Battery SOC query

[KJT]

For you electrics wizards, I have a query regarding my battery SOC indication.

 

My electrics consist of 2 x 220ah Victron AGM batteries, a Victron BMV600S battery monitor, both new last year. (Plus 530 watts of solar, two engine alternators and a 3.5 kva genny)

Usually my batteries SOC doesn't normally drop below around 70% and with solar and/or cruising are normally in the 90%-100% range.

A couple of weeks or so ago, for a couple of days, the indicated SOC dropped to the mid 30% range for no obvious reason and wouldn't rise above mid 50% range despite prolonged engine or genny charging. Then, after a nice sunny day of solar the SOC quickly recovered to 100%. Each day, over the last week, the same has happened, SOC dropping to the low 30% range and failing to increase above the mid 50% range. We are cruising on average about 3 hours a day. When I run the genny with the SOC around 30% the charging current is about 85 amps but gradually drops to about 15 amps when SOC is reading about 55%. The engine alternator charge indicates about 45 amps dropping to about 12 amps over the same 30 to 55% range.

Currently, after 3 hours cruising today, my SOC is showing 51% and the battery voltage is reading 12.56v. (No charge, negligible solar - it's raining, and all 12v users OFF) Am I correct in thinking my battery monitor is significantly under reading the SOC? I am led to believe that voltage is more like 90% SOC which is what I would expect by now.

I know I can re-synch the battery monitor but I don't want to re-synch it to an incorrect value.

Any ideas what might be causing this? Thanks in advance.

 

Ken

[Kae]

From your voltage reading it sounds like your battery monitor is way off.

 

This is probably one of those times that also having a smartgauge fitted would show the inconsistencies.

Edited August 29, 2014 by Kae

[nicknorman]

The gauge is showing AH taken out regardless of battery condition, so if recently you were at 100% and it rapidly fell to a low value, either you are using a lot of power that you don't realise, or (more consistently with the rest of your description) the gauge is for some reason working incorrectly. First thing I would check is the AH capacity set in the gauge. If for some inexplicable reason the set value of, presumably, 440AH had changed itself to some much lower value, that would explain why the gauge was rapidly falling off.

 

The way these gauges work is that, presuming a reasonably high charging voltage, once the charging current falls below the "tail current" of a few amps, the gauge resets to 100% even if it was reading nothing like that a moment ago. I think that explains your comment about quickly recovering to 100%.

 

So, things to check:

 

1) Check the battery capacity settings on the BMV, is it set to something like 440AH?

2) Have a look at the current readout on the gauge - is it normal? Does it go to zero when everything is switched off?

3) If you turn on a known load, does the current displayed increase by the expected amount? ( best turn off things with thermostats like the fridge for this test, otherwise you may get confused by the fridge cutting in and out.

 

If 1) is wrong, obviously correct it.

If 2) is wrong, you either have something using power that you don't realise, or the meter is over-reading on current, or (in the case of incorrect zero reading) has an offset

3) may help you to work out whether it is an over-reading gauge or not.

 

I think sometimes the shunts these gauges use can malfunction, so it might be worth checking that visually and perhaps tying to borrow someone else's if 2) or 3) are wrong

[KJT]

Thanks for your replies.

I have checked the parameters you suggested. The battery monitor is set correctly to 440ah. With everything switched off the ammeter function is showing a 1 amp charge which is commensurate with the solar which was indicating 1.2 amp charge. When I turned on the TV and other regular users (except fridge and freezer) it showed the usual and expected 3 to 4 amps draw. So it seems the battery monitor is behaving normally in all functions except the SOC. I assume because of the way the genny's charge current tails off that the actual state of charge is very probably closer to the 90-100% I would expect and not the 50% it is currently reading.

I plan on doing a fairly long day of cruising tomorrow which would normally restore my SOC to 100% or thereabouts, so would it be a good idea to re-synch my monitor shortly after switching off the engine?

I plan on installing a Smartgauge next month which hopefully should prevent this situation recurring, or at least provide an alternative method of determining SOC.

 

Thanks again

 

Ken

Edited August 29, 2014 by NB Ellisiana

[Arthur Brown]

If you can take rested battery volts readings several times a day for a while, you should be able to build your own table of Volts vs SOC for say 10 minutes rested batteries. They are AGM batteries so there are no SG figures to tally.

