Battery monitor error?

[frahkn]

I understand from pieces on here that all monitors have some error in readings, what I want to know is can they go completely wrong?

 

My sterling monitor shows figures for amps going in (or out) of the domestic bank and also the SoC which is a negative amp hour figure which normally reduces to 0.

 

I have been on shore power since Wednesday, when I started the monitor showed -45 Ah. Now, more than 48 hours later it has got down to -19 Ah. It says that the charger is putting in about half an Amp.

 

The batteries are brand new and have never been more than 25% discharged. Also while out last week with the alternator charging, the monitor never got below -7 Ah, though the engine was running for 8 hours per day.

 

So, can I assume that the monitor is off?

 

Thanks.

[Alan de Enfield]

19 minutes ago, frahkn said:

The batteries are brand new and have never been more than 25% discharged

How do you know ?

(Hopefully you are not relying on the monitor to show you)

How do you know when it is fully charged ?

 

What monitoring equipment do you have ?

 

20 minutes ago, frahkn said:

Also while out last week with the alternator charging, the monitor never got below -7 Ah,

If the monitor is correct, then it could imply that your alternator is faulty.

 

 

What sort of knowledge level do you have with regard to fault-finding ?

What sort of test equipment do you have ? (do you have a DC clamp meter ?)

 

[WotEver]

34 minutes ago, frahkn said:

It says that the charger is putting in about half an Amp.

At what voltage?

[Detling]

Battery monitors are usually wrong more often  than they are correct. There are many causes, monitoring the tiny voltages used to measure the Amps flowing through the shunt will pick up noise from the boats wiring leading to small errors. The monitor has parameters for charge efficiency, and peukert effect, these change with age and temperature of the batteries. I think quite a few monitors underestimate the charge efficiency deliberately, better for you to think you need a few more amps than to stop charging early.

[cuthound]

The battery monitor needs to know the actual capacity of your batteries for the SoC to be aanything like accurate, as they count amphours in and out, and then use a fiddle factor for charge efficiency.

 

Then the monitor needs to be synchronised by setting it to 100% when the battery is really fully charged, i.e. tail current at the end of charge down to 1-2% of battery capacity at 14.4 volts or more (the manual will tell you how to synchronise your monitor, usually by pressing two buttons at the same time for a short while).

 

As your new batteries lose capacity it is necessary to periodically assess the new capacity, set the monitor to this capacity and re-synchronise.

Edited March 20, 2020 by cuthound
Grandma

[frahkn]

3 hours ago, Alan de Enfield said:

How do you know ?

 

What monitoring equipment do you have ?

 

If the monitor is correct, then it could imply that your alternator is faulty.

 

 

What sort of knowledge level do you have with regard to fault-finding ?

What sort of test equipment do you have ? (do you have a DC clamp meter ?)

 

The battery bank is 550 Ah and I know, historically, don't use more than 110 Ah in 24 hours.

 

Just the Sterling monitor.

 

I am currently on shore power.

 

Some.

 

Yes, I have a DC clamp meter

3 hours ago, WotEver said:

At what voltage?

 

14.5 volts.

10 minutes ago, cuthound said:

The battery monitor needs to know the actual capacity of your batteries for the SoC to be aanything like accurate, as they count amphours in and out, and then use a fiddle factor for charge efficiency.

 

Then the monitor needs to be synchronised by setting it to 100% when the battery is really fully charged, i.e. tail current at the end of charge down to 1-2% of battery capacity at 14.4 volts or more (the manual will tell you how to synchronise your monitor, usually by pressing two buttons at the same time for a short while).

 

As your new batteries lose capacity it is necessary to periodically assess the new capacity, set the monitor to this capacity and re-synchronise.

Alas my monitor has no adjustment facility as far as I can see.

[WotEver]

51 minutes ago, frahkn said:

14.5 volts.

Then they are full

[frahkn]

1 hour ago, WotEver said:

Then they are full

 

Thank you, that's reassuring.

[Richard10002]

3 minutes ago, frahkn said:

 

Thank you, that's reassuring.

Not necessarily  

 

They may be full, but their capacity may be severely reduced.

[frahkn]

3 minutes ago, Richard10002 said:

Not necessarily  

 

They may be full, but their capacity may be severely reduced.

