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Battery Monitors and Capacity


GeoffS

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For 8+years I have used a Smartgauge to look after a set of AGM batteries. They failed during this summer and I replaced them with 4 x 115 ah Sealed Lead Acid batteries. Soon after I also installed a Victron Battery Monitor in addition to the Smartgauge as I wanted to see the battery actual usage while I was on an extensive cruise.

I am well aware of the short comings of Amp Hour counters and so always intended to use the Smartgauge as the SOC monitor. However I assume the Amp Hours consumed that is recorded (while there is no charging being done) should be accurate on the Victron. 

So I am a bit puzzled. If I take to Smartgauge to 100% SOC and discharge a portion of the batteries overnight the Smartgauge reads a far lower SOC than the consumed AH suggests. It is not just a small deviation but can be 15-20% suggesting that the actual capacity of these new batteries is far lower than their original quoted capacity.

If I discharge 60 AH I see a Victron SOC of 87% (straight maths) yet the Smartgauge will show around 70%.

Could this mean that my 4 x 115 AH batteries only actually have around 200 AH capacity? If so is there a way to check this as after just a few months I hope that this is not the case? Or can it be that even though it records 60 AH that more is actually consumed?

Thanks in advance for any information.

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What Ah capacity did you program into the Victron BM?

I don't have a Smargauge, but I think it needs to be 'reset' and will take time to learn the new batteries' characteristics.  Have you set both meters for SLAs?

Edited by mross
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The simplest way is to take a voltage reading before it gets light in the morning (no solar power) and preferrably no load on the batteries for an hour. Consult voltage vs SOC tables and that will give you an approximate (but good enough) SOC. I look at my voltage each morning before solar kicks in - much easier now it is darker - but there is usually -1.7A out from the inverter/240v fridge. I am now reasonably confident to estimate the 'no load' voltage from the '-1.7A drain' voltage. Then compare the AHr used vs the voltage derived SOC. I write it down each day and 4 months after putting in new lead acids, the numbers havent degraded significantly - if at all.

I assume you are getting the same voltage on both meters?

Edited by Dr Bob
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26 minutes ago, mross said:

What Ah capacity did you program into the Victron BM?

I don't have a Smargauge, but I think it needs to be 'reset' and will take time to learn the new batteries' characteristics.  Have you set both meters for SLAs?

I put 460AH in the Victron. As far as I know the Smartgauge should reset itself after a few discharges and I have had these now for 3/4 months. Both meters are set for SLA's.

12 minutes ago, Dr Bob said:

The simplest way is to take a voltage reading before it gets light in the morning (no solar power) and preferrably no load on the batteries for an hour. Consult voltage vs SOC tables and that will give you an approximate (but good enough) SOC. I look at my voltage each morning before solar kicks in - much easier now it is darker - but there is usually -1.7A out from the inverter/240v fridge. I am now reasonably confident to estimate the 'no load' voltage from the '-1.7A drain' voltage. Then compare the AHr used vs the voltage derived SOC. I write it down each day and 4 months after putting in new lead acids, the numbers havent degraded significantly - if at all.

I assume you are getting the same voltage on both meters?

The same voltage appears on both meters and looking at a SOC chart suggests the Smartgauge is correct.

Which is worrying in that it suggests the actual capacity is far far less than 460AH. As the batteries are less than 5 months old I am concerned and also need to decide if I go back to the supplier.

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How do you know that your batteries are actually reaching 100% SoC ?

Are you testing specific gravity of the cells, monitoring tail current, or using the Smartguage ?

 

You may have not been fully charging your batteries for the 5 months have had them - that is plenty of time to suffer from sulphation, and, if that is the case you could easily lose 50% of capacity in 5 months.

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The most likely scenario is that you didn’t charge to 100%. When your charging current at 14.4V or thereabouts has reduced to below 5A and has remained stable for about 45 minutes then you are close to 100%. If it continues to drop then keep charging until it doesn’t. Do not rely on SmartGauge to tell you when you’ve got there, it has no way of knowing. 

I see Alan got there before me. 