[Tony Brooks]

I would also like to know the charging voltage with engine revving and just before the final shut down of the day just to be sure the alternator is working properly.

[nicknorman]

I plan on doing a fairly long day of cruising tomorrow which would normally restore my SOC to 100% or thereabouts, so would it be a good idea to re-synch my monitor shortly after switching off the engine?

I plan on installing a Smartgauge next month which hopefully should prevent this situation recurring, or at least provide an alternative method of determining SOC.

Thanks again

Ken

Yes I think a re-sync to 100% is a good idea, presuming a decent charge voltage and low tail current before switching off the engine. The only suggestion might be a Microsoft Windows-style "if in doubt, reboot" ie disconnect the meter completely to power down the electronics leave for a minute or so, then reconnect, before you sync to 100%.

 

Once you have done all that, try to keep a frequent eye on discharge current, total ah taken out and indicated SoC, which may allow you to get a sense of what is going on.

[smileypete]

Could be helpful to a look at this post in this topic:

 

http://www.canalworld.net/forums/index.php?showtopic=69426&hl=smpt#entry1381778

 

Would recommend first spending a little time to learn to use volts and and amps alone, then moving onto other things.

 

With any monitor it's well worth carefully reading and understanding the manual.

 

cheers, Pete.

~smpt~

Edited August 29, 2014 by smileypete

[KJT]

If you can take rested battery volts readings several times a day for a while, you should be able to build your own table of Volts vs SOC for say 10 minutes rested batteries. They are AGM batteries so there are no SG figures to tally.

 

Thanks Arthur. That's seems a good idea, I will do that. Are you suggesting that because of the way the SG works I cannot compare Volts to SOC, or that AGM batteries aren't compatible with the SG? I hope it's not the latter, I have already ordered the SG!

 

I would also like to know the charging voltage with engine revving and just before the final shut down of the day just to be sure the alternator is working properly.

 

Thanks Tony. I will take both those readings tomorrow and post the results. That's a good point. Despite having two alternators, one 45amp and one 130amp I have never seen any charge current above about 50 amps when using the engine to charge, even when using the Victron Multiplus invertor to power the washing machine or Microwave whilst running the engine. If I use the genny I regularly see 90 amps charge current.

 

Yes I think a re-sync to 100% is a good idea, presuming a decent charge voltage and low tail current before switching off the engine. The only suggestion might be a Microsoft Windows-style "if in doubt, reboot" ie disconnect the meter completely to power down the electronics leave for a minute or so, then reconnect, before you sync to 100%.

Once you have done all that, try to keep a frequent eye on discharge current, total ah taken out and indicated SoC, which may allow you to get a sense of what is going on.

Thanks Nick. I will do as you suggest when I am (hopefully) certain that the batteries have been completely charged. I guess that's similar to what Arthur is suggesting also.

 

Ken

Edited August 29, 2014 by NB Ellisiana

[KJT]

Have a look at this post in this topic:

 

http://www.canalworld.net/forums/index.php?showtopic=69426&hl=smpt#entry1381778

 

Would recommend first spending a little time to learn to use volts and and amps alone, then moving onto other things.

 

With any monitor it's well worth carefully reading and understanding the manual.

 

cheers, Pete.

~smpt~

Thanks Pete. I had in fact seen that chart previously, which is where I got the idea the batteries were nearer 90% with the voltage of 12.56 rather than the 51% the monitor was suggesting. In fact for the last few days I have been using that table rather than the monitor which has allowed me to be a bit more relaxed about it.

 

Ken

[Bod]

Read the Smart Gauge instructions, carefully, the battery type setting is important.

Fit the gauge, ignore the reading for the first week, as the gauge settles down and calibrates it self.

Don't be surprised if the two systems give different readings.

If correctly set up, the Smart Gauge is most likely to be correct.

 

Bod

[Paul C]

I think he means SG = specific gravity, whereas you are reading it as SG = smartgauge. Obviously, one cannot read the specific gravity of a cell on a sealed-for-life battery or a gel battery.

[KJT]

Read the Smart Gauge instructions, carefully, the battery type setting is important.