The batteries are less than 2 months old, I am as certain as possible that they have never been below 75% and have always been charged to (say) 95% on the same day. So how could they capacity have been "severely reduced"?

[Richard10002]

4 minutes ago, frahkn said:

The batteries are less than 2 months old, I am as certain as possible that they have never been below 75% and have always been charged to (say) 95% on the same day. So how could they capacity have been "severely reduced"?

The rule where charging until current drawn is 1-2% of capacity, at 14.4V, only tells us that our batteries are full to whatever their current capacity is. I felt it worth pointing this out.

 

It is entirely possible to destroy a lead Acid battery bank in a couple of months but, based on what you say, your batteries are almost certainly full to their nominal capacity.

[MtB]

3 minutes ago, Richard10002 said:

It is entirely possible to destroy a lead Acid battery bank in a couple of months but, based on what you say, your batteries are almost certainly full to their nominal capacity.

 

I'd say that rather depends on the history and treatment of the batteries pre-purchase....

 

 

[frahkn]

1 hour ago, Mike the Boilerman said:

 

I'd say that rather depends on the history and treatment of the batteries pre-purchase....

 

 

 

OK Mike, obviously I don't know their pre-purchase history. So, is there any test I can  do to see if the batteries are (as I suspect) fully charged and therefore the monitor is at fault? Obviously I could buy another monitor but your own story suggests this might not be a solution.

[Tony Brooks]

From a rested voltage as close to 13.8 as you can get by charging - note rested, say for 24 hours - put a known load on one. Say a 21Watt car flasher bulb. Time how long it takes for the rested voltage to drop to about 11.8 volts. 21 watts/12.3V = 1.7 amps. So the time i hours times 1.7  the capacity in Ah.  12,3 chosen as an average voltage over the discharge time.

[nicknorman]

17 hours ago, frahkn said:

I understand from pieces on here that all monitors have some error in readings, what I want to know is can they go completely wrong?

 

My sterling monitor shows figures for amps going in (or out) of the domestic bank and also the SoC which is a negative amp hour figure which normally reduces to 0.

 

I have been on shore power since Wednesday, when I started the monitor showed -45 Ah. Now, more than 48 hours later it has got down to -19 Ah. It says that the charger is putting in about half an Amp.

 

The batteries are brand new and have never been more than 25% discharged. Also while out last week with the alternator charging, the monitor never got below -7 Ah, though the engine was running for 8 hours per day.

 

So, can I assume that the monitor is off?

 

Thanks.

These sorts of counters work by “counting the Ah out, and counting them back in again”. The count Ah by measuring current and multiplying it by time. This process is subject to an inevitable small error, and that error is cumulative (gets worse with time) as the error is multiplied by time. If the error takes the form of an offset on the zero point, it will count out a slightly different number of Ah from what it counts in. In other words, during charging it may either get to 100% SoC / 0Ah deficit early, or never.  
Ah counters generally have a big of logic whereby, once the charge current falls to near zero and with the voltage still up at a value that indicates it is still being charged, they will reset to 100% SoC / 0Ah deficit. I am not familiar with your Ah counter but I would suspect it has something similar, or a manual means to reset to 100%. I’ll try to have a look at the manual.

 

So I don’t think this indicates an issue with the batteries, more likely a configuration or “user” issue with the Ah counter, or just a poorly designed monitor.

 

ive had a look at the manual, it states a current measurement resolution of 0.1A which is pretty coarse. It doesn’t mention if/how it resets itself 100% to eliminate the cumulative errors. It doesn’t seem very good to me, I’d be inclined to get a BMV battery monitor which has lots of configurable options.

 

in the mean time, with the batteries fully charged (voltage up around 14.4v or higher, and current down to an amp or less) disconnect and reconnect the monitor’s power lead. It will come back up showing 0Ah deficit.

Edited March 21, 2020 by nicknorman

[frahkn]

20 minutes ago, nicknorman said:

These sorts of counters work by “counting the Ah out, and counting them back in again”. The count Ah by measuring current and multiplying it by time. This process is subject to an inevitable small error, and that error is cumulative (gets worse with time) as the error is multiplied by time. If the error takes the form of an offset on the zero point, it will count out a slightly different number of Ah from what it counts in. In other words, during charging it may either get to 100% SoC / 0Ah deficit early, or never.  
Ah counters generally have a big of logic whereby, once the charge current falls to near zero and with the voltage still up at a value that indicates it is still being charged, they will reset to 100% SoC / 0Ah deficit. I am not familiar with your Ah counter but I would suspect it has something similar, or a manual means to reset to 100%. I’ll try to have a look at the manual.