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The only sensible way, without monitoring Specific Gravity, is to charge until the amps drawn falls to between 1% & 2% of actual capacity, (5A and 2A on your case).

If you've been ending charging when Smartgauge reads 100% for 5 months, your batteries are probably sulphates and capacity reduced significantly.

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Not tested specific gravity, and they are sealed batteries. For Victron purposes I set the Tail current to be 1.5%. I have used a combination of the Smartgauge and the Tail current has fallen to 3-4A.

8 minutes ago, WotEver said:

Note that a couple of equalisation charges may well recover a substantial amount of sulphation. 

Can this be done with Sealed Batteries?

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49 minutes ago, GeoffS said:

Not tested specific gravity, and they are sealed batteries. For Victron purposes I set the Tail current to be 1.5%. I have used a combination of the Smartgauge and the Tail current has fallen to 3-4A

Can this be done with Sealed Batteries?

First point, is the charger still in absorbtion charge mode (circa 14.4 volts) when you are reading 3-4 amps? 

Also having set the battery type and capacity, you need to periodically "synchronise" the BMV when the battery really is fully charged, by presting the + & - keys simultaneously for 3 seconds or longer. (It is supposed to automatically synchronise whenever the battery is charged to 100%, but as the capacity falls due to sulphation it does this less and less giving an incorrect reading).

With regard to your second point,  it depends on the kind of sealed battery. Better quality AGM's can be equalised, as can the cheapo sealed batteries which contain a wet electrolyte.  With this type there is usually a sealed strip hiding the caps to the individual cells. 

Edited by cuthound
To unmangle the effects of autocorrect.
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21 minutes ago, cuthound said:

First point, is the charger still in absorbtion charge mode (circa 14.4 volts) when you are reading 3-4 amps? 

Also having set the battery type and capacity, you need to periodically "synchronise" the BMV when the battery really is fully charged, by presting the + & - keys simultaneously for 3 seconds or longer. (It is supposed to automatically synchronise whenever the battery is charged to 100%, but as the capacity falls due to sulphation it does this less and less giving an incorrect reading).

With regard to your second point,  it depends on the kind of sealed battery. Better quality AGM's can be equalised, as can the cheapo sealed batteries which contain a wet electrolyte.  With this type there is usually a sealed strip hiding the caps to the individual cells. 

The majority of the time I am charging via the engine alternators. I use a charger periodically, via a generator, and sometimes it could still be in Absorption mode.

Equalisation via an alternator is presumably not possible? I need to check the charger manual on if and how to make it do an equalisation charge.

 

Thanks for all the input

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As you’ve had 8 years from your last set of Batteries (if I read the OP correctly) then you’re not doing much wrong :)

As you’re going down to around 3A tail current from the alternator then, assuming it’s putting out over 14.1V, you’re fully charging the batts. Don’t worry about equalising at this stage.

Go down to around 60% SoC on the SmartGauge and see if the Ah used agrees more closely. 

Edited by WotEver
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Just a small point.  AH ratings of batteries are nominal and at a set temperature.  If I remember correctly (and I'm sure someone here will know). Its about 27 Deg C, below that capacity drops off.  Also the battery needs a number of cycles before it will be at optimum so to programme 460 AH is probably too high.  I have four new Trojan T105s nominally at 450 AH but have put 420 into my NASA (mind you the NASA SOC is useless).

 

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3 minutes ago, larryjc said:

Just a small point.  AH ratings of batteries are nominal and at a set temperature.  If I remember correctly (and I'm sure someone here will know). Its about 27 Deg C, below that capacity drops off.  Also the battery needs a number of cycles before it will be at optimum so to programme 460 AH is probably too high.  I have four new Trojan T105s nominally at 450 AH but have put 420 into my NASA (mind you the NASA SOC is useless).

 

Generally manufacturer's assess capacity at 25°C, (and then tell you that you will reduce life if you keep them above 20°C). Capacity increases with higher temperatures are tended reduces with lower ones.

You are correct that the plates are really other fully formed when a battery is new are rend it takes up to 5  cycles to complete the forming process.

