Odd voltage reading from three way voltage meter

[billrh42]

This weekend we prepped the boat for our first trip of the season. Our 2010 R29 was connected to shore power and we were getting the boat ready for a short ride. I went to the power panel to check the battery voltages, Bank 1 (house) showed 9v and the other two banks were around 13v. I measured the 4 bank batteries with a volt meter and saw 12.8v. I measured the voltage behind the helm station and it was 12.5v or so. All the house electronics worked fine, the engine started right up and the thrusters started right up fine. We went ahead and disconnected the shore power and took the boat for a short 1 hr cruise. We returned to the dock, I checked the voltage on the three way meter and bank 1 was now 11v. I connected the shore power and it jumped up to 14v. We stayed on the boat overnight and took it back out the next day and all was well with the voltages.

At this point I'm not sure what was going on but perhaps the bank 1 battery meter is not directly tied to the batteries but whatever it is tied to had an issue. What should I check if this issue pops up again in the future?

 

[NorthernFocus]

What you saw was likely in fact a dead battery. The ACR(automatic charging relay) will not connect the batteries if one of them falls below the threshold voltage(don't recall what that is. 11V?). Therefore it won't automatically charge with the other batteries. Somewhere along the way you must have closed the (manual) common switch which allowed the battery to take on a partial charge. When you reconnected the shore power the 11V was enough to allow the ACR to close. That's why you then saw charging voltage when you went back on shore power. Unfortunately that battery will have been damaged falling that low(9V).

 

[billrh42]

Solved:

I went back to the boat today and I was going to disconnect each of the house batteries and check each voltage until I found the one that was bad. Before I did that I spent some time measuring and checking out the three way voltage meter again. While looking at the meter the voltage would bounce between 9v to 12.5v, but my volt meter that was connected directly to one of the house batteries was a solid 13.4. So I started looking at the sense wire at the battery and it turned out that the sense wire had corroded down to just a few strands going to the battery connector. The resistance between the fuse holder and the connector about 5 inches away varied up to several 100 ohms instead of zero. This sure was a strange one but at least an easy and cheap fix.

 

[NorthernFocus]

...While looking at the meter the voltage would bounce between 9v to 12.5v, but my volt meter that was connected directly to one of the house batteries was a solid 13.4...

Yeah voltage bouncing around like that is a pretty could indicator of a bad connection/wire. Glad it turned out to be a simple and inexpensive fix for you. If you haven't already done so you might want to look up the specs on the ACRs to understand how they function. What you originally described sounds just like how a bad battery will behave. Though you did say that you had checked voltage directly at the batteries and it looked OK. But in forums like this it's difficult to interpret what's being said, skill set of those posting, etc. Doesn't lend itself to effective troubleshooting :?

When we bought our boat used the first thing I did was to go through all of the wiring in the battery compartment. On the older boats the lazarettes and generator compartment are notoriously leaky. Not a great area for things being doused periodically with salt water and frequently with rain water. I installed a small fuse box in a protected spot and got rid of all of the in-line fuses. That has worked well going on ten years. Also makes it easier to check fuses.

 

[billrh42]

Thanks for the feedback Dan. The ACRs are new to me and after I read your first message I dived into their specs. I have a better understanding of how they work and what to look for 'next time'. Good advice about the fuse block too. I'll be going through the rest of the wires on my next trip to the boat.

Thanks again.