I know this is a stupid battery question, but I cannot find the answer!

Kukri

Kukri

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This is probably the silliest question ever asked here, so I apologise in advance, but I can't remember the answer; the only textbook I have on hand is the excellent Nigel Calder, 4th edition, and I can't find a place where he states it.

I have a house bank of six Rolls S12-128 AGM 12v batteries, in parallel. When I got back to the boat after a fortnight away the resting voltage across the terminals was 12.58 volts.

There are also two identical starter batteries. They showed 12.67v and 12.68v respectively, which I take to be good figures.

Do I have a problem with the house bank?

Thanks!
 
No. I’d be quite happy with those values, especially as you don’t know what the resting voltages were when you left the boat.

Edit. If you still have lingering doubts, turn everything off and leave for a couple of hours before you next leave the boat, then make a note of the voltages. Compare those against the voltages next time you’re on the boat.
End edit.
 
Sailorman's link shows a SOC table for flooded lead acid. AGM batteries such as yours generally show slightly higher values for the same SOC. I haven't checked, but Rolls is a top-end brand; I'd have thought their website might give a table more relevant to your batteries.
 
Be careful with documents which provide SOC estimates using 2 decimal place voltages as battery chemistry can meaningfully alter them. In any event, AGM votages are often a touch higher than lead, so your batteries may be at only a 90%ish state of charge. Also, are the batteries connected to any kind of charging cycle when left alone? If not, you are risking some very fine kit there: even at 90% SOC sulfation can set in within the 10% of the battery chemistry not reactivated by the increasingly incomplete state of charge. Rolls recommend 13.6-13.8V http://barden-uk.com/wp-content/uploads/2015/10/Rolls-S12-128-Datasheet.pdf
 
Minn said:
Yes, I know. I ought to carry out a full discharge test...
Click to expand...

Why? Those values are quite consistent with a good set of batteries that are in use and not connected to a mains charger. Don’t forget that even hooked up to a mains charger will take the batteries over 24 hours before they’re fully charged, so the values you’re quoting are, generally speaking a good indication that the batteries are fine.
If you connect to mains for 24+, do a simple resting voltage check and then recheck a week or so later to see how much (or how little) voltage loss there’s been wil, tell you far more about health of your bank than any discharge testing.
 
I wonder whether it might be instructive, depending upon how much of a faff it would be, to test the voltage of each battery separately after they have all been charged and rested. It could be that one of them is a bit iffy and is dragging down the overall voltage by a fraction of a volt?

Richard
 
dom said:
Be careful with documents which provide SOC estimates using 2 decimal place voltages as battery chemistry can meaningfully alter them. In any event, AGM votages are often a touch higher than lead, so your batteries may be at only a 90%ish state of charge. Also, are the batteries connected to any kind of charging cycle when left alone? If not, you are risking some very fine kit there: even at 90% SOC sulfation can set in within the 10% of the battery chemistry not reactivated by the increasingly incomplete state of charge. Rolls recommend 13.6-13.8V http://barden-uk.com/wp-content/uploads/2015/10/Rolls-S12-128-Datasheet.pdf
Click to expand...

Thanks. I have left the Sterling 1250 CED charger on line with mains supply. I had not done this before. When I left the boat the LEDs were showing orange (AGM type detected) orange (constant current) and yellow (absorption phase), so, all going well I should see orange and green when I next go aboard.
 
RichardS said:
I wonder whether it might be instructive, depending upon how much of a faff it would be, to test the voltage of each battery separately after they have all been charged and rested. It could be that one of them is a bit iffy and is dragging down the overall voltage by a fraction of a volt?

Richard
Click to expand...

I've certainly had that problem even after replacing what I thought was my domestic battery bank. In fact the previous owner had added another battery into the bank in a nearby compartment between the bank and the engine which I had never spotted.

I expect domestic bank to be at 12.7 or 12.6 even after two or three months away but I do completely isolate batteries when leaving the boat.
 
Minn said:
Thanks. I have left the Sterling 1250 CED charger on line with mains supply. I had not done this before. When I left the boat the LEDs were showing orange (AGM type detected) orange (constant current) and yellow (absorption phase), so, all going well I should see orange and green when I next go aboard.
Click to expand...

Do you trust a charger left to its own devices in your absence :disgust:
 
In the blue corner, dom:

dom said:
Be careful with documents which provide SOC estimates using 2 decimal place voltages as battery chemistry can meaningfully alter them. In any event, AGM votages are often a touch higher than lead, so your batteries may be at only a 90%ish state of charge. Also, are the batteries connected to any kind of charging cycle when left alone? If not, you are risking some very fine kit there: even at 90% SOC sulfation can set in within the 10% of the battery chemistry not reactivated by the increasingly incomplete state of charge. Rolls recommend 13.6-13.8V http://barden-uk.com/wp-content/uploads/2015/10/Rolls-S12-128-Datasheet.pdf
Click to expand...

In the red corner, Sailorman:

Your batteries are well up now, why the need to keep them on charge
 
Minn said:
In the blue corner, dom:



In the red corner, Sailorman:

Your batteries are well up now, why the need to keep them on charge
Click to expand...
And just to add to the mix, temperature makes a big difference to resting voltage as well, so the could well be up near full. Though spending time sitting not completely full must be the no1 killer of boat batteries... A week or so on float isn't the end of the world, I've loads of voltage data recorded and no reason to doubt my sterling procharge ultra, watch the volts go up and down by a few mV as the temperature changes.
 
Minn said:
In the blue corner, dom:



In the red corner, Sailorman:

Your batteries are well up now, why the need to keep them on charge
Click to expand...

My 5 boat batteries and my car batteries, apart from the daily runabout, are all on permanent intelligent charge. The boat batteries are all 10 years old and still going strong and I've just changed the car batteries after 10 years.

Walking around my marina in winter in involves avoiding tripping over all the shore power cables powering battery chargers. :)

Richard
 
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