Battery monitor question

[linkshouse]

Not sure if this is the right forum or even the right site but here goes…

I have a victron battery monitor query for any experts in here.

I have a bmv700 installed in my van and have recently installed two 110ah lead carbon batteries. I’m just testing them on the drive at the moment just running the fridge and the mifi router.

According to the BMV700 they were fully charged yesterday and the solar charger was in float mode.

Checking the BMV700 this morning it reports that 13.5Ah have been consumed and that the batteries are at 96% charge. These numbers don’t seem to correlate.

I have the battery capacity set to 220Ah and the discharge floor set to 60% in the BMV700 settings.

Any suggestions as to how to get these two readings to correlate better, or failing that which one to trust the most.

Many thanks

Phill

 

[wildebus]

Hi Phil, there are a few things here ..... And I am not fully sure what you are questioning, so going to cover all bases ...

The actual battery capacity depends on how hard you pull a current from them when it comes to Lead batteries. The lower the current, the greater the capacity. It is a thing called the Peukert factor or effect or component (you see all terms used).
The Victron BMVs have a setting in them to set the Peukert value to tell it now much the batteries are affected by this. What this means is that if the battery draw has been very light, the State of Charge percentage is bigger than you think it should be for the amount of AHs draw.
Ignoring Peukert, the SOC of your batteries would be 94%, not 96%, but overnight the current draw would be pretty low in terms of the battery bank size which is why it is ended up larger than you might have calculated.


You may be questioning the SOC with reference to the setting you have for the discharge floor? The discharge floor setting on the BMV is a bit of a weird setting. It does NOT make any difference whatsoever to the State of Charge reading or calculation. It seems to have three possible purposes.
1) it is what is used to display the "Time to Go" value. This is the amount of time the batteries have, based on the usage over a period of time (think it is the last 5 minutes?) until the battery SOC drops to the discharge floor value.
2) there is a SOC setting option for the relay in the BMV which activates and deactivates the relay. It uses the discharge floor value as a starting point for this. The idea here is that someone may have a auto-start generator and when the discharge floor is reached, the relay is activated and a signal sent to the generator to turn on.
3) You can have a warning buzzer activated when the discharge floor is reached.
What people do with the setting depends on their needs. Personally, I have tended to use it to turn things on when the batteries are high, not low, so end up with a "discharge" floor of 85% or 90%!

If you were thinking the displayed SOC value is the AVAILABLE usable capacity based on the discharge floor setting, nope, it does not work that way - it is the batteries ACTUAL capacity where 100% is full, and 0% is dead as a dodo.

Were you maybe surprised how much current was drawn overnight when everything "off" maybe? I wish I was only using 13.5Ah overnight


Now few other things....
The Victron BMV battery monitor expects the battery bank capacity to be at the C20 rating for accurate measurements. Your batteries are 110Ah at C100 but they are 100Ah at C20, so you should have the capacity set at 200 and not at 220 (this difference will not be why there is an apparent mismatch of AHs drawn and SOC value.). When you make that change, IRRC the BMV will change the SOC reading dynamically.

Why did you chose 60% as the discharge floor? TBH it doesn't really matter what it is set at for most people, but if it is a "don't want to drop below that battery SOC" thing, your new batteries are perfectly ok below 60% or 50% for that matter.

There is a good Victron video (on YouTube) on how to setup the BMV depending on if you have lead or lithium, and if you have solar or not.
It is worthwhile watching that, especially the solar part, as if the BMV is not setup right, what can happen is it can prematurely reset the SOC back to 100% if certain things coincide, which is possible with solar charging when the BMV is not setup expecting solar. (It is about current levels at certain voltages, but the video explains it so no point in repeating it here).

 

[Nabsim]

If you just want the monitor to tell you zero is actually at 50% or 40% whatever don’t you just adjust the battery size setting David? I.e. 100ah on a 200ah bank would give zero SOC when the battery’s are 50% full?

 

[wildebus]

If you just want the monitor to tell you zero is actually at 50% or 40% whatever don’t you just adjust the battery size setting David? I.e. 100ah on a 200ah bank would give zero SOC when the battery’s are 50% full?

