Sunday, November 18, 2012

Temperature Compensation while charging FLA batteries

If you read most of the Marketing Literature out there, advanced charges will always make a major point that they Temperate Compensate the batteries.  I spent a little time looking into what this really was about and found this white paper: Temp Comp White paper.pdf

Long and short of it:  For FLA batteries, use:  -0.028 V  per  10ºF per  2 V cell.

That is it.  For every 10ºF away from the 'standard' 77ºF  / 12v battery I need to add or subtract 0.168v from the target charging voltage.  As temp rises, I compensate the actual voltage down.  For colder, compensate up.  This is applied to both the Bulk and the Float set points.  So, the actual 'magic' becomes:

                     setPointVolts += (77-batTemp) * 0.0168 ;

I have to admit -  rather anticlimactic.

Oh, during my research also picked up a warning about stopping Bulk Charging if the battery raises over 125ºF.  When that happens it is best to stop, or worst case drop down to the Float voltages.

It actually took more coding for that then the All Powerful Temperature Compensation!


  1. Have a look at the MAX6675 (digital thermocouple amplifier) although it is relativley expensive it does all of the hard work for you!

  2. Sam, Thank you for your comment, glad to know SOMEONE is reading these things :-)

    Yes, I did look into those type of devices for the K-type thermocouples. Both I2C connected as well as ones that would pass out simple 0-5v which could then be brought into an A/D port on the Amtel CPU.

    However, as you point out, these all are expensive. Mouser wants $14.85 for the MAX6675, and IIRC even the lowest cost Thermocouples IC all were north of $6. But all had 0.1 degree or better accuracy, and dealt with all the Thermocouples weirdness and non-linearity.

    I figure that for the EGT, having an error even as great as +/- 5 degrees would be more then acceptable. (I am expecting an operational range of 500-700f) And that is why I went with the $2.85 low-cost INA220. Plus, it gave me an extra Vbat sensor that I will use to sample VBat- at the battery and thereby be able to account for any voltage drops across the Ground line!