Home-made SOC gauge

cptdondo
cptdondo Solar Expert Posts: 105 ✭✭✭
in Solar Beginners Corner #1
I have a solar setup in my RV. I also have a lot of hardware left over from other projects; enough to make a really nice monitor for the voltage, current, and temp of the batteries.

I am looking for some software examples on how to calculate the approximate SOC based on monitoring voltage, current draw/charge and battery temp. I know about Peukert and I've found various ways to use it to approximate the SOC. I can't help but think someone has published the source to a program that does this, though....

Any suggestions?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,660 admin
    #2
    Re: Home-made SOC gauge

    The most information I have seen on this subject is on the SmartGauge website:

    http://www.smartgauge.co.uk/technical1.html

    And, there is another voltage only gauge from Midnite Solar:

    http://www.midnitesolar.com/products.php?menuItem=products&productCat_ID=27&productCatName=Battery%20Capacity%20Meters

    And here are some graphics about SOC and various battery voltages:
    New poster "leaf" has a really nice set of charts that compare battery voltage against different rates of discharging and charging (as well as resting voltage readings).
    leaf wrote: »
    Am trying to upload the charts I am using...

    Attachment not found.Attachment not found.

    attachment.php?attachmentid=3655

    attachment.php?attachmentid=3654

    I don't quite a agree with the resting voltage line (I think the voltage is a bit low)--But it shows how to estimate a battery's state of charge while operating.

    Note, where the charts "flatten out"--the room for error estimating state of charge is pretty high.

    That is the basics I am aware of...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    #3
    Re: Home-made SOC gauge

    I'm trying to do the same thing. So, I made a lot of observations on my batteries.

    When charging, this is absolutely unreliable. Probably +-30% is the best you can do.

    When discharging, you can more or less predict SOC. However, there's a "memory effect". When a load is suddently removed, it may take few minutes (if not longer) to restore the "correct" voltage level. When a load is applied, the voltage drop may be gradual and the voltage may dip further than it should. So, you need to be creative, monitor load history and do appropriate adjustments. You can use curves that BB has posted, but most likely you will need to adjust them to your particular batteries.

    I decided to relay more on AH-counting, which will give better results. When you do not get to fully charging state very often, AH-counting is likely to go haywire. For that, I will use SOC predictions to detect when the batteries are too low. I haven't written the software yet.
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