Low house battery voltage, intermittantly

[Mischief Managed]

Recently returned from a cruise where we went 17 days straight without a shore power connection. Three times during the 17 days, the house battery voltage would drop to 11.8V and stay there as the SOC meter dropped to 60% to 65% while the genset was not running. This issue would only resolve itself when we moved the boat from port to port on our cruise.



The steady current draw was relatively stable at 14 amps from the 615AH 12V FLA golf cart battery bank and the occasional big loads (180+ amps), like the microwave or coffee maker would draw, were easily delivered without excessive voltage drop regardless of the low voltage condition or not. In other words, if the voltage was at 11.8 with 14 amps and I ran the microwave which added 166 amps to the load, the voltage would drop to 11.5ish volts, which is the same as it would drop to if the starting voltage was 12.5 volts and the extra 166 amp load was applied.



I suspect I have one intermittently bad cell in one of my 6 GC batteries. They are 4 years old and have been used off shore power quite a bit. I ran a couple of equalization cycles on them, but it did not seem to matter.



Any thoughts would be appreciated. If I replace the batteries, it will be at the start of next season.

 

[twistedtree]

Tough to say, but I can suggest a few things to check when/if it happens again.

One possibility is that it’s just an issue with your meter. Try measuring battery bank voltage directly at the bank with a good hand meter to see if the bank voltage really is low.

If the bank voltage is really low, then you will need to measure individual battery voltage with at least one of the terminals disconnected. The open circuit voltage for all the batteries should be very close, say within .1v. If you find one that’s low, let’s look at how much low and go from there.

Other possibilities are bad connections. They can look good, but be bad. Really the best thing to do is to remove them, clean, and reconnect and tighten.

 

[Comodave]

Yup, easiest thing to do is check all connections. Clean then up and tighten then nice and snug. Then spray Boeshield on them to keep them clean. Then see what happens. At least then you will be able to eliminate dirty and loose connections.

 

[Larry M]

I remember when we were chasing a low battery voltage issue. I used an IR thermometer and found one battery was hot when charging. It turned out it was a shorted cell. I took that battery out of the bank and the other batteries came up as they should.

 

[twistedtree]

I remember when we were chasing a low battery voltage issue. I used an IR thermometer and found one battery was hot when charging. It turned out it was a shorted cell. I took that battery out of the bank and the other batteries came up as they should.

Another good diagnostic, and doesn't require disconnecting any batteries!

 

[Mischief Managed]

Took awhile, but I finally caught it in the act and figured it out. Before I explain the cause, I want to add a few more symptoms:


The DC breaker panel analog voltmeter agreed with the battery SOC meter voltage display


The batteries all measured normally and there was no hot battery when charging.


All wire connections are tight and clean


The DC voltage displayed on my inverter control looked normal and did not agree with the lower voltage intermittently displayed on the breaker panel voltmeter or the SOC meter. THAT was the key that got me looking for something else...



My boat has a Guest 230 AMP Rotary battery disconnect switch in the engine room for all the house battery loads, except for the inverter. My inverter has it's own Rotary battery disconnect switch.



Turns out the excess voltage loss was in the battery disconnect switch for the house loads (not used by the inverter) which is why the panel and the SOC meter agreed and the inverter did not. Exercising the switch improved the voltage, but it eventually dropped again. I ordered a new switch today. It's a pain to replace. I'll look like I fought off a mountain lion when I'm done.


Thanks for the replies.