Battery Woes !!!

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jez56uk

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I’m at a loss to understand why my domestic batteries are losing voltage very rapidly

I have 2 Mastervolt 090Ah AGM batteries for all domestic loads fitted to my motor cruiser. These were new 2 years ago. The starter is an identical battery which is stand-alone. It was new 6 months ago.
Charging is via a Waeco/Dometic smart charger when on shore-power. I’m using a Victron SmartShunt to monitor the usage.

After a full day cruising at the weekend both the domestic batteries suffered an unexpected major drop in voltage, going from fully charged at 13.7 volts at engine-off to 8.5 Volts in less than 24 hours. The only major load during this time was the Waeco fridge which draws around 4 ah. It cuts out when the voltage drops below 10.4 volts which we noticed after about 12 hours when it started to de-frost. I manually checked both batteries with a multi-meter and both show the 8.5v. The starter battery is unaffected and maintains a steady resting charge of 13.1 v

At 8.5v the battery monitor showed a total usage of only 29Ah within 24 hours with 88% capacity remaining

Does anyone have any thoughts on what the reasons are for such rapid battery degradation? I’m thinking that either both have a manufacturing fault – which is unlikely, or the 240V smart charger is faulty and is “cooking” the batteries… however the starter is fine and this charged from the same unit. I’ve included some screen shots of the SmartShunt monitor readout. Note that the voltage drops 0.38v in 5 minutes

I expect the domestic batteries are now unusable so consoling myself that I’ll have to replace them and the charger at some considerable cost.
 

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If the fridge low voltage cut-out operated at 10.4volts, something else must have continued to take current to get them down to 8.5volts. Worth a check?
 
It is not clear from your post whether the engine was running when the voltage dropped. Assuming not, your fridge will draw 4A, not ah, when running. If the ambient temperature was high it was probably running at least 50% of the time for an air cooled unit. 2 x 90 ah batteries is a very small capacity for this draw: I have 3 X 110 Ah for mine. Throw in lights, music, phone charging and you could easily account for the drop.

I know nothing about the monitoring system so cannot explain that.
 
The batteries are sulphated and you have not reset the battery monitor so it is telling you lies. You have not got 88% remaining because the 100% point has not been reset as the batteries have aged. At 8.5v unless you recharge them for a long time until they are really 100% charged they are scrap. Its probably too late for these batteries already.
You need to understand a proper battery charging regime. Read a battery primer and understand that it takes at least twice the time to put back what you take out.
 
The batteries are toast. They still can help you track down the cause though: Crack open the seals, pop the valve caps off and shine a flashlight in. If at least one cell is dry, they were cooked out, that is, something overcharged them (alternator regulator, faulty shorepower charger, etc.). If they're still wet, they died by deep discharge, so something did that while you weren't paying attention. Make sure you don't have any loads that bypass the shunt.

I hunted down such a mystery this spring on a catamaran. Once we'd found it, the owner slapped his forehead - he had installed a forced air diesel heater which was drawing 200mA when not in use (LCD control panel). He had directly connected this to the batteries (starter in this case) with just a fuse, bypassing the main battery disconnect, so when he disconnected it and left the boat a few months, the little LCD display noodled the battery empty each time, killing it.

The state of charge display (88%) is meaningless. It is calculated from a theoretical capacity that your batteries no longer have.
 
from the two pics of the smart shunt, we can (reasonably safely...) conclude that they only managed to provide circa 30Ah before dying.
Even so there was a .5A and 1A load even at that level, lights on? something else? That is ignoring things that may be wired straight on and bypass the shunt (almost always a few...).
Now if they were at 100% SoC and are now at 88% (theoretical means absolutely nothing now) or started at 60% and are now at 30% is also meaningless.

It's safe to conclude you need new batteries though. as 30Ah is 1/3 of the officially available capacity of your bank so batteries are not "holding" charge...
Would be nice to check everything and make sure there's not something emptying them or not something cooking them though so you don't get 2yrs (or less!) out of the new set.
fridge 4A @50% cycle means almost 50Ah on a 180Ah bank with say 50% usable so circa 90Ah. IOW, a fridge in a hot cabin in a hot summer day will consume half the available capacity of your service bank. I'd increase the bank if I could tbh.

sorry not saying anything different from the previous posters, just trying to get some numbers down so it makes a bit more sense...
 
No guarantee that the batteries are toast or sulphated. I had three AGMs bought new in 2006. Left for long periods (up to 5 years at one time) without being touched) and drained to absolute flat on several other ocassions. I replaced two this year and I still use one of the old house set as the starter battery.

