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ulfertg avatar image
ulfertg asked

Voltage difference between MPPT 100/20 & Smart Battery Sense

Hi all,

I noticed that the MPPT 100/20 (joined with a Smart Battery Sense) reports a lower voltage than the Smart Battery Sense.

It seems to be lower as a function of the current delivered to the battery like this:

V_MPPT100/20 = V_SmartBattery_Sense - 0.current(A)

Example:

Smart Battery Sense reported voltage: 13,50V (and most likely actual V, as verified with two different measuring devices).

MPPT 100/20 reported voltage: 13,40V

Current to battery at this moment: 10A


Could this be some sort of cable loss compensation in the MPPT 100/20 (which probably shouldn't kick in while the Smart Battery Sense is connected and reporting and could hence be a bug?)?

Can it be turned off, so that the MPPT 100/20 reads the exact voltage which is reported by the Smart Battery Sense?


I bought both devices a couple of days ago and they updated their firmware when I first connected them, so I assume they are on the latest FW.

bug report
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christian-bell avatar image christian-bell commented ·

How is the battery sense connected to the charge controller? Are they bluetoothed together?

If there was voltage drop the mppt would see a lower voltage than the voltage measured directly at the battery i think. Sorry i don't have answers just trying to work through it.

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ulfertg avatar image ulfertg christian-bell commented ·

Yes, they are connected in a VE.Smart Network via BT.

The MPPT should receive the voltage from the SmartBatterySense and work with it, but apparently it calculates a current dependant voltage drop into it.

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4 Answers
ulfertg avatar image
ulfertg answered ·

To sum it up: today I received a beta Firmware which eliminates the bug.


While I'm happy that I received a solution, I must say it was a very time consuming and frustrating journey to get there.

During the last couple of weeks I wrote mails back and forth with support.

I explained the issue over and over again.

Wish there was an easier way to report simple bugs like this.

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rpetrovski-forum avatar image rpetrovski-forum commented ·

what firmware version did you get? For which gizmo? I'm having similar troubles with 4.03 BMV712 and 1.50 MPPT100/20. At the moment they're supposedly talking via DVCC but I think there was the same issue when I used VE.Smart via bluetooth group.

Also, Despite being configured to go into absorbtion (see time markers) at 14.1V, MPPT did that at 14.2V which at the time was 14.3V on BMV-712.

I guess I can live with it, but looks extremely lame on Victron side.

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ulfertg avatar image ulfertg rpetrovski-forum commented ·

I got 1.52_A05F for MPPT 100/20.

I guess / hope that the bugfix will be incorporated in future FW releases.

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rpetrovski-forum avatar image rpetrovski-forum ulfertg commented ·

FYI, the 1.53 rolls back that change. No idea why, but once you upgrade Victron Connect, you won't be able to change settings unless you update the firmware. Current is 1.54


v1.53 – 06 October 2020

Released only for all models.


Changes:

  • Remove improvement, added on v1.52, of the remote voltage sense compensation to speed up filtering. That improvement is postponed for now.


Bug-fix:

  • Text messages now always sent every 2 seconds and no longer interrupted when a hex message is received. That could reduce the risk of the MPPT control to get unresponsive when on the “connecting”, as well as the “fetching history” screens.
  • History total data now always announced together with the history day0 data reducing the risk of the MPPT control to get unresponsive when on the “fetching history” screens.


v1.52 – 15 September 2020

Released only for all models.


Changes:

  • Include low power mode for the small models. Idle current consumption (when PV is around 0V) is reduced by around 70%.
  • Improve remote voltage sense compensation to speed up filtering when necessary and improve voltage control response.


Bug-fix:

  • Fix remote voltage compensation for small models. The voltage compensation of small MPPTs did not consider some current depended voltage drops, causing the MPPT to have a higher voltage setpoint, up to 200mV, depending on the MPPT current. This FW version fix that.


Add new models:

  • Include support to BlueSolar 150/45 rev3 and SmartSolar 150/45 rev3.
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ulfertg avatar image ulfertg rpetrovski-forum commented ·

I just checked my devices (MPPT 100/20 FW1.54 & Smart Sense), while charging with 8A they both report the same voltage (previously the MPPT would report an 80mV lower voltage while charging with 8A)

As far as I understand the change-log Victron removed:

Changes:

  • Improve remote voltage sense compensation to speed up filtering when necessary and improve voltage control response.

in FW1.53, but not:

Bug-fix:

  • Fix remote voltage compensation for small models. The voltage compensation of small MPPTs did not consider some current depended voltage drops, causing the MPPT to have a higher voltage setpoint, up to 200mV, depending on the MPPT current. This FW version fix that.

