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