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Battery Monitoring

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.

Duffy Energy Gauge

There is an "Energy Gauge" on the instrument panel that looks like a fuel gauge, with Empty and Full markings at each end.

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. A volt meter, especially one that does not show voltage markings, is not useful for determining remaining power. Here are some things it can provide an indication of:

• State of charge
  When disconnected from shore power, the key switch is on, 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 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 higher voltage than the maximum combined voltage of your battery bank (e.g. 40V 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.

Battery Chemistry and Motor Controller Effects on Monitoring

Lead acid batteries exhibit a significant voltage drop under load, which is exaggerated in older batteries with less remaining power. The motor controller will do its best to provide an optimal power profile to the motor regardless of battery state. As a result, everything will seem fine when motoring until the motor controller can no longer compensate. You are unlikely to detect this from the Duffy energy gauge.

Proper Battery Monitoring

Third party marine battery monitors are available that will show battery capacity and remaining power (search for "marine battery monitor with shunt" or "Coulometer". These require some rewiring and periodic calibration to reflect the remaining maximum capacity of your batteries.

If you switch to a LiFePO4 battery, it will have a battery management system that will be able to provide an accurate remaining power measurement

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