3.14.2012 – Wednesday
With time and experience comes knowledge… err, well, at least that is the hope. In my quest for knowledge I have revisited my old charts/graphs on state of charge (SOC) and specific gravity (SG). With new research and a more precise battery meter I took some time to update my charts. I determined that this was a worthwhile use of my time because of the unique set of conditions winter presents an off-grid system. The inability to reliably recharge the battery bank to 100% in winter added to the reduced capacity of cold batteries means that using the SOC readout of the battery meter is unreliable. SG is time consuming and not useful for daily monitoring of battery status. As a result, battery voltage, the old fallback, is the most useful measure of battery status.
My first task was to collect some data on batteries. I found some reference values for battery specific gravity and then extrapolated (most references list by 10% interval: 50%, 60%, 70%… so I used some simple math to make my chart list by 1% intervals: 50%, 51%, 52%, 53%, 54%…). After a bit more tweaking I ended up with a specific gravity chart that allows me to input the temperature (1st orange cell) and then auto-calculate the temperature-corrected SG from 10-100% at 1% intervals. This was step one. With specific gravity I can accurately find the SOC of the system. With that data in hand I can compare the current voltage on the battery monitor under specific conditions and prepare to make the next chart.
For chart number two I once again looked up reference values, this time for voltage, and then compared it with our system. It turns out that the open circuit voltage matched out system quite well. Under a load I could then measure voltage and compare to the true SOC (determined by SG). This next chart shows off my findings. I highlighted the reference temperature values (blue) and then created two input cells.
- The green “input corrected V reading :: ” will correct for differences between reference values and actual system readings. Input one number, chart auto-calculates values
- The red “corrected for 20 Amp load :: ” could be relabeled for any load. This input does the same as above but now makes the voltage readout more reliable under normal use (batteries under a load). I picked a normal system load for the cabin and used it (20 Amp) for this value.
Here is the PDF file of above as well as an excel version (with some additional graphs)
To sum it all up: we use SOC on the battery monitor in summer (system reaches 100% charge often, batteries are warm, functional capacity of the battery bank is known) and use voltage in winter (system rarely reaches 100% SOC and batteries have diminished capacity from the cold).
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Hello, I have enjoyed your site. I am considering building a cabin about the size of yours in the Pacific Northwest (at 4800 ft) well away from any power. I intend to have a hot water heater, stove and generator fueled by propane. I am considering 4 6v golf cart batteries and an inverter to run the electrical (primarily lights and laptop) when the generator isn’t running. Can I expect to power 4-6 LED lighs for a meaningful period of time on the batteries and if so, will I be able to top them off in a matter of hours? I hear much about inverter loss, does that play into this much? If so, in your view would I be better off just wiring the lights 12V to avoid inverter inefficiency? I suspect that during the cold winter (when I will likely not visit frequently) I will store the batteries back in my city garage as I won’t be able to keep them charged and prevent freezing. Perhaps AGM rather than acid lead would be a preference to avoid that? Just curious as to your thoughts, if you have a few minutes to share any. Thanks for writing the blog.
First, batteries and freezing – I have a great graph and chart for this that I should post soon. To sum up, a fully charged FLA needs to get below -93°F and at 50% needs to drop below -4°F to freeze. As far as inverter loss it appears that for low loads it may become significant. Our inverter uses about 30-50 watts to “idle” – so for one light or a battery charger we aren’t coming out ahead (that’s why we have the 12V outlets and charging station for our AA batteries and phones). Smaller inverters will use less power at “idle”. I’m not sure if there is an advantage to AGM vs. FLA for cold weather performance. You’ll have to read the manuals for the inverter of your choice for efficiency data. So far I’ve found AC LED lights to be more reliable than DC powered LED lights (2 burned out already – some sort of failure or design flaw I suspect). For run-time numbers you’ll need to crunch numbers and familiarize yourself with amp hours, watts, volts, and SOC. I consider 50% SOC time to recharge. Cheers – SW.