Fit the gauge, ignore the reading for the first week, as the gauge settles down and calibrates it self.

Don't be surprised if the two systems give different readings.

If correctly set up, the Smart Gauge is most likely to be correct.

 

Bod

Cheers Bod. From what I have heard/read, it seems the Smartguage should be easier and more reliable than the Victron monitor.

 

I think he means SG = specific gravity, whereas you are reading it as SG = smartgauge. Obviously, one cannot read the specific gravity of a cell on a sealed-for-life battery or a gel battery.

Ah! Thanks for pointing that out. That makes sense. I freely admit to being a muppet when it comes to electrics!

 

Ken

[Bod]

Smart Gauge is easy to fit, 3 wires, direct to battery terminals.(Will mean that Smart G does not go through the shunt.)

 

3 Buttons, main battery bank voltage, main battery bank State of Charge,(shows % of remaining capacity, not % of new capacity!!) voltage of starter battery.

 

The trick to interpreting all these gauges, is to watch how quickly the state of charge falls and how quick it can be recharged.

 

Quick to fall, and quick to recharge = low battery capacity. (May be small battery, or knackered large battery.)

Slow to fall, and slow to recharge = large battery capacity.

 

Bod

[Arthur Brown]

Sorry, I was referring to Specific Gravity.

Rest the batteries for 10 minutes frequently, read the battery volts. You will soon get a table of your battery's volts at full and less charged. Use this table to check the approx SOC whenever you need.

 

You don't actually need a SmartGauge, but lots of people find them handy for an instant SOC reading, so far they seem to be the most accurate method of determining SOC instrumentally.

[smileypete]

Thanks Pete. I had in fact seen that chart previously, which is where I got the idea the batteries were nearer 90% with the voltage of 12.56 rather than the 51% the monitor was suggesting. In fact for the last few days I have been using that table rather than the monitor which has allowed me to be a bit more relaxed about it.

 

Ken

 

OK sounds good,

 

With the BMV, what is in the settings for: charged voltage, tail current, charged detection time etc?

 

Also when the batts are being charged, what is the max charge voltage when using genny, solar and alt, and what's the lowest charge current reached?

 

My guess is that the charged voltage is set too high for engine/genny, and/or the tail current set too low, so that it never syncs to 100% when on alt or genny.

 

Once the sync is sorted out, if after a full charge/sync the SoC reading is materially different from resting voltage SoC, then probably batt capacity has declined and needs updating in the settings.

 

Smartguage isn't perfect but at least the manual is smaller. If there's understanding that sub 50% SoC doesn't mean the end of the world, just a voltmeter and ammeter might do fine.

 

cheers, Pete.

~smpt~

Edited August 29, 2014 by smileypete

[KJT]

I have taken some readings today, and have recorded the following. I first covered the solar panels all day, so they won't interfere with the readings.

 

First thing this morning the indicated SOC was 25.9% and the battery voltage was 12.05. This voltage was taken after switching off all 12v users for about 15 minutes prior to taking the reading.

 

After engine start the charge current @ 1500 rpm was 60amps and the charge voltage was 13.02, @ 2000 rpm the charge current was 64 amps and the voltage 13.05.

 

After 2.25 hrs the readings were @ 1500 rpm 49 amps/ 13.7v and @ 2000 rpm 55 amps/ 13.9v. SOC was 41.7% indicated.

 

After 4.00 hrs the readings were @ 1500 rpm 26 amps / 14.17v and 2000 rpm 26 amps/ 14.17v (identical). SOC was 46.6% indicated.

 

After switching off the engine and all 12v users, after 15 minutes the indicated SOC was 47.1% and the voltage 12.77

After a further 30 minutes the SOC remained 47.1% and the voltage was 12.73

 

I figure that with the resting voltage around 12.73 the correct SOC was nearer 100% than 47% so re-synced the BMV to 100%.

I hope I was correct in that assumption.

The only thing that puzzles me was the charge current of 26 amps after 4 hours, which I think is higher than one would expect if the batteries were indeed fully charged. Would an incorrect indicated SOC have any bearing on that? I personally think it wouldn't, but then again I know pretty much zilch when it comes to electrics!

 

Any observations?

