[ausev] determining 50% SOC

[Marc Kohler]

Technically you have to wait 24 hours or so to get a true OCV (open circuit
voltage) reading versus SOC (state of charge).
But who has the time?
I've always liked voltage under load since that's what we have while we are
using it.
I don't have a "Voltage under a certain load vs SOC graph" handy, but try to
keep your pack above 42v (5.25V per battery) under your normal driving load
(100A?).
Cruise around Trojan's website (http://www.trojan-battery.com/)
I believe they have some sort of SOC under load information.
Marc

-----Original Message-----
From: ausev-bounces at austinev.org [mailto:ausev-bounces at austinev.org] On
Behalf Of Joby Wieser
Sent: Thursday, November 20, 2008 6:32 AM
To: ausev at austinev.org
Subject: [ausev] determining 50% SOC


The discussion of how cold weather affects batteries prompts a few questions
I have been pondering as I drive, recharge and watch the volt meter.

My main question is how I operationally know when it gets to 50% SOC.   I
understand that when it gets down to nominal voltage then it is 50%
discharged.  The problem is that from the moment load is removed the voltage
slowly increases.  I have also observed that in the first 6 miles or so the
recovery time is very quick (a few seconds).  As I drive past 6 miles the
loaded voltage drops lower and lower, and it may take a few minutes after
load is removed to recover.  

For reference my little truck weighs about 2000 pounds and is currently
powered by 8 T105 type 6 volt batteries giving me a nominal voltage of 48
volts.  Typically in the morning after being fully charged I start out with
52 volts before applying load.  During the first 6 miles, with a 100 amp
load the voltage stays above 45 volts, with 200 amps it stays above 42 volts
and when I remove load it pops back up to 49 volts or more in less than 5
seconds.  As I continue to drive a 100 amp load may lower the voltage to 38.
I have seen 300 amps drop the volts to as low as 32.  When load is removed
the voltage may start out as low as 41 volts and take 5 minutes to recover
to above 48 volts.  The performance of the truck at this level of discharge
is reduced but not substantially.  

My question is how long do I wait to check the voltage to determine if I
have gone past 50% SOC, or is there a better way to determine this, perhaps
checking the voltage at 100 amps?  


Joby Wieser
Fredericksburg



      
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