Question: Here are the batteries and bow thruster on my boat. Recently it seems that when I activate the thruster, the motor is turning slower than it used to.

If you run cables from your charger all the way forward to your thruster batteries, make sure to use a gauge big enough to avoid a voltage drop.

If you run cables from your charger all the way forward to your thruster batteries, make sure to use a gauge big enough to avoid a voltage drop.

The batteries are only four years old and it just seems like they should last longer than four years. I only use the thruster occasionally. What about the charger for these batteries? I can’t find one anywhere close to the batteries and I’m wondering if maybe there is a problem with the battery charger that serves these batteries. Is there an easy way to check this out?

Answer: I’ve seen this matter of re-charging bow thruster batteries handled several ways. One way is to provide a small, relatively inexpensive charger dedicated to these batteries and mounted in the same compartment somewhere close to the batteries. The other less expensive and therefore more common way is to deal with this is to simply run cables from the primary on-board battery charger all the way to the bow and connect them to the thruster batteries. This method can work fine as long as the wiring is sized properly for the extended cable run to ensure that excessive voltage drop does not occur between the charger and the point of connection to your batteries.

The easy way to check this is to perform a two-step voltage test using your multimeter set to the DC volts scale. Here’s how:

  1. Access the thruster batteries, and with everything shut off on your DC electrical panel board, measure the standing voltage in your battery(s). (The red lead from the meter touches the positive terminal on the battery and the black (com) lead from the meter touches the negative terminal.)

  2. In doing the above, you are simply checking what we call the “open-circuit voltage” in the battery in question. There is a correlation between this value and the battery’s state of charge; 12.6 volts and above indicates a full charge. 12.3 volts represents about 75% charged. 12.2 volts equals about a 50% state of charge. So, if the motor is cranking slower than normal, it may be the battery state of charge.

  3. In any event, once you get the static reading, turn on the boat’s battery charger and recheck your readings at the battery terminals. You should see a minimum of a 0.5-volt increase in the voltage reading if the charger is functioning. If there is no change in the reading, there is something wrong with either the battery charger or the cable run between the charger and the battery. Further work will be needed to find the problem.

  4. If the voltage reading with the charger turned on increases over the static reading, then you know the charger is doing something and the problem may in fact be with the batteries themselves. You can try a cranking voltage test.

  5. Have someone activate the bow thruster while you measure the voltage across the terminals. If the voltage drops below about 10.5 volts, the battery may indeed need replacement. To be sure you will need to have a comprehensive test performed on the battery to confirm this.