Victron BMV-712 Charge Efficiency Factor Setting

[richwest3]

I have a new BMV-712 battery monitor installed with my new LiFePO4 system. In using the system for the past two months, I've been watching the monitor as the system reaches 100% SOC and adjusting the charge efficiency factor setting to try to get the monitor to read 100% at about the same time that the battery reaches its 100% SOC voltage (system set to 14 volts). I keep making 2% adjustments down (rather than to put in the brain work required to calculate a more precise value) and am now at 91% and I think I might still need to drop it to perhaps as low as 87%.

What do you think about this? Reading has led me to believe that this number should be more like 95% but that's not what I'm experiencing.

I was going to build my own batteries from cells from AliExpress but I couldn't get them shipped to NZ. Instead, I bought no-name 12 volt, 200AH batteries locally. I took the top off the beautiful stainless steel case thinking I would find a mess that I could rework (the batteries were about $750 each so my expectations were low) but instead I found what looks like excellent quality and workmanship. The BMS units have very large balancing resistors - perhaps 2 watt resistors. Is it possible that high balancing current might be causing this suspected inefficiency?

The batteries (with internal BMS units) are running pretty cool during charging at about 20C. I'm charging the 400AH bank with 500 watts of solar and when we move the boat, my 90 amp alternator (which outputs about 60 amps). I have the Peukert exponent set at 1.05. I calibrated the BMV for zero amps during installation.

 

[rmaddy]

Victron states that a Charge Efficiency Factor of 99% should be used for Lithium batteries.

 

[richwest3]

Victron states that a Charge Efficiency Factor of 99% should be used for Lithium batteries.

That's all well and good but that doesn't work, at least in my case. With that setting, the BMV-712 reads 100% well before the batteries are fully charged. They still accept high charging current for quite some time with the voltage well below anything that could be considered a fully charged voltage.

 

[rmaddy]

In my case my BMV's idea of 100% SOC was quite off the first time I had it all hooked up. I then did a full charge with the proper settings and the BMV then got a proper idea of what 100% SOC really was. Subsequent charges result in it showing 100% SOC pretty late in the charge cycle which seems about right. If you are having the wrong SOC even after a few full charge cycles then most likely you have other settings setup incorrectly for the BMV. It's probably not the charge efficiency factor causing the issues.

 

[mikefitz]

Leave the charge efficiency at 99%, this does not determine the sync to 100% point.

The BMV syncs to 100% when a certain voltage has been exceeded and the current has fallen below a certain value for a certain time.

You can set BMV712 to 100% capacity, either manually, or by adjusting the settings in the BMV712 to match the charger settings.
Manual setting.
Stop the automatic synchronizing by adjusting settings out of range, for example, setting the voltage to a higher value than the charger maximum. Charge the battery bank to 100%, monitor the voltage and current to establish this.
If you are charging to 14.4, then one 100Ah battery is full when you are in absorption (voltage constant at 14.4) and the current drops to 4 amps, with 400Ah batteries that's 16 amps. Once this state is reached, using the app, set to 100%.
Since the efficiency of the lithium is high, the BMV should track fairly well for a while, however a manual sync to 100% every so often is useful.

To use the automatic sync of the BMV the following parameters need setting in relationship to the charger settings,
voltage, just below the charge voltage value to allow for device calibration variation, with a 14.4 charge voltage , 14.20 to 14.3 volts
tail current, % of capacity, with 14.4 charge you need 4 amps per 100Ah, that's 4% (400 x 4% = 16 amps) The current must fall below this value for a sync to take place.
Time to go, this is the period that both the voltage and current settings must be met before sync takes place, typically 5 minutes.
Efficiency 99%.

With a variable charge source like solar, sometimes the current drops below the tail current set value before full charge has been reached and the BMV syncs. This is more pronounced where the BMV set voltage is much lower than charge voltage.

The BMV works quite well but there are always small tolerances that an accumulate over time, so even in auto, a manual sync when the battery is known to be at full capacity is advisable from time to time.

to establish full charge of 100Ah battery use the following values,
(scale for actual battery capacity)

13.8 charge, terminate when the current falls to 1.4 amps
14.0 charge, terminate when the current falls to 2.3 amps
14.2 charge, terminate when the current falls to 3.2 amps
14.4 charge, terminate when the current falls to 4.1 amps
14.6 charge, terminate when the current falls to 5.0 amps

If you are not too worried about a full 100% charge then any charge voltage over 13.8 volts with a charge current maximum of less than 20 amps per 100Ah will charge well over 95% capacity with no absorption period. Charge to voltage target and stop.

Mike

 

[JeepDaddy]

Have you followed the synchronization procedure in the factory Victron manual? Section 2.2 IIRC.

 

[JeepDaddy]

This thread may help as well:

BMV-712 Thread

 

[mikefitz]

factory Victron manual? Section 2.2 IIRC.

The problem is that it, the manual, states the sync voltage is 13.2 volts. Since this is the voltage of a lithium 12v pack at around 60% capacity the BMV can sync to 100% even when not being charged.

