# question

## MPPT 75/15 over charging battery

Hi

I have a 200 watt panel with a MPPT 75/15 controller to charge 2 x 90 AH batteries in parallel and I set up the the user defined voltage for absorption 14.6 for 6 hours, float at 13.6 volts, however both my MPPT and BMV712 are reading voltage above the 14.6, especially when we are having a sunny day!, I have re set both items, but it’s still happening, any ideas please?

Thanks

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is it possible that it's cold where the setup is mounted? (temperature compensation makes the voltages higher when it's below 20 degree's C)

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Sounds like it is a result of temperature of compensation.

When temperatures get below the MPPT baseline temperature of 25°C (77°F), you will see higher charging voltages for Absorption and Float than the user defined values.

https://www.victronenergy.com/live/victronconnect:mppt-solarchargers/#temperature_compensation

In case you're not familar with it, this link may also be of help.

https://www.sunwize.com/tech-notes/battery-temperature-compensation/

Another thing, some batteries may require a different setting than whatever the preset value on your charge controller is.

So first, you will want to check the spec sheet for your battery and find the temperature compensation coefficient. My battery was listed as +/- 3mV / °C per cell.

When changing the value on the MPPT, it will display, "The configured coefficient is in mV per degree Celsius for the whole battery bank, not per cell".

Your battery specification may give you required the mV per degree Celsius for the cell, so you'll need to do the math for the whole bank.

Example:

My 12v battery required -0.003V/°C/cell x 6 cells = -0.018V/°C

OR

My 12v battery required -3mV/°C/cell x 6 cells = -18mV/°C

My charge controller preset value is -16.20mV/°C so I had to change that to -18mV/°C.

Summary

For my MPPT, the Absorb voltage setting is set to 14.5V (MPPT baseline temperature of 25°C).

However, today my temperature is at -3°C, so it is charging in Absorb at greater than 14.5V, I think it was between 14.8V and 15.00V as a result of temperature compensation.

So that might be why it charges at a higher voltage.

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Many thanks

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