 

So I don’t think this indicates an issue with the batteries, more likely a configuration or “user” issue with the Ah counter, or just a poorly designed monitor.

 

ive had a look at the manual, it states a current measurement resolution of 0.1A which is pretty coarse. It doesn’t mention if/how it resets itself 100% to eliminate the cumulative errors. It doesn’t seem very good to me, I’d be inclined to get a BMV battery monitor which has lots of configurable options.

 

in the mean time, with the batteries fully charged (voltage up around 14.4v or higher, and current down to an amp or less) disconnect and reconnect the monitor’s power lead. It will come back up showing 0Ah deficit.

Thanks very much, appreciate the detail and completeness.

[Dr Bob]

17 hours ago, frahkn said:

I understand from pieces on here that all monitors have some error in readings, what I want to know is can they go completely wrong?

 

 

I have been on shore power since Wednesday, when I started the monitor showed -45 Ah. Now, more than 48 hours later it has got down to -19 Ah. It says that the charger is putting in about half an Amp.

 

The batteries are brand new and have never been more than 25% discharged. Also while out last week with the alternator charging, the monitor never got below -7 Ah, though the engine was running for 8 hours per day

I am very confused about the facts.

You say it never got below -7Ahr. Did you park up overnight and draw power on the inverter for the night? If so it should have gone below -7Ahr and should have been nearer -100 Ahr.

It sounds like when on shore power you are fully charged. To test the meter I would do the following.

Synchronise the meter to 100%SoC as it sounds like you are full. If you can't do this....chuck it out and buy a Victron BMV.

Disconnect shore power and turn on inverter. Turn on something on the boat to drain the surface charge i.e. the ebersparky for an hour. That will take about 5Ahr if done for an hour. Turn off everything for an hour and then take a voltage reading. It should be around 12.70V. Measure this on the sterling and your multimeter.

Run on the inverter for the night and in the morning turn everything off for an hour and check the voltage and the Ahr meter. If everything is ok then the Ahrs out should show -100Ahr ish (that's what you normally use) and the voltage should show approx the %used (refer to a voltage SoC table). On mine with 660Ahr capacity I would see a voltage of 12.50V with -100Ahr out. Turn shore power on and measure current in against time as it charges to full. That will tell you most of what you need.

This is what I do from time to time but I ignore the 1 hr rest and adjust the voltage up knowing the effect on voltage of a 1.7Ahr draw ...my typical draw with the inverter on with all the gibbons I run, the rasp pi, Alexa, router etc etc. Temperature of batteries does have a effect if you are estimating at rest voltage from a voltage obtained with a current draw.

If you have solar then do the above only when dark as the solar confuses everything.

[Alan de Enfield]

7 minutes ago, Dr Bob said:

You say it never got below -7Ahr. Did you park up overnight and draw power on the inverter for the night? If so it should have gone below -7Ahr and should have been nearer -100 Ahr.

I took the 'never gone below -7Ah' as being -6Ah, -5Ah, -4Ah etc etc

If the device is never getting (below / above ?) -7Ah then either the device is out of calibration, or, the battery is never getting fully charged.

 

I would have thought if it went to -8Ah to -100Ah then that would be 'above' 7Ah.

 

One of us must be wrong.

 

Its a bit like the knob in the fridge - do you turn it 'up' or 'down' to make it cooler ?

[Dr Bob]

38 minutes ago, Alan de Enfield said:

I took the 'never gone below -7Ah' as being -6Ah, -5Ah, -4Ah etc etc

If the device is never getting (below / above ?) -7Ah then either the device is out of calibration, or, the battery is never getting fully charged.

 

I would have thought if it went to -8Ah to -100Ah then that would be 'above' 7Ah.

 

One of us must be wrong.

 

Its a bit like the knob in the fridge - do you turn it 'up' or 'down' to make it cooler ?

Well that is why I am confused. In every scenario -100Ahrs is below -7Ahrs.

