There are roughly 720 hours in a month, so at 40 amp hours you are drawing an effective continuous 55 milliamps, and this is not an unreasonable amount of power. It is significant when the boat is in storage however. I don't know how large your battery bank is, but if a typical single battery can supply about 100 amp hours, you would draw this battery down in just over two months of storage. It is handy to monitor batteries with your system, but you may want to shut down the monitor when the boat is on the hard.

Your battery reading of 12.87 volts is essentially an open-circuit measurement, likely the only load on the batteries is the monitor and this is a pretty small load. After you have drawn 40 amp hours, I think the battery voltage would drop dramatically if connected to a more significant load such as a cabin light.

The fact that your charger is only providing a float is also not surprising with such a high battery voltage. However, you could have a problem where the battery charger is hooked up-the start bank vs. the house bank-or requires that the switchgear be in a BOTH position.

As I said, I don't think you have a problem, but I would allow the charger to top off your battery banks and then power down the monitor until spring.