 

Thanks again

 

Ken

 

I would also like to know the charging voltage with engine revving and just before the final shut down of the day just to be sure the alternator is working properly.

[nicknorman]

With a charge current of 26A the batteries are not fully charged. More like 80 - 85%. We have 440AH of Trojans and I noted today that when fully charged the current at 14.5v was just over 1A. I did a quick equalise and even at 15.5v the current was only 4A. To get a good sense of SoC after charging and without discharge, you need to leave the batteries to settle for a long time - many hours.

 

Ed: 14.17v is quite a low charge voltage. It would be interesting to know what the charge voltage is when the batteries finally reach fully charged (current < say 4A).

Edited August 30, 2014 by nicknorman

[KJT]

Thanks Nick. I suspected they might not be fully charged with a 26 amp charge current. I will keep an eye on it over the next few days and do another re-sync when the charge current is less than 4 amps.

If the charge voltage of 14.17 is too low is that indicative of a faulty charging system? As I mentioned earlier, my main alternator is 130 amp but have not seen more than 60 amps charge current, even if a high draw item like a microwave is being used. I would have thought it might produce more than that when required.

 

Ken

[ditchcrawler]

I am just wondering how you measure your alternator output current. My ammeter shunt is in the main negative line of the domestic battery bank, so this measures the current going into and out of the battery, it doesn't measure what is coming from the alternator and going to the inverter.

[nicknorman]

Thanks Nick. I suspected they might not be fully charged with a 26 amp charge current. I will keep an eye on it over the next few days and do another re-sync when the charge current is less than 4 amps.

If the charge voltage of 14.17 is too low is that indicative of a faulty charging system? As I mentioned earlier, my main alternator is 130 amp but have not seen more than 60 amps charge current, even if a high draw item like a microwave is being used. I would have thought it might produce more than that when required.

Ken

I missed that your alternator was 130A. So with such low batteries (12.05) and adequate engine rpm I would have expected close to full output, at least for the first few minutes. The relevant figures you have presented are 60A at 13.02v (or the 2000 rpm figure if you like). With the voltage pulled that low, I would expect full output. Something is wrong.

 

1/ the alternator is working properly but there is excessive resistance / voltage drop between the alternator and the batteries.

 

2/ the alternator is damaged - blown diode or two causing the alternator to operate well below par.

 

To determine what is going on, when you next charge from a low SoC, check the voltage right at the alternator, ie between the main stud on the alternator and its case, then compare to the voltage across the battery terminals. They should of course be similar, within say 0.3v. If there is a significant difference, put the meter on the alternator casing and the battery -ve terminal to check the drop in the -ve wires, and again between the alternator +ve stud and the battery +ve terminal.

[smileypete]

I have taken some readings today, and have recorded the following. I first covered the solar panels all day, so they won't interfere with the readings.

 

First thing this morning the indicated SOC was 25.9% and the battery voltage was 12.05. This voltage was taken after switching off all 12v users for about 15 minutes prior to taking the reading.

 

After engine start the charge current @ 1500 rpm was 60amps and the charge voltage was 13.02, @ 2000 rpm the charge current was 64 amps and the voltage 13.05.

 

After 2.25 hrs the readings were @ 1500 rpm 49 amps/ 13.7v and @ 2000 rpm 55 amps/ 13.9v. SOC was 41.7% indicated.

 

After 4.00 hrs the readings were @ 1500 rpm 26 amps / 14.17v and 2000 rpm 26 amps/ 14.17v (identical). SOC was 46.6% indicated.

 

After switching off the engine and all 12v users, after 15 minutes the indicated SOC was 47.1% and the voltage 12.77

After a further 30 minutes the SOC remained 47.1% and the voltage was 12.73

 

I figure that with the resting voltage around 12.73 the correct SOC was nearer 100% than 47% so re-synced the BMV to 100%.

I hope I was correct in that assumption.

The only thing that puzzles me was the charge current of 26 amps after 4 hours, which I think is higher than one would expect if the batteries were indeed fully charged. Would an incorrect indicated SOC have any bearing on that? I personally think it wouldn't, but then again I know pretty much zilch when it comes to electrics!

 

 

OK looks like the alt charges up to 14.2V or thereabouts, any word on what the BMV settings mentioned in post #16 are?

