Thruster battery being charged from home bank…continuously.

[Lake Effect]

I am the new owner of a 2021 RT25LE. I recently experienced a reduction in function of my thruster. The battery tested lower voltage. Upon examination, I observed the thruster battery was not grounded. A check with the factory schematic showed the cable from the thruster battery negative to the engine ground had been removed. I inquired and received verification the thruster battery is required to be grounded with all other batteries. I added the ground cable. The thruster battery was then being charged.
However, here is the problem. The house battery bank began showing a 6A discharge to the thruster battery. At first I thought little of that but became concerned after 12 hours of continuous discharge and the house bank being reduced from 100% to 82% with mostly full sun solar exposure. The thruster battery was showing full charge yet was still being charged from the house bank.
I didn’t feel I could let the house bank completely discharge so unhooked the negative from the shunt. This of course stopped the discharge.
Is the thruster battery even supposed to be charged from the house bank? Do I have an ACR not working properly?
I think this problem is why someone had removed the ground cable from the thruster battery previously. To stop the continuous house bank discharge.

https://cdn.prod.website-files.com/6801675a4c7c632ec66cbd6a/681230f1c417bbb940d5385b_r-25-owners-manual_compressed_100.pdf

 

[Lake Effect]

When I disconnected the thruster battery negative and reattached the house bank ground, systems again worked properly to and from the house bank.

 

[SKing]

Have the thruster battery checked or put it on a trickle charger while it's separated from the house bank to see if it's bad.

 

[Lake Effect]

Thank you for the directions SKing. This morning I checked the voltage of the ungrounded thruster battery and found the state if charge to be 12.6v. After charging the battery with a separate charger (not the boat’s charger) the voltage was 13.46.
Does this indicate a good battery?

 

[SKing]

AGM yes, if it holds your voltage without the charger attached. As far as how the charging system works on the 25 I am not familiar with that. I have a c31. But now that you know you have a fully charged battery what happens if you hook it back up in the system?

 

[William Sanborn]

Just learned a lot about the four Battery wiring on my 2011 RT 27. The Solar has two outputs one for house set at 90% and one for the Starting Battery set at 10%. Works well cause only need Starting to "Start" and Thruster and Starting Batteries are recharge by the engine, and engine is typically running when thruster is used. The A/C Battery charger has Three wire positions all the same; one for 2 House batteries, one for Start battery, and one for Thruster battery. All the Batteries are grounded together, but the positives are seperated to the proper circuits, and from there is where the magic happens and that is beyond my paygrade.

 

[Lake Effect]

I load tested the thruster battery. It tested very good. I then charged the thruster battery with a charger other than the on board charger until FUL. It had a surface charge of 13.4v.
My BMV monitor showed 1.29A 17W on the house bank coming in. I then grounded the thruster battery. The house bank BMV went to -6.7A and -90W after attaching the ground to the thruster battery! I could hear the KISAE Dc to Dc charger fan kick in.
These discharge numbers continued until I once again disconnected the ground to the thruster battery. The numbers continued on the BMV until I momentarily disconnected the house bank then reattached. After that, normal charging through solar showed on the house BMV. 3.4A and 45W running radio and refrigerator. Just like before grounding the thruster battery.
What’s not working?

 

[SKing]

I'm assuming the parallel switch is open and the thruster switch as well?

Without conneting the ground does turning on the parallel switch cause the same discharge?

Does turning on off the thruster switch change the discharge?

If I was there I would be using the electrical prints to isolate different things while watching the discharge.

 

[Lake Effect]

Yes, parallel and thruster switch was off.
I’ll get back with you later with the other testing when I’m again at the boat. Your help is much appreciated!

 

[Lake Effect]

I first reported there was no ground cable on the thruster battery, I was referring to a ground cable connecting all batteries. There is a ground cable going to the thruster motor and probably the windless from the thruster battery.

 

[Levitation]

I first reported there was no ground cable on the thruster battery, I was referring to a ground cable connecting all batteries. There is a ground cable going to the thruster motor and probably the windless from the thruster battery.

So far none of this answers the issue of the excess power drain.
All of the batteries should go to a common ground as should the engine alternator and its starting battery.
Only the positive leads will be segregated into separate banks by the Blue Seas relays.

The engine should charge and run off the starting battery with everything else disconnected.
The Blue Sea relays will then connect the engine to the house bank, et. al. to charge them once the engine is running and the starting battery has recharged to the "set" level.

The thruster battery should (imo) be part of the house set. There are those who disagree, but I want nothing connected to the starting battery that can discharge it leaving me stranded.
There is the argument that with the thruster battery being part of the house set and discharged, then you will not be able to raise the anchor.
My argument is that a running engine will raise the charge on the house+thruster set in less than 20 minutes to allow raising the anchor.

Now to your actual problem.
If the thruster battery takes a charge and tests normally, it should not / will not put a continuous drain on the house set.(or the engine starting battery if connected that way)
I am strongly suspicious that someones fine fingers were into the wiring.
Let me suggest that you remove the thruster battery (ya, ya, ya I know it's a pain - wanna see my battle scars)
Verify that the routing of the positive and negative leads to that battery make sense.
If they make sense put a different battery temporarily in it's place and see if the parasitic drain is still present (I'm suspicious it will be)
1. If the drain is there with the replacement battery, then search and destroy.
2, If the parasitic drain is not there, then you have a very strange battery on your hands..