Ray, I have never considered the effect of battery voltage at low temperature under load, only capacity loss, so you got me thinking. Here's a simple idea: why not put a battery in a freezer with your expected load and monitor voltage, or better yet, chart to see where it lands. I would be doing this test right now but I don't have a freezer at the shop.
Larry On Mar 30, 2014, at 12:26 PM, Ray Walters <r...@solarray.com> wrote: 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) <cighdiif.jpg> 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 _______________________________________________ List sponsored by Home Power magazine List Address: RE-wrenches@lists.re-wrenches.org Change email address & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out participant bios: www.members.re-wrenches.org _______________________________________________ List sponsored by Home Power magazine List Address: RE-wrenches@lists.re-wrenches.org Change email address & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out participant bios: www.members.re-wrenches.org