If you read further, section 2.3, it does suggest that the sync voltage may need to be higher.

Mike

 

[chrisski]

I had to adjust my tail current to keep my712 jumping from high 80%s to 100% charged.

This is a picture of a lead acid battery charge settings. Never changed the efficiency factor. With tail current set to 2% and a 458 ah battery system, when my charging voltage is reached of 14.6 and my batteries can only take in less than 9 amps, my meter will go to 100%.

Originally, it was set much higher like 8%, so once the charging reached 14.6 volts, and the amps reached 36 amps, it would be a high 80s percentage state of charge, but jump to 100%

Stepping down my tail current helped with this. It’s pretty spot on now. Now it may jum from 98% to 100%, but that’s much better than it used tp.

 

[JeepDaddy]

The problem is that it, the manual, states the sync voltage is 13.2 volts. Since this is the voltage of a lithium 12v pack at around 60% capacity the BMV can sync to 100% even when not being charged.

If you read further, section 2.3, it does suggest that the sync voltage may need to be higher.

Mike

True. It says 14.2, which would be better.

There is no shame in reading the factory manual.

 

[richwest3]

Leave the charge efficiency at 99%, this does not determine the sync to 100% point.

The BMV syncs to 100% when a certain voltage has been exceeded and the current has fallen below a certain value for a certain time.

You can set BMV712 to 100% capacity, either manually, or by adjusting the settings in the BMV712 to match the charger settings.
Manual setting.
Stop the automatic synchronizing by adjusting settings out of range, for example, setting the voltage to a higher value than the charger maximum. Charge the battery bank to 100%, monitor the voltage and current to establish this.
If you are charging to 14.4, then one 100Ah battery is full when you are in absorption (voltage constant at 14.4) and the current drops to 4 amps, with 400Ah batteries that's 16 amps. Once this state is reached, using the app, set to 100%.
Since the efficiency of the lithium is high, the BMV should track fairly well for a while, however a manual sync to 100% every so often is useful.

To use the automatic sync of the BMV the following parameters need setting in relationship to the charger settings,
voltage, just below the charge voltage value to allow for device calibration variation, with a 14.4 charge voltage , 14.20 to 14.3 volts
tail current, % of capacity, with 14.4 charge you need 4 amps per 100Ah, that's 4% (400 x 4% = 16 amps) The current must fall below this value for a sync to take place.
Time to go, this is the period that both the voltage and current settings must be met before sync takes place, typically 5 minutes.
Efficiency 99%.

With a variable charge source like solar, sometimes the current drops below the tail current set value before full charge has been reached and the BMV syncs. This is more pronounced where the BMV set voltage is much lower than charge voltage.

The BMV works quite well but there are always small tolerances that an accumulate over time, so even in auto, a manual sync when the battery is known to be at full capacity is advisable from time to time.

to establish full charge of 100Ah battery use the following values,
(scale for actual battery capacity)

13.8 charge, terminate when the current falls to 1.4 amps
14.0 charge, terminate when the current falls to 2.3 amps
14.2 charge, terminate when the current falls to 3.2 amps
14.4 charge, terminate when the current falls to 4.1 amps
14.6 charge, terminate when the current falls to 5.0 amps

If you are not too worried about a full 100% charge then any charge voltage over 13.8 volts with a charge current maximum of less than 20 amps per 100Ah will charge well over 95% capacity with no absorption period. Charge to voltage target and stop.

Mike

Wow Mike, THANK YOU for that great writeup! It should be placed somewhere others can find it easily.

I understand what you are saying and agree but when we're out cruising in our boat, it can be many days or even weeks between syncs. I'd like the monitor to reflect the actual charge state between those syncs and that's why I've been adjusting the charge efficiency factor. I think that it's also interesting that I'm not seeing anywhere near the 99% efficiency that I thought I should see with lithium batteries.

 

[SteveSS]

Leave the charge efficiency at 99%, this does not determine the sync to 100% point.

The BMV syncs to 100% when a certain voltage has been exceeded and the current has fallen below a certain value for a certain time.

You can set BMV712 to 100% capacity, either manually, or by adjusting the settings in the BMV712 to match the charger settings.
Manual setting.
Stop the automatic synchronizing by adjusting settings out of range, for example, setting the voltage to a higher value than the charger maximum. Charge the battery bank to 100%, monitor the voltage and current to establish this.
If you are charging to 14.4, then one 100Ah battery is full when you are in absorption (voltage constant at 14.4) and the current drops to 4 amps, with 400Ah batteries that's 16 amps. Once this state is reached, using the app, set to 100%.
Since the efficiency of the lithium is high, the BMV should track fairly well for a while, however a manual sync to 100% every so often is useful.

To use the automatic sync of the BMV the following parameters need setting in relationship to the charger settings,
voltage, just below the charge voltage value to allow for device calibration variation, with a 14.4 charge voltage , 14.20 to 14.3 volts
tail current, % of capacity, with 14.4 charge you need 4 amps per 100Ah, that's 4% (400 x 4% = 16 amps) The current must fall below this value for a sync to take place.
Time to go, this is the period that both the voltage and current settings must be met before sync takes place, typically 5 minutes.
Efficiency 99%.

