Patrick avatar image
Patrick asked

MPPT 100/30: Please explain how the temperature compensation works?

I found out that the temperature compensation does not work correctly. It is too high by a factor of 6! I tried out some things and found out that I need to enter the compensation value per cell and NOT per battery bank as the app is asking for.

Test situation:

  • The manufacturer of the used lead acid batteries prescribes a compensation value of -4mV/°K per cell.
  • I have 6 cells, thus I use the MPPT controller at 12V.
  • The floating voltage is set at 13.36V and the controller is in floating mode.
  • Temperature and battery voltage is measured directly at the battery through a BMV-712 battery monitor and send to the MPPT via bluetooth. They are interconnected with each other (which is a pretty cool feature anyway).

The app is asking the temperature compensating coefficient explicitly as per bank and not per cell.

Thus I multiplied the -4mV/°K per cell by 6 and get -24.00mV/°K per bank which I entered in the app.

Today the battery temperature was measured at 19°C. This is 1°C below 20°C (20°C means no compensation).

Expected result:

13.36V + (19°C - 20°C) * -0.024V = 13,384V

Observed result:

13,50V (which is obviously too high for a lousy delta of 1°K)

Then I entered the coefficient as per cell (-4.00mV/°K) and got 13,39V which is what I wanted all the time but never got.

For testing, I also tried to switch off the compensation and got exactly the configured 13.36V.

It looks like the controller is multiplying the entered value internally by 6 as it knows I am using the controller at 12V with lead acid cells (can only be 6 cells as 12V / 2V = 6).

Because of this I killed my new bank last winter as the voltage was too high all the time!!!


0°C in winter means (0°C - 20°C) * (-0.024V * 6 [6 because of the bug]) compensation

= -20 * -0.144V = 2.88V (WTF?!?)


Floating voltage: 13.36V + 2.88V = 16.24V

Absorbing voltage: 14.40V + 2.88V = 17.28V

Balancing voltage: 15.10V + 2.88V = 17.98V


I updated the MPPT to FW v1.50 today which has still the same bug. :-(

Did anyone noticed the same?

MPPT Controllersbug reporttemperature compensation
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5 Answers
klim8skeptic avatar image
klim8skeptic answered ·

Where did you read that battery temperature compensation was always based on 20 degree celsius?

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Patrick avatar image
Patrick answered ·

All parameters for a battery are given at 20°C by the manufacturers. So why should Victron define a different centre point?,%20back-up%20and%20island%20systems_rev%2013_EN_web.pdf

14. Effect of temperature on charging voltage The charge voltage should be reduced with increased temperature. Temperature compensation is required when the temperature of the battery is expected to be less than 10°C / 50°F or more than 30°C / 85°F during long periods of time. The recommended temperature compensation for Victron VRLA batteries is -4 mV / Cell (-24 mV /°C for a 12 V battery). The centre point for temperature compensation is 20°C / 70°F.

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klim8skeptic avatar image klim8skeptic ♦ commented ·

The centre point for temperature compensation is 20°C / 70°F.

Never assume anything, as there are always more than 1 way/opinion for everything.

Victron mpt temperature compensation is based around 25c (battery industry standard?).

The recommended temperature compensation for Victron VRLA batteries Victron VRLA AGM/Gel

Always consult the relevant documents.

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Patrick avatar image
Patrick answered ·

First, thank you very much for the links!

This makes this case really interesting because both documents describe the same products, the Victron gel and AGM batteries. The whole text is identical except the centre point part "20°C/70°F" vs. "25°C/70°F". But which document is correct?

I think none of it because did you check the Fahrenheit values:

20°C is not 70°F, it's 68°F

and 25°C is 77°F

So, if 25°C is the correct centre point, it should be 25°C/77°C

The document describing the settings of the MPPT chargers say only at this one point that the centre point is 25°C. If this is the case, I would assume, that also all other parametres are based on 25°C.

As a consequence, the parameters of manufacturers who base them on 20°C (like HOPPECKE, Germany) have to be converted to 25°C before entering in the settings.


Add a reference temperature setting, the entered values are based on as there are multiple (industry standards?) used in the world.

This would explain why the measured voltages are always incorrect and too high. But it does not explain, why the voltages in winter are too high. I saw 16V and more and voltages above 17.5V were logged as maximum. The destilled water went down from MAX to the MIN mark during winter (more than a litre per cell) and the bottoms were covered by 2-3mm of lead dust (the cells were brand new!).

The max voltages at 0°C and 15.08V (@25°) balance voltage should be then (delta = -25) 15.68°C but not above 17°C.

If I have time, I will do a reverse engineering using a potentiometer instead of the temperature sensor and record the measurings for each degree step.

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espen avatar image espen commented ·

the high voltage: How is this observed, and where (on the BMV or the MPPT)?

For example:

-On the output of the MPPT, the voltage can rise for a couple of second if the battery is disconnected.

-When using the BMV to measure battery voltage, the MPPT can rise the voltage up to 2V to compensate for cable loss.

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espen avatar image
espen answered ·

You have written that you have a MPPT, a battery and using an app. So you need to read the manuals on all those three for the correct setup.

In the VictronConnect manual, it clearly says ”The base temperature for the compensation is 25°C (77°F)”:

(there’s also a help-link from the app, with video explanations.)

In the manual for the MPPT, it also says where to place the MPPT if not using an external temp.sensor to get correct temp.reading. And if the internal temperature is used, its using the temperature read on the morning before the mppt itself starts to heat up (for example if its 10°C in the morning, and your batteries is getting up to 20 degrees in the evening, they still get charged with the 10°C value the whole day).

The data you have given:

Battery @20°C:

Abs: 14,4V

Float: 13,36.

Temp.comp. coef: -4mV/°K /cell

6 cells.

You then need to do the calculation to get the values needed in VictronConnect app:


Abs: 14,28V

Float: 13,24V

Temp.comp.coef: -24mV/°C

Then, this chart shows MPPT charge, temperature compensated:

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espen avatar image espen commented ·

I see now that you also have a BMV712, so then the charge voltages from the MPPT gets compensated with the actual batteri temperature throughout the day, according to the chart.

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rasch1966 avatar image
rasch1966 answered ·

Hello everyone,

I have a solar system with Victron components since the beginning of 2020. I, too, have been busy entering the correct values in the components. This is not so easy. I read this chat with great interest. It is very funny for me that aspen quotes the Victron documents and refers to 25 ° C as the basis (see also his hardcopy "14. Effect of temperature on charging voltage).

If I open the Victron documentation today, the base is 20 ° C !!!!


But in the documentation of the MPPT 100/30 the reference temperature is 25°C


A little confusing for me......

what do the Victron experts think about this?

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