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Hi folks;<br>
<br>
I have pondered LVD set points to protect batteries from freezing,
and realized I don't have enough information.<br>
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)<br>
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 <u>Decreases</u> at cold temps. (Look at
voltage at bottom of chart)<br>
<img src="cid:part1.02060605.06070102@solarray.com" alt=""><br>
<br>
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?<br>
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?<br>
<br>
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.<br>
So voltage for a lead acid battery at room temperature at 40% SOC
is about 12 volts (open circuit, at rest)<br>
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.<br>
Basically I want set points that protect the battery, but aren't
overly conservative.<br>
<br>
Anyone have a temperature compensation chart (or coefficient) that
applies to different Discharge rates?<br>
<br>
<pre class="moz-signature" cols="72">--
R.Ray Walters
CTO, Solarray, Inc
Nabcep Certified PV Installer,
Licensed Master Electrician
Solar Design Engineer
303 505-8760</pre>
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