question

(-) sign is correct. Here is Trojan's info:

https://www.trojanbattery.com/tech-support/battery-maintenance/

The (-) sign indicates an inverse mathematical relationship. If it was (+), voltage would go up with temperature.

If -2.7 is indicated, it may mean -2.7mV/°F/cell, which may be the default for °F.

Do you have a temperature sensor with it? If not, you may not want to enable temp comp as the units typically work simply by taking a "snapshot" temp when it starts charging in the morning and keep that relationship all day.

Thanks Snoobler i can see Trojan used to display the figure with a minus figure . Manual for the Blue Solar shows default temperature is C which i confirmed when i went through the settings and as my system is 24 volt and the default is 48 im even more confused because cant explain why it wont change to 5mv per cell . 0.005 x 12 = 0.060

Am i missing something ?

The post I made is from their battery maintenance instructions that's independent of model. If you look at the datasheet you provided, it has to be negative to be able to satisfy the "ADD" side and the "SUBRACT" side. Forumula:

Vcomp = Vspec + (Temp - 25°C) * (-.005mV/°C/cell)

Yes. For a 24V system, your temp comp should be -.060V/°C.

Where are you getting your temperature value, i.e., what is measuring temperature?

The setting is in mV, so you should be entering -60. I just tried this in the demo library in Victron connect, it works. You can do the same if you wish. I think the reason it's not saving is that your value isn't valid.

Thanks both . Managed to save the compensation value at its maximum -3.5 however that is still almost 30% lower than recommended by both Rolls and Troijan batteries at 0.42 mv for a 24 volt battery bank .

Snoobler , i only connected it to power so that i could set up the parameters and check out everything before installing it .

Kevgermany , there is no -60 , the adjustment range only goes from -35 to 0 to 35 so at its max setting its still only just over 50% of what's needed . I found the demo you mentioned and tried myself and it did accept -60 explaining it was for the Battery bank rather than the cell , my Mornigstar uses cells voltage which is all im used to so thats that explained .

I did just realize that the name Blue Solar is also used on the latest MPPT charge controllers that look very different that mine so that might also create problems .Bit like trying to work on a Ford Fiesta with the manual for the current model .

Thats what i dont understand , there are more batteries that would fall outside those parameters than they would cover .

Thank you. I wanted to make sure you're using an external temperature sensor. The default behavior w/o a temp sensor is undesirable.

-35 is better than nothing. You can always opt to seasonally adjust your default voltages to compensate.

I dont know how critical these compensation voltages are to the lifespan of batteries , im concerned that if they were not important then the Batterie manufacture's would not bother to give them .Charging voltages i know are one of the most important settings when setting up and Charge controller and compensation adjustments form part of the voltages .

At 25c theres no problem but any temperature either way ,up or down and the user is using incorrect settings so their batteries will either be overcharged or undercharged , for AGM either is a life shortening problem .

I cant understand what Victron could be thinking off , by imposing these limits , its defeating the basic purpose of a charge Controler which is to charge batteries as efficiently as possible while protecting them from harm so as to get the longest possible life from them .