[ausev] determining 50% SOC

[Joby Wieser]

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