 

If doing a full charge which may need doing at least weekly, the charge current needs to tail off to a low level, usually 1-2% of batt bank capacity, so 4 to 8 amps in your case.

 

Sounds like the alt isn't giving out as much current as expected, could also be worth checking the belt is tight enough.

 

cheers, Pete.

~smpt~

[KJT]

I am just wondering how you measure your alternator output current. My ammeter shunt is in the main negative line of the domestic battery bank, so this measures the current going into and out of the battery, it doesn't measure what is coming from the alternator and going to the inverter.

Very basic and non scientific I'm afraid. It's just what the BMV600S tells me is going into the batteries. We don't normally have the inverter running when cruising unless we have a particular requirement. And today in particular I had all the non essential 12v users switched off in an effort to provide as accurate picture as possible as to what was going into the batteries.

 

Ken

[KJT]

OK looks like the alt charges up to 14.2V or thereabouts, any word on what the BMV settings mentioned in post #16 are?

 

If doing a full charge which may need doing at least weekly, the charge current needs to tail off to a low level, usually 1-2% of batt bank capacity, so 4 to 8 amps in your case.

 

Sounds like the alt isn't giving out as much current as expected, could also be worth checking the belt is tight enough.

 

cheers, Pete.

~smpt~

 

Hi Pete. Sorry I forgot to post the figures requested in post 16.

 

The Charge Voltage is 13.2v, the Tail Current is 1% and the Charge Detection Time is 3 mins.

 

I did a long charge using the genny this afternoon until the charge current reached 4 amps. I then re-sync'd the Battery Monitor to 100% so hopefully it should now be correct.

 

I am having the engine serviced in a couple of weeks and all the belts replaced at the same time. Although they appear in good condition and tight.

 

 

I missed that your alternator was 130A. So with such low batteries (12.05) and adequate engine rpm I would have expected close to full output, at least for the first few minutes. The relevant figures you have presented are 60A at 13.02v (or the 2000 rpm figure if you like). With the voltage pulled that low, I would expect full output. Something is wrong.

1/ the alternator is working properly but there is excessive resistance / voltage drop between the alternator and the batteries.

2/ the alternator is damaged - blown diode or two causing the alternator to operate well below par.

To determine what is going on, when you next charge from a low SoC, check the voltage right at the alternator, ie between the main stud on the alternator and its case, then compare to the voltage across the battery terminals. They should of course be similar, within say 0.3v. If there is a significant difference, put the meter on the alternator casing and the battery -ve terminal to check the drop in the -ve wires, and again between the alternator +ve stud and the battery +ve terminal.

Thanks again Nick. I am having the Smartguage fitted in a couple of weeks, along with an engine service/belt change so will have the alternators checked at the same time. It does look as though the alternators aren't pulling their weight.

 

Ken

Edited August 30, 2014 by NB Ellisiana

[smileypete]

 

 

Hi Pete. Sorry I forgot to post the figures requested in post 16.

 

The Charge Voltage is 13.2v, the Tail Current is 1% and the Charge Detection Time is 3 mins.

 

I did a long charge using the genny this afternoon until the charge current reached 4 amps. I then re-sync'd the Battery Monitor to 100% so hopefully it should now be correct.

 

OK,

 

When the genny charge current reached 4 amps, can you remember what the charge voltage was?

 

Also would be helpful to run the engine til the alt charge current reaches 4 amps, with solar not operating/panels disconnected, then find out what the alt charge voltage is.

 

From the info given so far, I'd set the Battery Capacity to 440Ah, Charged Voltage to 14.1V (at least initially), Tail Current to 2%, Charged Detection Time to 3 mins (as it is), Charge Efficiency Factor to 85%.

 

If the batts are in daily use, they still need a looonng weekly charge to minimum tail current, especially AGMs which should reach less than 0.5-1%.

 

Sounds like the charge voltage is set way too low at 13.2V, so when the solar was dribbling in current at say 13.5V under low light levels, the meter would sync to 100% even though the batts were nowhere near charged.

 

Once it's syncing OK, all that needs is a couple of simple checks then it's all good.

 

cheers, Pete.

~smpt~

Edited August 30, 2014 by smileypete