Determining SOC for small solar set-up.

[Andy Rhody]

I have a small solar set-up for a remote cabin. 160 watts of solar panels, Morningstar controller, Morningstar "Sure Sine" inverter, with two 12 volt Walmart 105 amp hour marine batteries in parallel. I guess that gives me 210 amp hours.

Anyway, the "Tesla" thread got me thinking about State of Charge "SOC" and wondering about how to monitor this. These are lead acid batteries without twist-off caps so it's not practical to pry the tops off regularly and check the specific gravity with a hydrometer. I did install a voltmeter.

Can I just use the voltmeter to make sure I don't ever take down the battery bank below 50%? With a 12 volt battery bank, I don't think that 50% means only 6 volts.

My inverter shuts down at 11.5 volts. I use the system sparingly and I've never taken it down to the low voltage disconnect.

http://www.walmart.com/ip/EverStart-27DC-6-Marine-Battery/16795212

[Rob Beckers]

Hi Andy,

How're the wind turbine and PV system at your house doing? Hopefully everything is working as it should!

Voltage is a poor indicator of state-of-charge for batteries, and you would have to let them sit with no load for an hour to get an actual SOC reading from Voltage. That said, it still gives an idea of what's going on, even under load. Rolls/Surrette published this graph:

1150

Somewhere around 11.8 Volt would be 50% SOC, though the Voltage really fluctuates quite a bit based on loads (and internal resistance of batteries, wiring etc.).

Another one that I can recommend is MidNite Solar's MNBCM. They give some idea of what's going on with the batteries, and it is cheap. Best of all it tells you when the batteries didn't see a full charge in the past 2 weeks, and that's the no. 1 reason people kill perfectly good batteries in short order (lead-acid batteries should see a full charge at least once a month, preferably once in two weeks, to avoid sulphating).

-RoB-

[Andy Rhody]

The grid-tie Wind turbine and PV system are working flawlessly but of course they were designed by solacity. This cabin system was designed by me. LOL!

Thanks Rob. I couldn't understand how to read the graph but the number of 11.8 that you gave me should really help me gauge what's going on.

When sun is on the panels without a load, the voltmeter may read as high as 14.4 volts. At night around 13 volts or so. With the inverter running the lights, in a short while it goes down to around 12.4 and then levels off to around 12.2 volts and stays there for hours. I should be pretty good.

http://i41.photobucket.com/albums/e295/andy47bottles/Science/a40b946b-8991-4110-8d42-8203ea05eff0_zps489f91d8.jpg (http://s41.photobucket.com/user/andy47bottles/media/Science/a40b946b-8991-4110-8d42-8203ea05eff0_zps489f91d8.jpg.html)

[Rob Beckers]

That's a good inverter! (Morningstar SureSine)..

The graph shows the Voltage per cell (a 12V battery would be 6 cells), while under load. The different lines are for different loads: There's a line that shows Voltage while the battery is being discharged in 20 hours, another line shows what happens if the current is higher, discharging the battery in 10 hours etc. For regular off-grid use most should look at the 10 to 20 hour rate, and looking at the x-axis the 50% discharge point is around 1.98 to 2.00 Volt per cell. That would work out to nearly 12 (11.90 - 12.00 Volt) for a 12 Volt battery.

In practice I've seen Voltage vary greatly, there are more losses due to wiring (lowering the Voltage a bit), and the graph really is just an indication (and made for Surrette's battery chemistry, which will be a little bit different from other brands). Helpful, but not something I would use to set the auto-gen-start with. Voltage just is not that good of an indication for state-of-charge. In your case, 11.80 Volt should be close enough for 50% SOC.

-RoB-

[Andy Rhody]

Thanks Rob, at least I have a good idea where I'm at with SOC.

Yeah, the SureSign inverter is real nice and under 300 dollars (US). I had a 2000 watt Harbor Frieght inverter which could run power tools but wasn't pure sign wave and the buzz in my AM radio and CB radio made them unusable. The SureSign is only 300 watts but that's plenty for lights and also doesn't need to have a fan so it's totally silent and fans have a failure rate. It also is on all the time on standby and wakes up when you use 8 watts or more so you just walk into the cabin and flick a light switch just like at home.