[RE-wrenches] Battery Temp, SOC, and Voltage

[Ray Walters]

Hi folks;

I have pondered LVD set points to protect batteries from freezing, and 
realized I don't have enough information.
While temp compensation is fairly straight forward when charging ( 
increase charge voltage when cold, reduce when hot, manufacturer's #s 
are close to charge controller assumptions....no problem)
However, it is not at all clear what the battery voltage is at low 
temperatures, when the battery is at rest or during discharge. I've 
actually found a chart that shows battery voltage at rest for a fully 
charged battery _Decreases_ at cold temps.   (Look at voltage at bottom 
of chart)


  I know battery capacity is reduced, but the voltage actually goes 
down, while charge voltage goes up?  what happens during discharge? is 
the voltage higher or lower at low temps?
I know that it becomes complicated with discharge rate, so that higher 
discharge rates create artificially lower voltages, hence the wonderful 
LVD settings of the old Trace SW 4024, which had 3 different voltage set 
points for 15 minutes, 2hrs, and 24 hrs periods.  However, were these 
set points temperature compensated? and if so, was it a negative or 
positive compensation?

Example:  I want to keep electrolyte specific gravity above 1.15 
(roughly 40% SOC) as this has a freeze point of about 5 deg F.
So voltage for a lead acid battery at room temperature at 40% SOC is  
about 12 volts (open circuit, at rest)
Just looking at this chart, and applying the same voltage ratio to a 
battery at 40% SOC,  I'm seeing a set point of 11.8 v for a battery at 
rest at 5 deg F.  But this doesn't account for discharge rate, and I 
don't know whether to fudge voltage up or down based on discharge rate 
at low temps.
Basically I want set points that protect the battery, but aren't overly 
conservative.

Anyone have a temperature compensation chart (or coefficient) that 
applies to different Discharge rates?

-- 
R.Ray Walters
CTO, Solarray, Inc
Nabcep Certified PV Installer,
Licensed Master Electrician
Solar Design Engineer
303 505-8760

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org/attachments/20140330/ef3bbdc6/attachment-0002.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: cighdiif.jpg
Type: image/jpeg
Size: 119664 bytes
Desc: not available
URL: <http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org/attachments/20140330/ef3bbdc6/attachment-0005.jpg>