I have to admit though that their description of the bug fix seems misleading to me. It would make more sense to me if they had written something like: "The voltage compensation of small MPPT's did falsely calculate some current depended voltages drops while receiving the accurate voltage readings remotely ".


Side note: thank you for making me aware that there is a Victron Professional sub section which provides change logs and other useful information.

Didn't discover that previously.

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Kevin Windrem avatar image
Kevin Windrem answered ·

If the voltage drop varies with current then it is most likely due to voltage drop in the wiring from the MPPT. 0.1 volts @ 10 amps means there is 0.01 ohms in that circuit. You can minimize this drop by either shortening the distance between the MPPT and battery or increase the wire size. 10 feet of #12 wire (5' distance to the MPPT) is 0.016 ohms.Upping this to 10 gauge drops resistance to 0.0 ohms (50% less resistance). With 8 gauge, resistance is only 0.006 ohms.

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ulfertg avatar image ulfertg commented ·

But this is exactly the reason why there is a Smart Battery Sense in the network: to read the exact voltage directly at the battery, without any drop due to the resistance of the wiring from battery to MPPT.

If it was the issue you describe then the voltage reported by the MPPT should be higher than the voltage at the battery (and without the Smart Battery Sense and thin/long cable it is). In my scenario it's the exactly the opposite, the MPPT reports a lower voltage than the Smart Battery Sense reads directly at the battery.

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ulfertg avatar image
ulfertg answered ·

Is there any way to directly contact Victron about it?

It really looks like a software bug to me.

The way it's implemented now makes voltage analysis worse with the Smart Battery Sense in the system.

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Leslieanne avatar image Leslieanne commented ·

Hi, I can confirm what you are seeing and have reported exactly the same issue with mppt100/20 and BMV in smart network configuration. I have described it in great detail here:

https://community.victronenergy.com/questions/33706/smartsolar-10020-showing-incorrect-low-battery-vol.html

This problem appeared with the major update around 1.39 firmware. Before that the mppt would switch to absorb too early, after the update would switch at too high a voltage even though in the app it was reporting the correct voltage.....so unless you looked at the reported BMV voltage at the same time carefully and/or measured things you would not notice!!!!

Definitely a bug. No response from Victron at the time, may have just gone under the radar. I simply do not use the smart network anymore for the sake of my lithium batteries!!

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Leslieanne avatar image Leslieanne Leslieanne commented ·

https://community.victronenergy.com/questions/33706/smartsolar-10020-showing-incorrect-low-battery-vol.html?childToView=33732#comment-33732

Last link seemed not to work. This was from 11 Nov 2019

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ulfertg avatar image ulfertg Leslieanne commented ·

Thank you for confirming this issue.

And also helping me understand why there are not much more reports about it - it must be much harder to notice when the MPPT is hooked to solar.

I connected it to a programmable power supply for testing purpose.

Because of the stable power input to the MPPT it was quite easy to see the difference between the reported voltages. And to understand the linear dependancy of the reported voltage drop to the amount of electric current flowing to the battery.


Now we only need a way to make Victron aware of it.

If my theory is correct this is probably easy to fix: the assumed "voltage drop due to cable loss compensation" subroutine should not be called if there is a voltage reporting device present in the network which reads directly from the battery.

Victron, can you hear us?

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wkirby avatar image
wkirby answered ·

Hi

I have a 100/20 and a Smart Battery Sense. All have latest firmware.
I did manage to replicate your problem in my setup. Sorry for the poor photography.

After some time this does settle to the correct Voltage I have noticed.

Victron are already aware of this issue.


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ulfertg avatar image ulfertg commented ·

Thank you for confirming this issue.

You wrote that it did settle to the correct voltage after some time. Could it be that the battery was full then and there was no significant current to the battery anymore? This would confirm my theory of the erroneously applied current depending voltage compensation.

Since when is Victron aware of this issue? I'm asking because I'm still in the return window of my Smart Battery Sense unit and I'm trying to figure out if I better send it back. Since it makes the charging process worse the way it is programmed now.

If there is any chance that Victron is going to fix it in about the next couple of weeks I'd love to keep it though.

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wkirby avatar image wkirby ♦♦ ulfertg commented ·

I did have the battery full in absorb stage, but I also had a load on it, so the MPPT was still charging with 10A at the absorb Voltage. After some time both MPPT and SBS were showing the same Voltage of 14.7V.

This problem has been identified with some MPPT's, it's not the Smart Battery Sense. I have a different MPPT and another smart charger working with the same SBS, no problems.
I wouldn't know the timescale, but I understand that they are currently working on a firmware update.

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