State of charge formula [RE-wrenches]

Carl Emerson, Free Power Ltd. freepower at freepower.co.nz
Mon Jul 7 15:02:48 PDT 2008


Hullo again,

I think I posted this at a bad time just before the Independence Day
holiday.

Comments needed on a simple SOC formula given that we monitor power into and
out of the bank. Is this harder than it seems to get say 5% accuracy ??

Thanks,

Carl E.


==================================

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


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