You could do that, but I would not recommend it as you have no way of seeing the usage IF you do continue using the battery power. It won't show negative SOC.
And then you get into the next problem.. if the battery capacity is set at 100AH but is is really 200AH. When you start to charge, each 10Ah is will show up as an increase in SOC of 10%, instead of 5%
. So if you DID take the battery down to say 40% REAL SOC, it will say fully charged when it is not as it will be 10% out. Plus you are not really starting from 0% but actually from -10%. So when it has recharged a bit and says 50% SOC in that configuration (and you think it is at 75% of actual capacity), the battery will really only be 65% of actual capacity - 20Ah out.
And if the battery isn't able to recharge (wild camping with solar will often not get you back to full each day), you will end up with a constantly false reading and potentially - if you do use the battery again below it's reported 0% on multiple occasions - the gap between the reported SOC and the REAL SOC getting bigger and bigger and you may end up with a battery sitting at a low level in reality but the monitor telling you all is fine and actually potentially unknowingly end up with the exact situation you are trying to avoid.

Using the discharge floor setting with the alarm buzzer would be the way to go - or if you wanted to go really strict, have a power relay going from battery to loads and when the discharge floor is reached, have the BMV relay activate and switch OFF the power relay to remove the load. This approach is available (although rarely used) on quite a few devices where there is a 'standard' load output which is disconnected on a low voltage (or if available, low SOC), and a 'critical' load output where the circuit stays on - and where keeping the load running is more important than looking after the battery (a medical device maybe? Or in a MH, could be the beer fridge )

 

[linkshouse]

Hi Phil, there are a few things here ..... And I am not fully sure what you are questioning, so going to cover all bases ...

The actual battery capacity depends on how hard you pull a current from them when it comes to Lead batteries. The lower the current, the greater the capacity. It is a thing called the Peukert factor or effect or component (you see all terms used).
The Victron BMVs have a setting in them to set the Peukert value to tell it now much the batteries are affected by this. What this means is that if the battery draw has been very light, the State of Charge percentage is bigger than you think it should be for the amount of AHs draw.
Ignoring Peukert, the SOC of your batteries would be 94%, not 96%, but overnight the current draw would be pretty low in terms of the battery bank size which is why it is ended up larger than you might have calculated.


You may be questioning the SOC with reference to the setting you have for the discharge floor? The discharge floor setting on the BMV is a bit of a weird setting. It does NOT make any difference whatsoever to the State of Charge reading or calculation. It seems to have three possible purposes.
1) it is what is used to display the "Time to Go" value. This is the amount of time the batteries have, based on the usage over a period of time (think it is the last 5 minutes?) until the battery SOC drops to the discharge floor value.
2) there is a SOC setting option for the relay in the BMV which activates and deactivates the relay. It uses the discharge floor value as a starting point for this. The idea here is that someone may have a auto-start generator and when the discharge floor is reached, the relay is activated and a signal sent to the generator to turn on.
3) You can have a warning buzzer activated when the discharge floor is reached.
What people do with the setting depends on their needs. Personally, I have tended to use it to turn things on when the batteries are high, not low, so end up with a "discharge" floor of 85% or 90%!

If you were thinking the displayed SOC value is the AVAILABLE usable capacity based on the discharge floor setting, nope, it does not work that way - it is the batteries ACTUAL capacity where 100% is full, and 0% is dead as a dodo.

Were you maybe surprised how much current was drawn overnight when everything "off" maybe? I wish I was only using 13.5Ah overnight


Now few other things....
The Victron BMV battery monitor expects the battery bank capacity to be at the C20 rating for accurate measurements. Your batteries are 110Ah at C100 but they are 100Ah at C20, so you should have the capacity set at 200 and not at 220 (this difference will not be why there is an apparent mismatch of AHs drawn and SOC value.). When you make that change, IRRC the BMV will change the SOC reading dynamically.

Why did you chose 60% as the discharge floor? TBH it doesn't really matter what it is set at for most people, but if it is a "don't want to drop below that battery SOC" thing, your new batteries are perfectly ok below 60% or 50% for that matter.

There is a good Victron video (on YouTube) on how to setup the BMV depending on if you have lead or lithium, and if you have solar or not.
It is worthwhile watching that, especially the solar part, as if the BMV is not setup right, what can happen is it can prematurely reset the SOC back to 100% if certain things coincide, which is possible with solar charging when the BMV is not setup expecting solar. (It is about current levels at certain voltages, but the video explains it so no point in repeating it here).

Thanks, that is a great explanation and pretty much answers all the issues I had in mind.

i had indeed set the discharge floor to 60% so as not to go below that. Given what you’ve said this was pointless, which is fine now I know.