 

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3 hours ago, WotEver said:

As you’ve had 8 years from your last set of Batteries (if I read the OP correctly) then you’re not doing much wrong :)

As you’re going down to around 3A tail current from the alternator then, assuming it’s putting out over 14.1V, you’re fully charging the batts. Don’t worry about equalising at this stage.

Go down to around 60% SoC on the SmartGauge and see if the Ah used agrees more closely. 

Yes 8+ years and I have always been diligent in making sure I never deeply discharged them. They started to struggle at the beginning of this year but I dithered about what to replace with (I wanted to go Trojan or another set of AGM but I couldnt find the correct physical size and didnt want to do a lot of reworking of the positioning which would not have been easy). In a bit of a hurry I went for Exide ER 550 which were certainly not 'cheap' at Around £150 each.

I have been down to the mid 60's Smartgauge SOC and it is close to 20% lower than the Victron shows when set to 460AH. If I set to 200AH then it starts to get much closer.

I think a discussion with the supplier/installer may be the next thing to do.

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42 minutes ago, GeoffS said:

I have been down to the mid 60's Smartgauge SOC and it is close to 20% lower than the Victron shows when set to 460AH. If I set to 200AH then it starts to get much closer.

A simpler way of doing it...

  • Charge to 100% and zero the Victron. 
  • Discharge to somewhere around 50% to 60% on the SmartGauge and note the Ah taken out according to the Victron. 

Some simple maths will now tell you the true capacity of the bank. 

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Are you absolutely certain that the Victron is “seeing” all the currrent going out of the batteries? Since you installed the Victron after changing the batteries, there is no track record of it working properly. Check that the domestic battery negatives are connected to each other only, but not connected to the engine battery negative. Check that the only things connected to the domestic battery negative is the shunt, and the Smartgauge negative. Nothing else. No, not even that little wire you didn’t really notice!

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2 minutes ago, nicknorman said:

Are you absolutely certain that the Victron is “seeing” all the currrent going out of the batteries? Since you installed the Victron after changing the batteries, there is no track record of it working properly. Check that the domestic battery negatives are connected to each other only, but not connected to the engine battery negative. Check that the only things connected to the domestic battery negative is the shunt, and the Smartgauge negative. Nothing else. No, not even that little wire you didn’t really notice!

Good point :)

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As per my previous post, check the voltage at rest first thing in the morning - which will give a reasonable estimate of SoC - at least to 5% accuracy. If you are fully charged from the day before (run for 4 or 5 hours and then use the genny) then you can compare the SoC's. I only have a BMV meter and the SoC from that is just rubbish.  Use your volts first thing in the morning as your measure of 'goodness' of the battery. Track it day to day. Look for trend changes rather than individual readings. Even if your voltage meter is inaccurate, the trend change is useable. That's what your SG is doing - but but can an individual do it better with their own set up? The more I hear about SG's, the more I question a global model that is fit for all set ups. I am happy with monitoring voltage on a daily basis vs A's out (but not in) and totally ignore SoC data.

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7 minutes ago, Dr Bob said:

As per my previous post, check the voltage at rest first thing in the morning - which will give a reasonable estimate of SoC - at least to 5% accuracy. If you are fully charged from the day before (run for 4 or 5 hours and then use the genny) then you can compare the SoC's. I only have a BMV meter and the SoC from that is just rubbish.  Use your volts first thing in the morning as your measure of 'goodness' of the battery. Track it day to day. Look for trend changes rather than individual readings. Even if your voltage meter is inaccurate, the trend change is useable. That's what your SG is doing - but but can an individual do it better with their own set up? The more I hear about SG's, the more I question a global model that is fit for all set ups. I am happy with monitoring voltage on a daily basis vs A's out (but not in) and totally ignore SoC data.

Unless the Smartgauge is out of calibration, I think determining the SoC by looking at the morning voltage will yield the same results as the Smartgauge, within the accurcies of both methods. Depending on where you look on the internet, there are different tables of SoC vs rested voltage, with a difference of more than 5%. Which one is right?

