Battery Meter

The information provided is for a 2002 Duffy 18' Classic. Many systems and components are common across models and years.
It is up to you to determine if this information applies to your boat or not.

There is an "Energy Gauge" on the instrument panel that looks like a fuel gauge, with Empty and Full markings at each end. It operates when boat power is turned on by the key.

The gauge is misleading because in order to show the amount of available power (empty/full), the boat would need a monitoring system that would
1) know the amp hour capacity of your battery bank and
2) be able to constantly monitor power consumption and remaining power.

This is not what the Duffy gauge is indicating. It is actually a volt meter, showing the voltage of your battery bank in its current state when the key switch is on.

Here's what you can determine from your Duffy volt meter:

State of charge
When disconnected from shore power and all systems are off, the meter will show available battery bank voltage when not under load. Since there are no voltage markings on the gauge, you will need a reference point on the meter to determine if your batteries are at a normal state of charge.

Voltage while under load and a rough idea of available battery power
For a given load (e.g. full speed throttle), the voltage will be lower on a battery bank that has less available power. With a new set of batteries, you will see the voltage drop when at full throttle and then slowly decline as you continue motoring. With batteries that are near end-of-life, you will see a greater drop in voltage initially and a more rapid voltage decline as you continue motoring. This will correspond with reduced speed and range as your batteries are unable to provide the required power.

Whether your battery charger is working or not
Charging the batteries requires the battery charger to produce a slightly higher voltage than the maximum combined voltage of your battery bank (e.g. 38V for a 36V system). This is why Duffy advises to remove shore power before turning on boat power with the key.
To be safe, turn off all lights and other systems. When you turn on the key, the volt meter will show a higher voltage while charging than when not.

Note that when motoring at less than full throttle, the motor controller may temporarily compensate for low battery power. This would create a situation where you are powering at a constant speed, but the battery bank is degrading rapidly. Your speed would then drop-off rapidly once the motor controller can no longer compensate.

Third party marine battery monitors are available that will show battery capacity and remaining power, but they are not inexpensive and are unnecessary for our short-range Duffys.

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