I agree with you that Frank probably meant it never went above -7Ahrs ie up to zero in which case it could just be a syncronisation issue or the Sterling has confused itself to the point where it wont auto syncronise when full. If he followed what I said above he could identify if the meter is working 'approximately' (and a rough guess at battery capacity) and then when recharging we can see if it does resync when full. Before I got my lithiums, I used to check my LA's this way every 3 months or so......no extra work other than just looking at the meter at the right time of day and writing down the results.

 

 

eta...oh, forgot the fridge.

I always leave my knob alone.

 

Edited March 21, 2020 by Dr Bob

[MtB]

 

Having always accepted forum wisdom that AH counters drift out of sync and can't be trusted, I've never used one properly. But intuitively I'd expect one to display a positive number of AH in the battery then count down toward zero as the battery gets progressively more discharged.

 

If the OP's is designed to work like this, but is actually displaying in the negative range i.e. is believes the battery SoC is actually below zero, I'm not surprised it doesn't seem to work right!

[nb Innisfree]

8 minutes ago, Mike the Boilerman said:

 

Having always accepted forum wisdom that AH counters drift out of sync and can't be trusted, I've never used one properly. But intuitively I'd expect one to display a positive number of AH in the battery then count down toward zero as the battery gets progressively more discharged.

 

If the OP's is designed to work like this, but is actually displaying in the negative range i.e. is believes the battery SoC is actually below zero, I'm not surprised it doesn't seem to work right!

I don't think ah counters get out of sync, it's just that they can't know what ah are in a battery before it begins its countdown, they're just dumb counters. 

[nicknorman]

5 minutes ago, nb Innisfree said:

I don't think ah counters get out of sync, it's just that they can't know what ah are in a battery before it begins its countdown, they're just dumb counters. 

They do get out of sync for the reasons I explained earlier. However most have a means to regain synchronisation by changing the display to 100% / 0Ah when they detect a “fully charged” condition by other means.

21 minutes ago, Mike the Boilerman said:

 

Having always accepted forum wisdom that AH counters drift out of sync and can't be trusted, I've never used one properly. But intuitively I'd expect one to display a positive number of AH in the battery then count down toward zero as the battery gets progressively more discharged.

 

If the OP's is designed to work like this, but is actually displaying in the negative range i.e. is believes the battery SoC is actually below zero, I'm not surprised it doesn't seem to work right!

The Sterling manual indicates that it starts at zero, and shows negative Ah as the battery discharges. Just Charles’ logic, different from most other people’s logic!

[MtB]

8 minutes ago, nicknorman said:

The Sterling manual indicates that it starts at zero, and shows negative Ah as the battery discharges. Just Charles’ logic, different from most other people’s logic!

 

Charles is Higgsy, and I claim my five pounds!!

 

 

 

 

[nb Innisfree]

22 minutes ago, nicknorman said:

They do get out of sync for the reasons I explained earlier. However most have a means to regain synchronisation by changing the display to 100% / 0Ah when they detect a “fully charged” condition by other means.

The Sterling manual indicates that it starts at zero, and shows negative Ah as the battery discharges. Just Charles’ logic, different from most other people’s logic!

But 'fully charged' is no indication of batt capacity, so counter still doesn't know what % level batt is at, though I realise you know that. 

[Tony Brooks]

2 hours ago, nb Innisfree said:

But 'fully charged' is no indication of batt capacity, so counter still doesn't know what % level batt is at, though I realise you know that. 

and that's the killer. It thinks you still have (say) 440Ah of battery capacity so when you have drawn off 220Ah it says 50% charged but if the capacity has reduced to  220Ah it should really show 0% charged but it won't.

 

The only readings most boaters should  trust in my view are volts, amps and Ah out.

 

To give another answer to your earlier question about finding battery capacity. If you fully charge the battery as per the ammeter being at 1 to 2% of battery capacity and 14.4ish volts   and the current failing to rise over a hours or so of extra charging then you can be sure the batteries are as fully charged as they can be.

 

Next zero the Ah out counter and get on with your evening. Next morning take the rested voltage with nothing running and the Ah out reading. Use the rested voltage to infer the batteries state of charge so say its around the 12.2 to 12.3 that is roughly a true 50% discharged. Now use the Ah out to calculate the capacity so say the Ah out was 40Ah it means 40Ah is 50% of battery capacity so the actual capacity is 80Ah.

Edited March 21, 2020 by Tony Brooks