State of charge formula [RE-wrenches]

[boB Gudgel]

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.

boB



Carl Emerson wrote:
> 
> Hi there gang,
> 
> It is a while since I have posted.
> 
> I have just finished a remote cellphone site for Vodafone here in NZ.
> 
> We installed an Eoltec Scirocco 6 kW wind turbine and 2.1 kW of Canon 
> thin
> film solar panels.
> 
> You can see the site live at...
> 
> http://view2.fatspaniel.net/Vodafone/HostedAdminView.html?&eid=103633
> 
> Now we have used Fatspaniel to monitor the site and they have done a 
> great
> job. However have a 4000 Ah AGM 48 volt bank and I would like to monitor
> state of charge, and this function, Fatspaniel have not offered.
> 
> We could have installed a Bogart or similar but we want to derive the 
> SOC
> from the data available.
> 
> The parameters we have access to are...
> 
> Cumulative Load
> Cumulative total power generated
> Battery temperature
> Full charge condition (from MX60)
> 
> Data is recorded every 15 minutes with min max and average for the 
> period.
> Now the bank is not new in that it has been in storage for several 
> years, a
> cap test revealed that the worst cells were functioning at about 80% of 
> the
> rated capacity.
> 
> The question is can I estimate, to say within 5% accuracy, the state of
> charge with a formula like (4000 - load + power) * 0.85 (to allow for
> efficiency) and express as a percentage.
> 
> Am I dreaming or will this or a similar formula give reasonable accuracy 
> ??
> 
> Appreciate any comments,
> 
> Carl Emerson
> Free Power Ltd.
> Auckland NZ
> 
> 
> 



boB Gudgel K7IQ
MidNite/Magnum  Solar/Energy
Washington AC


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