Batteries charged to 80% yet volts dropping to 11.3 ?

[scr0che]

Noticed an interesting pattern with my battery bank which I can't understand.

Victron smart battery monitor, 4x100Ah AGM Exide batteries, less than 12 months old. What I notice is that during overnight passages, when the nav electronics etc are running, once the battery capacity drops to around 80% I see the volts in the monitor at 11.3 (or thereabouts). This doesn't look right, shouldn't it remain at 12.5v until my capacity is being exhausted? The upshot is, my AIS unit is beeping at me with a 12v alarm...

Can anyone explain this behaviour or a possible cause, where to check, what to check? Surely not duff batteries?

 

[roaringgirl]

Capacity is calculated by counting the Ah in and out through the shunt. With lead-acid batteries, there's a fudge factor included for Peukert's equation so they can get a bit out of sync. The voltage *at rest* is a good indicator of state of charge. You may need to check your state of charge this way and reset the battery monitor.

However, when I started seeing a big discrepancy between calculated charge capacity and voltage, it was a good sign my batteries were on the way out

 

[Sandy]

Don't get at rest voltage and in use voltage mixed up.

What happens when you switch off everything, make your morning tea, go on deck and greet the new day, check your log, have a wee then check your voltage? Or just wait 30 mins, but that is much less fun.

 

[Neeves]

Could be 'duff' batteries. If you have allowed your batteries to be used and they drop, regularly. below 50% charged or have run near flat then their life expectancy would be reduced (and they would perform as yours are doing). This has nothing to do with the BATTERIES being 'duff'........

Jonathan

 

[Bobc]

You may have 1 duff one, dragging the others down.

 

[GHA]

no way anyone can say anything with any degree of certainty without a lot more data. (won't stop them though ?)
How do they normally get charged?
If they spend a lot of time (like over a week seems to be general rule of thumb) not fully charged then they basically get less & less capacity.
Also battery voltages vary a lot with temperature.
First big question is have they definitely been getting fully charged regularly?

 

[geem]

We have a similar sized battery bank to you. We never see voltages that low on an overnight passage. Something very wrong. Could be a number of things but a lot more info needed

 

[Graham376]

We have a similar sized battery bank to you. We never see voltages that low on an overnight passage. Something very wrong. Could be a number of things but a lot more info needed

The monitor figures given don't make sense. It's showing 80% remaining capacity, so the battery voltage should be around 12.6v, but 11.3 indicated = flat battery, zero remaining capacity.

As the AIS alarm is giving battery warning and the batteries were 100% or near at start, either there's been some hefty power drain bypassing the shunt or they're knackered.

 

[PaulRainbow]

Noticed an interesting pattern with my battery bank which I can't understand.

Victron smart battery monitor, 4x100Ah AGM Exide batteries, less than 12 months old. What I notice is that during overnight passages, when the nav electronics etc are running, once the battery capacity drops to around 80% I see the volts in the monitor at 11.3 (or thereabouts). This doesn't look right, shouldn't it remain at 12.5v until my capacity is being exhausted? The upshot is, my AIS unit is beeping at me with a 12v alarm...

Can anyone explain this behaviour or a possible cause, where to check, what to check? Surely not duff batteries?

Ignore the 80% reading for now, it's a calculated value and can easily be wrong. There could also be a current draw that's bypassing the shunt, so that needs checking at some point.

So, your current reading (amps) may not be correct too. Check to make sure that nothing is connected to the battery negative (except the shunt), the battery connection to the shunt, or anywhere else in between.

The only reading you can trust (short of ensuring the above is all correct) is the voltage. This should be directly read from the batteries and should be reliable, assuming it isn't connected elsewhere or otherwise bodged.

First job is to check the batteries.

1) Fully charge the batteries, preferably an overnight charge with a shore charger.
2) Turn the charger off and leave the batteries for 15 mins.
3) Are any batteries hot, hotter than the others ? If so, it suggests that one is faulty.
4) Disconnect the interconnecting negative cables, so the batteries are separate.
5) Note the voltage of each battery.
5) Check the voltage again after an hour or two.

If there has not been a big voltage drop on any battery you'll need to carry out a drop test on each battery.

 

[Bav32]

Suggest you split the 4 batteries up eg use only 2 a d repeat, then try the other 2.
If there is one duff one dragging the others down, this might help to identify.
Have you actually heched to battery voltage with a voltmeter, on a d off load?

 

[scr0che]

Thanks for all these great replies, I shall investigate further when I'm next ashore in a week or so and report back.