This weekend, I spent most of the time on the boat and as I have seemingly sorted the solar issues, I watched the voltage and charge climb slowly but steadily in the house bank. (New batteries) The one old battery still in use is connected only to the alternator and a set of charging diodes and it is up around 13v despite it's age and history...

You may not be able to get them to charge from a smart charger as it may perceive a fault and cut out. I had an old caravan charger which brought mine back to life over a few days.

I would pull fuses and eliminate as many circuits as possible and check things like automatic pumps for the power drain if you are not satisfied that the fridge is the culprit. .
 
pandos said:
Here is an interesting read on sulphation.

Fighting Sulfation in AGMs - Practical Sailor
Click to expand...
Good article.. I learned a lot. Whilst it's logical that a sailing boat's batteries could possibly never be fully charged, that seems unlikely to be my problem as 99% of the time the boat is either being charged by the AC charger via shore power or via the engine alternator. Rarely do I operate the 12v systems using the stand-alone batteries. One option is the charger is faulty and is never getting the batteries to full charge. Although if this were the case I imagine the starter battery would also be affected.. which it isn't.

I did a full check today and there is no parasitic draw so this is not causing the problem. Whilst experimenting with various loads I made an interesting discovery. Turning on a 12v iPhone charger which draws 1 amp when the battery showed 11.12v reduced the voltage to 8.4v in less than 1 minute. Unplugging the charger saw the voltage recover to 11.11v in 2 minutes. See the screen shots

I'm still at a loss to fully understand what has caused the batteries to be trashed in 2 years when they have had relatively little use and my usage profile has not changed nor have I added or replaced any 12v gear. The previous ones lasted 5 years and they were still capable of running the fridge for 18 hours at the time of replacement.

I've also attached the SmartShunt settings page as requested by Kompetent Krew
 

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jez56uk said:
Good article.. I learned a lot. Whilst it's logical that a sailing boat's batteries could possibly never be fully charged, that seems unlikely to be my problem as 99% of the time the boat is either being charged by the AC charger via shore power or via the engine alternator. Rarely do I operate the 12v systems using the stand-alone batteries. One option is the charger is faulty and is never getting the batteries to full charge. Although if this were the case I imagine the starter battery would also be affected.. which it isn't.

I did a full check today and there is no parasitic draw so this is not causing the problem. Whilst experimenting with various loads I made an interesting discovery. Turning on a 12v iPhone charger which draws 1 amp when the battery showed 11.12v reduced the voltage to 8.4v in less than 1 minute. Unplugging the charger saw the voltage recover to 11.11v in 2 minutes. See the screen shots

I'm still at a loss to fully understand what has caused the batteries to be trashed in 2 years when they have had relatively little use and my usage profile has not changed nor have I added or replaced any 12v gear. The previous ones lasted 5 years and they were still capable of running the fridge for 18 hours at the time of replacement.

I've also attached the SmartShunt settings page as requested by Kompetent Krew
Click to expand...
I would look to the charger, can you switch the feed from the charger for the start battery to the house bank and see if there is any change in the behavior of the house batteries?.

Or even isolate one battery and charge that from a standalone charger and see if it improves, this will at least tell you if the batteries are actually toast.

The charger might be simply not doing it's job, or it might be actually doing damage to the batteries.

Either way something is wrong and I suspect it's something that is reputed to be "smart" but is misbehaving....
 
Try fully charging with a normal 3 stage profile (starting at 14.6 - 14.8 volts). Check that the charger follows the correct profile and that the current ends up essentially 0 with a float voltage of 13.4V or so.

When fully charged, try equalisation by charging at low current and at 15.4V for 12 hours or so. If the current is high, then you have a dead cell. That battery is almost certainly unrecoverable.
This might help. You have nothing to lose by trying again if it did not help much the first time.

If not, your batteries are dead. As others have said you need to find out why. It may well be that the batteries got cooked from overcharging too long at too high a voltage. It only needs one cell to fail in one battery. But I would replace both batteries anyway as having different performing batteries is a recipe for disaster.

Note that if you have a dead cell, then the other battery in parallel will discharge trying to charge the battery with the dead cell. This is not good for the surviving battery!
 
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This is in the manual, as are instructions to set the charged voltage (which seems to be right on yours), but IMO not prominently enough - it's easy to overlook.

In some circumstances, e.g. if you disconnect the batteries, perhaps using a master battery isolator or switch, the Smart Shunt will show the batteries as 100% charged when they are not, if this setting is enabled. It would be evident that they are not from the battery voltage, if there's no draw and you happen to notice it's lower than it should be.

My thread on Victron's help site: SmartShunt showing the wrong percentage - Victron Community
 
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