Edited by nicknorman
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7 minutes ago, nicknorman said:

Unless the Smartgauge is out of calibration, I think determining the SoC by looking at the morning voltage will yield the same results as the Smartgauge, within the accurcies of both methods. Depending on where you look on the internet, there are different tables of SoC vs rested voltage, with a difference of more than 5%. Which one is right?

....a difference of more than 5%.....?

The OP is saying he is seeing differences of of 20% plus. I fully agree that the SoC/Volts tables vary, as does temp when you measure it, the accuracy of your volt meter etc, etc, etc. Lots of possible variances. That's why I do a check each day and keep a track of data. Trending is the key as it then overcomes some of the variables you cant tie down. For the last few weeks, now the solar is useless at 7.30am, I am seeing 12.45 +/- 0.03 Volts on my BMV with a -1.7A current as we are creatures of habit and usually use circa 70Ahrs each night (+/- 10Ahr). Doing it this way you should be able to be within 5% of the answer which for me is fine. Being pessemistic I may be 10% out but again that is fine.

I did ask if both meters were the same for voltage and the OP said they were - so using the Volts at rest should give an accuracy better than 10% and hopefully 5%.

I would believe my SoC as calculated from V at rest over the SoC the BMV gives me anyday.

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8 hours ago, Dr Bob said:

As per my previous post, check the voltage at rest first thing in the morning - which will give a reasonable estimate of SoC - at least to 5% accuracy

But he already knows the SoC at rest or during discharge because the has a SmartGauge. We also know it’s not faulty as he got 8 years out of his last batts. His only query here is the capacity of the bank and I’ve already explained the simplest way of calculating that.  

Edited by WotEver
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8 hours ago, Dr Bob said:

....a difference of more than 5%.....?

The OP is saying he is seeing differences of of 20% plus. I fully agree that the SoC/Volts tables vary, as does temp when you measure it, the accuracy of your volt meter etc, etc, etc. Lots of possible variances. That's why I do a check each day and keep a track of data. Trending is the key as it then overcomes some of the variables you cant tie down. For the last few weeks, now the solar is useless at 7.30am, I am seeing 12.45 +/- 0.03 Volts on my BMV with a -1.7A current as we are creatures of habit and usually use circa 70Ahrs each night (+/- 10Ahr). Doing it this way you should be able to be within 5% of the answer which for me is fine. Being pessemistic I may be 10% out but again that is fine.

I did ask if both meters were the same for voltage and the OP said they were - so using the Volts at rest should give an accuracy better than 10% and hopefully 5%.

I would believe my SoC as calculated from V at rest over the SoC the BMV gives me anyday.

I think we are slightly at cross purposes. The 5% came from you, my point was that your method can’t necessarily give a result within 5% since the source information (the SoC vs voltage tables ) vary by more than 5% depending on which table you look at on the internet.

i have a Smartgauge and an AH counter, they agree during discharge within a few %. If the OP’s don’t, it is either because his AH counter isn’t seeing all the current, or because the batteries are well under capacity (presuming the SG voltage reading is correctly calibrated). Checking the SoC via voltage measurements isn’t going to help as the results will be pretty much the same as the SG gives. No point in having a dog and barking yourself = no point in having a SG and checking the SoC via rested voltage.

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2 hours ago, WotEver said:

But he already knows the SoC at rest or during discharge because the has a SmartGauge. We also know it’s not faulty as he got 8 years out of his last batts. His only query here is the capacity of the bank and I’ve already explained the simplest way of calculating that.  

Yes, agreed and your method should throw a lot of light on the problem but if the Soc via smart gauge and amps drawn give similar results to the op initial results then is the capacity low? One possible explanation is the SG is telling porkies. Using the V at rest allows guessing the SoC at any state of charge.....but as Nick says with limited accuracy, but good enough for this question and all it needs is a quick glance in the morning. To the OP, what was your voltage and Amps draw this morning when you got up?

As Nick says though, vital to ensure all the wires go through the shunt.

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