 

[scr0che]

As an aside, the BMV monitor reads 100% after a charge in the morning from solar panels (360w), so my bank should be pretty much charged up all the time, I've never seen it go below 75%.

I can confirm there are no negative cables coming from the battery other than the one to the shunt.

 

[Alex_Blackwood]

As an aside, the BMV monitor reads 100% after a charge in the morning from solar panels (360w), so my bank should be pretty much charged up all the time, I've never seen it go below 75%.

I can confirm there are no negative cables coming from the battery other than the one to the shunt.

Even if the monitor reads 100% have you tried taking a voltage reading from the batteries with a multimeter and see what that is. Off chance there could be a problem with the monitor and the reading is incorrect. What is the monitor voltage reading in comparison?

 

[QBhoy]

Are we meaning after an overnight passage under sail and without and generated charge to the batteries during the passage ? Is the voltage reported taken afterwards and without any draw from the batteries at the time ? Or is it taken whilst in a state of draw from nav lights and instruments etc ?

 

[roaringgirl]

Battery voltage at rest will tell you real charge state:

Charge fully
Isolate batteries from all charge and loads
Leave for as long as you can (ideally, I think it's 2 hours)
Check voltage with multimeter
Load batts without charging until the monitor reads 80%
Isolate fully and leave for as long as you can again.
Recheck voltage

Fully charged AGMs are 12.9V, at 80% should be 12.5V, if batteries are lower, then your real capacity is less than you thought - could be a dud battery bringing the rest down, so you could test each individually; or the whole bank could be degraded. I've heard of being able to revive AGMs with a gadget that does spikes of high voltage to clean the plates, but I haven't used one myself.

 

[PaulRainbow]

Battery voltage at rest will tell you real charge state:

Charge fully
Isolate batteries from all charge and loads
Leave for as long as you can (ideally, I think it's 2 hours)
Check voltage with multimeter
Load batts without charging until the monitor reads 80%
Isolate fully and leave for as long as you can again.
Recheck voltage

Pretty much what i posted in post #9

 

[KompetentKrew]

Is the "starts synchronised" setting turned off in the Victron app? Very easy to get a wrong 100% reading and knacker your batteries if this is enabled.

 

[GHA]

Battery voltage at rest will tell you real charge state:

Charge fully
Isolate batteries from all charge and loads
Leave for as long as you can (ideally, I think it's 2 hours)
Check voltage with multimeter
Load batts without charging until the monitor reads 80%
Isolate fully and leave for as long as you can again.
Recheck voltage

Fully charged AGMs are 12.9V, at 80% should be 12.5V, if batteries are lower, then your real capacity is less than you thought - could be a dud battery bringing the rest down, so you could test each individually; or the whole bank could be degraded. I've heard of being able to revive AGMs with a gadget that does spikes of high voltage to clean the plates, but I haven't used one myself.

Bearing in mind those voltages can vary substantially with temperature, age & chemistry between manufacturers.
Voltage as a state of charge indicator is limited, apart from being very limited accuracy it can take overnight for voltage to settle.
Those gadgets are snake oil unfortunately. ..

 

[Neeves]

As an aside, the BMV monitor reads 100% after a charge in the morning from solar panels (360w), so my bank should be pretty much charged up all the time, I've never seen it go below 75%.

I can confirm there are no negative cables coming from the battery other than the one to the shunt.

I don't, quite, understand how your battery mon defines 100% fully charged after 'only' a morning in the sun. You have, at least, been running the fridge overnight, and presumably domestic lights (and maybe sailing and then all the nav gear, autopilot etc etc). The solars are operating in the UK which is not renowned for its sunshine. You may have a large solar bank - but it only operates at full power at midday, assuming no shade and assuming all the panels are fully operational.

My thoughts are the Mon needs to be zeroed as its indication of 100% and the very low voltage implies to me - the batteries are not being fully charged, though you think they are fully charged, they are constantly being used not fully charged and maybe very little charge - basically you have been using uncharged batters (as if full) and this has destroyed the batteries.

As is being said repetitively - you need to fully charge the batteries, which you will only sensibly achieve from shore power - your alternators may never achieve full charge, or take 12 hours. Once you have achieved full charge - simply leave on charge for a very long time - then try the 'test' protocols suggested.

Jonathan

 

[scr0che]

Turns out the %age reading is a calculation on the BMV and shouldn't be relied on, consumed amp hours is a better measure but is not displayed by default! Mostly likely my reading of the %age has knackered my batteries, ie thinking they are full when they are not. New batteries installed, all working correctly, lesson learnt!