Intermittent Bow Thruster Failures on 2019 R-31

[STS]

As a new boater, I rely heavily on my thrusters when docking and undocking. The stern thruster has been working well so far, but the bow thruster has stopped responding twice before I was able to return to my slip.
The second time this happened, the thruster started working again after a few minutes but stopped responding after just a few bursts. When I checked it 15–20 minutes later, it was functioning again.
My best guess is a faulty thruster battery, but this doesn’t explain why the stern thruster still works. According to the manual, both thrusters are wired to the same battery and protected by a 250A fuse. I’ve noticed that the stern thruster is wired with 1/0 gauge wire, while the bow thruster uses 3/0 gauge wire. Perhaps the stern thruster requires less amperage, allowing it to function even when the battery is too discharged for the bow thruster to spin? Any other possible explanations?
The previous owner upgraded the engine battery to 103AH/1150 CCA in 2021, but I’m not sure what type of thruster battery is installed—likely the factory original. I’ve ordered a MOTOPOWER MP0515A 12V Car Battery Tester and will be inspecting the battery visually and with the tester tomorrow. In the meantime, any recommendations for a new AGM battery brand and capacity would be greatly appreciated.

 

[watson1987]

The run to the bow is longer, so requires larger gauge to prevent voltage drop.

Beyond a bad battery, the thrusters also have a thermal cutoff, which might explain why they start to work after a few minutes.

 

[Grey25]

As a new boater, I rely heavily on my thrusters when docking and undocking. The stern thruster has been working well so far, but the bow thruster has stopped responding twice before I was able to return to my slip.
The second time this happened, the thruster started working again after a few minutes but stopped responding after just a few bursts. When I checked it 15–20 minutes later, it was functioning again.
My best guess is a faulty thruster battery, but this doesn’t explain why the stern thruster still works. According to the manual, both thrusters are wired to the same battery and protected by a 250A fuse. I’ve noticed that the stern thruster is wired with 1/0 gauge wire, while the bow thruster uses 3/0 gauge wire. Perhaps the stern thruster requires less amperage, allowing it to function even when the battery is too discharged for the bow thruster to spin? Any other possible explanations?
The previous owner upgraded the engine battery to 103AH/1150 CCA in 2021, but I’m not sure what type of thruster battery is installed—likely the factory original. I’ve ordered a MOTOPOWER MP0515A 12V Car Battery Tester and will be inspecting the battery visually and with the tester tomorrow. In the meantime, any recommendations for a new AGM battery brand and capacity would be greatly appreciated.

I had a similar problem with intermittent Bow thruster cleaned connections at thruster motor and have had no problems since

 

[Brian John]

Have you checked the connections at the battery and thruster? A simple and effective way to test your battery is to place your digital multimeter (DMM) test leads on the battery and have someone operate the thrusters while you watch the battery voltage. Another option is if your DMM has a min/max capture setting.

 

[Mastercraft]

I just had my stern thruster fail intermittently in one direction only. It was corrosion on the 4-pin connector. The remote for thruster helped diagnose the problem. Joystick and remote were both intermittent.

 

[Jcat2010]

Check the joystick connections also, but first the battery. Check the battery is fully charged, then check under load. If it pulls down below 10v, that is a sign it's being stressed.

 

[Brian John]

Have you checked the connections at the battery and thruster? A simple and effective way to test your battery is to place your digital multimeter (DMM) test leads on the battery and have someone operate the thrusters while you watch the battery voltage. Another option is if your DMM has a min/max capture setting.

 

[STS]

Thank you for all the suggestions! I checked the battery today, and the test tool showed "Great" for both the engine and thruster batteries. However, the thruster battery has a lower CCA and a smaller State of Health (SOH) value compared to the engine battery. Pictures attached. Should I replace the thruster battery?
Both batteries are NorthStar Ultra High Power, with the following specifications:

• Marine Cranking Hours: 1,370
• Pulse Cranking Hours: 2,150
• Cold Cranking Amps (CCA): 1,150
• Reserve Capacity: 220

They were installed at the end of May 2021, so they’re about four years old now.
Next, I plan to check the thruster connections and clean them, as Grey25 suggested. Then, I’ll inspect the joystick wiring.
The thruster is an SE40/125 S model—should I proactively replace the carbon brushes and/or the thermal switch?
On a side note, battery access (house, engine, and thruster) is awful! The batteries are strapped into plastic bins, which are secured to a wooden platform with 6–8 screws. It looks like the platform itself is permanently glued to the floor. I’ve seen another boat where the platform has two screws at the front, allowing the entire set to slide out—but not on mine.

 

[dexmot74]

Thermal switch is known to fail. Mine did. Wasn't intermittent though.