my battery failure diagnosis correct?

[Cutwater28GG]

took the boat to dinner. left house battery switch on to keep fridge running. sun set so no amps from solar panel and no shore power.
no other electronics on apart from accidentally leaving underwater LED lights on.

came back to boat after 3 hours, start engine fine, alarms from autopilot, sonar low voltage. check voltage on house - 8 volts - crikey.
engine and thruster battery both show close to 13 volts like normal.
engine charging showing 14.1 volts on volvo control panel. cannot run house electronics for garmin as low voltage. garmin turns off initially after engine starts.

batteries were all fully charged before we left, shore power and engine charging.

I'm pretty sure the house batteries are toast. end of life and they just couldnt handle 3 hours of the fridge. i replaced the engine battery last year as it was EOL. not sure on age, but I bet its a number of years. maybe even 5 and they are the original ones in the boat.
even if they werent toast and it was some weird power draw, I bet they are toast now!!

if I had a weird power draw on the battery whilst I was away, my gut tells me there would be some signs like a blown fuse as to draw down that many amps so quickly would have been a large load.

I'm not going mad right and I shouldn't be checking something else causing the power draw?

 

[Levitation]

Well, for starters how many amps do the underwater lights draw?
Have you put a clamp-on meter on the house set to see how much draw there is?
Are there loads you don't suspect (as you asked) I call them vampire loads because they suck your blood (electrons) in the dark.
Tough to make recommendations without some basic numbers - are they watered / how many times have they been discharged below 50% / how much current is being drawn / etc.

Now, it is likely the house set has gotten tired but we don't know that. Top off the water. Give them a good charge and do a load test. From there you can decide of they need replacing or if they are being loaded beyond the rating.

 

[knotflying]

Denny's recommendation is spot on. And for future reference, if any of the batteries go below 9.5 volts the relays will not allow a charge to go to the "bad battery". As you said, with such low voltage the Garmin equipment would not work. However, if once you start your engine you could have put your parallel switch to the on position and this would have bypassed the relays and allowed power to go to the house batteries and your equipment would have worked.

 

[McThomas]

Knotflying, is that correct about the parallel switch? I always thought it was "one way", in that it could back up the engine battery with the house batteries if necessary but not the other way round. If it can back up the house with the engine that is a great tip and good to know!

 

[NorthernFocus]

Your logic seems solid aside from not knowing how many amps the LED lights draw or if there were any additional "hidden" load at the time. But in any case it would take one heck of a load to pull healthy batteries down to 8V in three hours. I'd do a simple, practical test before getting too carried away. I'd put a good charge on them and put a moderate, (relatively")known load on them with equipment on the boat. Then check the voltage(at the battery terminals) every couple of hours or so. If the voltage does a similar nose dive then the batteries are functionally useless to you. Or you've got some other load on them that you're unaware of.

Knotflying, is that correct about the parallel switch? I always thought it was "one way"...

No it's just a "dumb" switch that connects the positive side of the batteries. Current can flow either way. Which, by the way, also means that if you close the switch with low voltage on the house bank and without the engine running you'll pull the engine battery down pretty quickly.

 

[Cutwater28GG]

I can't believe the underwater led lights draw that much power. They are led after all. I have left the boat like this a number of times in the past with no problem. . These batteries are an unknown since it was a used boat and is now 5 years old. I have decided to replace them.

1. Should I replace thruster batt at same time? It Seems prudent.
2. Anything I need to do when disconnecting the batteries to replace them? Other breakers I should turn off? Order of disconnection other than ground first?
I was thinking shore power disconnected, main breakers off. But didn't know if there was anything on the batt charger or acr's I need to fiddle with. (I didn't when I did engine battery replacement. )

 

[NorthernFocus]

If you think the batteries are all five years old it would be prudent to also replace the thruster batt. Nothing special required other than removing the load prior to disconnecting them. I just shut off all of the main switches and make sure to turn off any equipment that is connected directly to the buss(several items on my boat including the heater). Do yourself a favor and apply "tune up" grease to the leads/terminals when you install the new batteries. It's hateful stuff to work with but virtually eliminates issues associated with corrosion.

 

[stwendl]

If your batteries are like on a r27 classic then you may have two batteries in parallel. I am would separate them and charge individually and test them individually to see if one of them is bad and draws down the other. If you replace paralleled batteries replace both of them with the same model and manufacturing date or one battery will always trying to charge the other. If one or both house batteries are bad using the parallel switch would just empty the paralleled battery as quickly as the other. You also might consider adding a battery tester to your collection of tools. Of all odds harbor freight sells a small electronic tester (yellow hand held device) that got surprisingly good reviews. Use in single batteries only and yes that means you have to disconnect parallel batteries for the test

I had a similar situation on my diesel truck where one battery was bad and discharged the good battery. Battery at 8v often means bad cells.

 

[Cutwater28GG]

thanks all. have ordered two new AGM's for the house. will also add a new AGM for the thruster. lets get to a clean baseline. because I already replaced the engine battery with the AGM last year I have to keep them all the same as the charger cannot handle different battery chemistry across the bank.

I will also install the Balmar meter at the same time (its been on the project list for some time. this will give me a view on the house SOC and capacity much more accurately than just voltage. )

Amazon turned out to be the best source with basically free shipping. posting link for those interested https://www.amazon.com/gp/product/B0088L5AKS/ref=od_aui_detailpages00?ie=UTF8&psc=1

urgh 3 of these weigh 200 pounds!

 

[knotflying]

If your batteries are like on a r27 classic then you may have two batteries in parallel. I am would separate them and charge individually and test them individually to see if one of them is bad and draws down the other. If you replace paralleled batteries replace both of them with the same model and manufacturing date or one battery will always trying to charge the other. If one or both house batteries are bad using the parallel switch would just empty the paralleled battery as quickly as the other. You also might consider adding a battery tester to your collection of tools. Of all odds harbor freight sells a small electronic tester (yellow hand held device) that got surprisingly good reviews. Use in single batteries only and yes that means you have to disconnect parallel batteries for the test

I had a similar situation on my diesel truck where one battery was bad and discharged the good battery. Battery at 8v often means bad cells.

Keep in mind that I said, "once the engine is started you can put the parallel switch to on" . The need for the parallel switch is usually a dead battery not providing enough current to do its job. Never leave your parallel switch on and think you will be okay. As stated a bad battery will drain your good ones. That is why if it is your house battery that is bad you turn it on after the engine starts. If your engine battery is bad you need to turn it on before you start startthe engine and then leave it on while traveling. The boat is not wired like a car where you can use booster cables (similar to using your parallel switch) and then remove them. Because of the relays the bad battery, including the engine battery if that is the bad one, will not receive the charge from the alternator until it reaches the required threshold voltage.

 

[PhilR]

Hey Mike, If the house battery switch, the thruster switch and the parallel switch are on and the engine switch off, does that accomplish the same thing as connecting the house and thruster as you have described?

Phil

 

[Red Raven]

The stock parallel switch only connects the house to start. It will not add in the thruster batt. You have to add a second parallel switch the connect the thruster to house.

Curt