I understand, miraculously, what you’re saying about the Peukert component. This makes me think that the Ah consumed is probably the most accurate measurement.

Thanks

Phill

 

[wildebus]

Thanks, that is a great explanation and pretty much answers all the issues I had in mind.

i had indeed set the discharge floor to 60% so as not to go below that. Given what you’ve said this was pointless, which is fine now I know.

I understand, miraculously, what you’re saying about the Peukert component. This makes me think that the Ah consumed is probably the most accurate measurement.

Thanks

Phill

I think both are useful when viewed together in fact, as the SOC does take into account HOW you are using the battery and not just how much.
The two are a bit different - think about a car fuel tank... drive carefully and you get more miles. drive hard all the time and your range plummets. do the occasional boy racer trip and you get a dip.

The great thing about the Lead Carbons is that they don't mind the occasional "Boy Racer" session and dropping down below 50%, 40% or even 30% and it does them no real harm. Sure, the charge cycle count reduces but if you were to do that big drop say 10 times a year, you might be knocking off 3 months over a 5 or 10 year service life. is that really significant?
It is like Lithiums ... don't discharge them low and you get an extra 1,000 cycles. So what, it just means they may last 15 years instead of 20 (based on sellers data sheets). The amount of money paid for the bit of battery you never used is just wasted as chances are you will not be using those same batteries in 15 years anyway let alone 20.

 

[linkshouse]

I think both are useful when viewed together in fact, as the SOC does take into account HOW you are using the battery and not just how much.
The two are a bit different - think about a car fuel tank... drive carefully and you get more miles. drive hard all the time and your range plummets. do the occasional boy racer trip and you get a dip.

I linkage that analogy, bit like range on the trip computer vs using the fuel gauge.

Thanks

 

[Nabsim]

I had seen people posting about lowering battery capacity setting as a means of ‘staying safe’ which is why I asked the questions above. I haven’t done it myself as I have no need. If I take mine to zero SOC they are still likely to outlast me lol

 

[linkshouse]

I linkage that analogy, bit like range on the trip computer vs using the fuel gauge.

Thanks

Just noticed! Linkage = like

 

[wildebus]

I had seen people posting about lowering battery capacity setting as a means of ‘staying safe’ which is why I asked the questions above. I haven’t done it myself as I have no need. If I take mine to zero SOC they are still likely to outlast me lol

I think overall the BMVs are slightly pessimistic (but that might be I tend to try and use good batteries and avoid ones like Banner Bull wet cells.)
Something that is not a bad idea is to have the "charged" voltage that is used to define a fully charged battery set at a higher voltage then would happen. This stops any possibility of a premature jump to 100% SOC.

I think it is better to configure right in terms of capacity and know the reality and manage it rather then setting something to deliberately show the wrong values, even if done with best intentions.

 

[Phantom]

I think it is better to configure right in terms of capacity and know the reality and manage it rather then setting something to deliberately show the wrong values, even if done with best intentions.

I have wet lead batteries totalling 210Ah, though I would not expect them to have that capacity after a several years. I have my battery monitor set to 210Ah capacity and it always gets back up to read 100%/210Ah eventually, though I guess that cannot be quite right? What do you think is happening there?

 

[wildebus]

what you can do to update the monitor and maybe get a more accurate capacity is fully charge the battery (leave on charge until current is minimal) and then do a discharge test, and when the monitor reports 105Ah taken, stop the discharge, leave for around an hour and then see what the voltage is (should be around 12.05V if you still have 210Ah.)
Alternatively do it via the voltage - do the discharge, stop when the voltage is 12V, then after an hour rest, check the voltage is around 12.05V and see what the amount of AHs taken is. double it to get the current battery capacity.

The Battery monitors are not really able to find out what the capacity is, they just report on power in and out. you could set the capacities to anything you like and the monitor wouldn't know any better, just the info it shows wouldn't make sense.

 

[Phantom]

what you can do to update the monitor and maybe get a more accurate capacity is fully charge the battery (leave on charge until current is minimal) and then do a discharge test, and when the monitor reports 105Ah taken, stop the discharge, leave for around an hour and then see what the voltage is (should be around 12.05V if you still have 210Ah.)
Alternatively do it via the voltage - do the discharge, stop when the voltage is 12V, then after an hour rest, check the voltage is around 12.05V and see what the amount of AHs taken is. double it to get the current battery capacity.

Thanks David, I will disconnect solar and do that, it will be useful to know the real battery capacity.