State of charge formula [RE-wrenches]

[Todd Cory, Mt. Shasta Energy Services]

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Kid of steeling this thread here, but I do have a gripe. One of my 
biggest problems with the new Xantrex XW system is their lack of 
attention to including an accurate amp hour battery state of charge 
meter. This system is designed to be used with batteries and the 
elephant in the living room (an amp hour meter that communicates with 
the changer controller) still has not been addressed.

How often have us grid tie with battery back up installers been 
contacted by a customer that calls to say the amp hour meter says the 
batteries are not charged and what can they do. This happens when the 
charge controller and amp hour meter see different "full battery" 
characteristics and get out of sync with one another. Yes, Outback has 
their flexware meter but their grid tie inverter only has one input so 
it is off my list of acceptable devices to use for grid ties now that 
that XW is available.

Xantrex folks, is this even on the drawing table and if not why not?

Todd




boB Gudgel wrote:

>Hi Carl...
>I would think that the normal equations might be OK if you use the 
>actual capacity of the battery bank as it is now, (4000 X 0.8), as your 
>input.  Maybe you can determine the actual capacity and make the numbers 
>work.
>
>Normally, for SOC calculations, Amp-Hours taken out and A-H put back in  
>are used. (numbers multiplied by the proper inefficiency abuse constants 
>attributed to the famous dead scientists they are named after of 
>course)... Or, ~around~ 95% efficiency for AH methods of SOC 
>calculations, although your eff. might be lower than that.
>
>The problem is knowing how many Amp-Hours (or even Watt-Hours) are going 
>in and out of the battery itself, and not having the load A-H get in the 
>way, which will happen when the CC is looking at the whole system and 
>not just the battery Guz'inta and battery Guz'outa.
>
>If you can get to within 5% of what a Ralph-O-Metric reads, you're doing 
>very good.
>  
>


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