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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.

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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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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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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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Alexandra avatar image Alexandra ♦ ulfertg commented ·

@ulfertg

The difference is the battery sense is not carrying amperage it is just reading. Resistance increases under load and voltage drop happens that is why the mppt will read differently. Also, as was mentioned cable length and size, as well as strand count matter.

In any case is 0.1v a huge enough difference to cause an issue?

Also the mppt is also not a measuring instrument. So the difference may even be a rounding issue. Battery sense rounding up and the mppt rounding down.

The information given will be a pretty good indicator of what the system is doing for most users.

Have you measure the terminals yourself at both points?

Most managed batteries have systems in place to prevent any problems in any case, in my experience. This drop has not been an issue for charging there are many other factors with greater impact on system performance

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ulfertg avatar image ulfertg Alexandra ♦ commented ·

@Alexandra

Well, either I still haven't managed to explain the issue properly or I'm completely missing the forest through the trees.

In case it's the former I'll try to explain it again:

I have a MPPT 100/20 connected to solar and a battery.

I have a Smart Battery Sense connected directly to the battery's terminals.

The MPPT 100/20 and Smart Battery Sense are connected wirelessly via Bluetooth in a VE.Smart Network.

The MPPT reads the voltage and the temperature wirelessly from the Battery Sense.

It applies a voltage correction to the reported voltage of the Battery Sense as a function of the electric current delivered to the battery (V_MPPT100/20 = V_SmartBattery_Sense - 0.current(A)).

By doing this it renders one of the two functions of the Battery Sense useless: the reading of the voltage directly at the battery and hence bypassing any potential cable losses of the MPPT.


In case I'm mistaken: What am I missing?

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Alexandra avatar image Alexandra ♦ ulfertg commented ·

@ulfertg

I think I also misunderstood what you were questioning there as well. I will assume that you know why there is a difference in the voltage readings.

Do I understand correctly that you are saying that the mppt is not using the voltage reading from the smart battery sense for its charging calculations? Even though it has been smart networked?

Or that it is using it and now the mppt is charging the battery incorrectly?

My other question is which way around are the terminations on the battery, as i also know this makes a difference with resistance etc. Do you have the terminals for the battery sense under or over the mppt terminals?

I would verify what the data is showing with an actual reading with a meter. Graph reading and data are dependent on sample rates. Sometimes a meter is just better.

Just another piece of info. Most batteries with battery management prevent over charging in any case. For instance a well known brand we use has winston cells and they can be charged up to 3.85v (according to their datasheet) a cell but the bms will limit them to about 3.35v per cell. The bms will say it is 100% but in reality it is not. (I appreciate that lithium are not meant to be kept or charged to 100% - but this is off topic). But for the consumer it is 100% capacity. What I am really saying is the difference is not a problem for a well self managed bank. And if the bank is still drawing charge from the mppt then it is probably trying to internally balance and needs the power to do so, whether passively or actively balanced, so mppt's algorithm is also based on what the battery is taking the amps or not among other things not just voltage readings, or at least that is what I understand at this point.

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ulfertg avatar image ulfertg Alexandra ♦ commented ·

@Alexandra

The thing is: I believe there shouldn't be any difference in voltage reading in the first place, that's what the Smart Battery Sense is for in the system.

It should read the Voltage directly at the battery's terminal and send it via Bluetooth to the MPPT, so any potential cable loss should be left out of the equation.

But nevertheless the MPPT assumes a lower voltage (while connected with the Battery Sense) than is actually present at the terminals.


If the issue was in any way related to cable losses and resistance, the voltage reported by the MPPT should be higher than the voltage at the battery's terminal (during charging of course).

But it's lower.


And this is indeed a problem in my opinion. Of course certain BMS (not all of them) can prevent an unhealthy overcharging of LFP batteries, but I understand that to be a fail safe rather than a regular function.


And no, I don't know why there is this difference. At this point and without looking at the code I can only guess. And my guess is, that it is a software bug (you could look at my reasoning in the opening post).


Of course it still is possible that I'm completely mistaken and I'm open to this option.

But then it would be very helpful and appreciated if someone could explain the behaviour of the MPPT, which to me doesn't make sense at all.

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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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