With a variable charge source like solar, sometimes the current drops below the tail current set value before full charge has been reached and the BMV syncs. This is more pronounced where the BMV set voltage is much lower than charge voltage.

The BMV works quite well but there are always small tolerances that an accumulate over time, so even in auto, a manual sync when the battery is known to be at full capacity is advisable from time to time.

to establish full charge of 100Ah battery use the following values,
(scale for actual battery capacity)

13.8 charge, terminate when the current falls to 1.4 amps
14.0 charge, terminate when the current falls to 2.3 amps
14.2 charge, terminate when the current falls to 3.2 amps
14.4 charge, terminate when the current falls to 4.1 amps
14.6 charge, terminate when the current falls to 5.0 amps

If you are not too worried about a full 100% charge then any charge voltage over 13.8 volts with a charge current maximum of less than 20 amps per 100Ah will charge well over 95% capacity with no absorption period. Charge to voltage target and stop.

Mike

Good stuff! I have a Bogart battery monitor and new SOK LiFePO4. I've decided to keep them below 100% SOC most of the time, which means the battery monitor isn't able to auto-sync. No problem, I've been fully charging them every 2-3 weeks in order to manually re-sync. However, I've noticed the "old" SOC only reaches 85-90% at full-charge (14.6V). I've been using Bogart's default efficiency setting of 96% (likely for lead-acid). Thanks to this post, I'm going to try 99% to see if I can go longer between manual re-syncs requiring full charges.

Questions:
1) If a 14.6V full charge for 100Ah should terminate at 5A, would 400Ah of the same terminate at 20A?
2) Renogy tells me their DC2DC lithium settings, regardless of battery Ah or V-target, terminate at about 10% of a DC2DC unit's amp rating. For for my 60A DC2DC that would be 6A. Should I worry if I miss the 20A mark?

 

[mikefitz]

If a 14.6V full charge for 100Ah should terminate at 5A, would 400Ah of the same terminate at 20A?

Yes , assuming you were charging with 0.5 C constant current up to the point where the constant voltage occurred. If the charge current in the bulk stage was very much less that 0.5C then terminating with a short absorption time (constant voltage stage) would be less stressful.

Should I worry if I miss the 20A mark?

Perhaps not, once you hit target where the charging current is small compared to capacity, the current falls quickly to a low value.

Mike

 

[SteveSS]

Thanks, @mikefitz . My DC2DC charges at 0.15C. Do similar guidelines (or rules of thumb) exist for charge rates other than 0.5C?

 

[mikefitz]

exist for charge rates other than 0.5C?

its somewhat difficult to quantify. You could carry out charge and capacity tests under various applied voltages and currents to establish the optimum.
I have only carried out limited tests but my results agree with published/internet similar tests. The cell manufactures quote full charge is reached when charging at 0.5C to a target of 3.65 volts and maintaining an absorption voltage of 3.65 per cell until the current falls to 0.05C.
I found that when charging with low currents compared to capacity, once the battery reached the target volts, 14 volts or over for a 12v battery, 3.5 per cell, the capacity was within a few % of maximum.
For most hobby applications it would seem 'a good idea' to terminate charge at target volts in this situation, or to add a short duration constant voltage, absorption time, 'just in case' and/or give more balance time for cell balancing.
General opinion suggests that having the cells at high voltage for long periods or charging at high current, reduces service life.

Cell balancing may be an issue with some batteries that require a longer time with the cell voltage over 3.4 v , ( the typical voltage at which the BMS enables balancing). In practice with low charge currents the cell will spend a longer time above 3.4v before the charge target volts is reached, so additional absorption time may not be needed. There seems to be a general trend among quality 'ready built' battery manufactures to recommend a short absorption period rather than the longer duration previously suggested.

Mike

 

[SteveSS]

Thanks, @mikefitz , that clears it up nicely. I will try to keep absorb stage short, but not sweat it too much.

Your comment about cell balancing brings up another question. I have SOK lithium; their spec sheet and tech support guy both tell me their BMS does continuous cell balancing up to 14.4V. But I'm hearing from you and others that cell balancing usually (historically?) only happens above 14.x volts. Have you ever heard of continuous cell balancing? That would sure be cool. No more need for occasional full-charging.

 

[mikefitz]

their BMS does continuous cell balancing up to 14.4V.

It would seem odd to continuously balance under all conditions. Most of the popular BMS use passive balance, only under charge, and above 3.4 cell volts (13.6 for a 12v battery).

Mike

 

[SteveSS]

It would seem odd to continuously balance under all conditions. Most of the popular BMS use passive balance, only under charge, and above 3.4 cell volts (13.6 for a 12v battery).

Mike

It seemed odd to me, too. I'll try posting the question in related topic area to find out if anyone else knows about it.