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.
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).