A sunshine lesson in TRNP

The north unit of Theodore Roosevelt National Park (TRNP) is disconnected from the south, separated by 70 miles of rolling terrain near the edges of North Dakota’s current oil boom.  There’s a strange confluence occurring in the area between the yellow hayfields and the gray derricks, trucks, and tanks that represent the rush to plunder the oil reserves below.

The north unit of the park is of course protected from all that, and I am glad to say that it is beautiful and worth the trip.  Still, not many people go there. I suppose this is because there’s less “stuff” to see and do compared to the south unit, and it lacks the convenience of a nearby tourist town (the south unit has the town of Medora).  There’s also no cell phone service in the campground, and no hookups or services, so you have to be self-sustaining and OK with being disconnected for a while.  I didn’t mind.

The weather finally turned on us, soaring up to 95 degrees during our one-night stay in the north unit. Without air conditioning we had to revive some hot-weather countermeasures we haven’t dug out in years.  Mostly the trick is to limit activity, have cold drinks, deploy all the awnings, run fans, and take cold showers.  We lazed around in the trailer for a while reading books, but eventually fled the Airstream to drive the scenic road through the park and soak up some chilled air in the car.

TRNP north CCC building

In the photo you can see Eleanor walking toward a CCC-constructed shelter on a bluff overlooking the Little Missouri River.

Emma picked up another Junior Ranger badge in the morning, and then we moved on to Fort Union National Historic Site, about an hour away on the North Dakota/Montana border.

Fort Union NHP

We parked the Airstream in the Montana parking lot, had lunch, and then walked to the North Dakota side to tour this interesting reconstructed fort and trading post. In the photo above, note the unplanned comparison between the work truck of the 19th century and the work truck of the 21st century.

Airstream Montana sign Fort Union

After all four nights of boondocking I was worried about getting back online to do some work, and getting the batteries re-charged.  We had drained the trailer battery below a certain point and I wanted to get it back up to 100%.  (The following discussion gets technical but those of you who are interested in solar stuff might want to consider this.)

Practically speaking, we can collect a maximum of about 55 amp-hours in a single sunny summer day with our two 115-watt fixed-angle rooftop solar panels.  In theory such a pair of panels could collect 150+ amp-hours in eight hours of full sun, but in the real world it doesn’t happen for various reasons: clouds/haze, sun angle in the morning and afternoon, time of year, wiring losses, efficiency losses in the charge controller, battery resistance, dust on the panels, etc.

We also have a pair of 120-watt GoPower portable solar panels, which I use when we need extra charging capacity or when the Airstream’s rooftop panels are shaded by trees—as they were at TRNP. They can add another 30-35 amp-hours in real world use.  (They are more efficient than the rooftop panels because they can be angled to capture morning and afternoon sun).

It’s not the goal to recharge to 100% every day. If the batteries manage to make it to 100% that’s nice but it indicates either we didn’t need much recharge or the panels are oversized relative to the batteries (or the solar charge meter is inaccurate, which is virtually guaranteed for voltage-based meters).

Since the last 10% of charge takes a disproportionate amount of time, our goal is to get the battery to 85-90% each day, and draw it down it to no lower than half its capacity overnight. Our battery is rated at 255 amp-hours with a rated duty cycle of 50%, which gives us a usable capacity of 127.5 amp-hours.  (We can discharge it lower but if we do we’ll be buying a new battery much sooner.)

On this trip the Tri-Metric amp-hour meter was telling me that we were using a lot of power (due to heavy use of 3 vent fans and watching a movie each night using the inverter), so we had a net loss each day. By the first morning we had used 74 amp-hours (Ah) and regained 51 Ah during the following day, netting out at -23 Ah from full.

The second night we were more cautious with our power use, but still by morning the meter showed that we were down to -93 Ah and we recharged to only -64 Ah.  The third morning it was down to -104.5 Ah and we recharged to -64 Ah again.

This was an unsustainable trend. At this point the battery was well below our theoretical maximum daily charge of 55 Ah and it would take two days of full sun to get back to 100%.

I was disappointed because we have been deploying the portable solar panels in addition to the rooftop panels, to capture morning and afternoon sun.  My calculations suggested we should have gained much more power than the Tri-Metric was telling me.  But I had a theory about why, and it was confirmed the next day when we plugged into city power at a campground in Culbertson MT: the batteries took almost no charge, despite the Tri-Metric showing that they were still at -104 Ah.

The problem stems from the fact that I’ve got two solar charge controllers in the Airstream and they’re fighting each other.  There’s a built-in BlueSky MPPT controller that takes the power from the two roof panels and puts it into the battery, and the portable GoPower panels have their own controller that does the same thing.  Each controller “sees” the other one, by detecting the voltage at the battery.  The GoPower controller gets confused and backs off on charging (thinking the battery is already fully charged) which results in it putting less power into the battery than it should.

Worse, the Tri-Metric gets baffled in this situation.  It uses a calibrated shunt at the battery to measure the current going in and out. For some reason, when I plug in the portable panels it only measures some of the electrical current, resulting in an inaccurate reading.  Thus, each day I was getting an inaccurate state-of-charge report, and I thought we were low on power even when we weren’t.  In actuality we were recharging to nearly full each day!

Fortunately the Tri-Metric is smart enough to know when it is wrong.  It uses an algorithm to sense when the battery is fully charged, and resets to 100% under those specific circumstances, thus compensating for any long-term inaccuracies.  It was interesting to see it “realize” that our battery was not at 50% but rather 100%, and after just a few minutes of being plugged in the Tri-Metric conceded and ‘fessed up.

I’ve got a fix planned for this.  I’m going to install a plug for the portable panels that ties into the existing wiring for the rooftop panels. This plug will go in the refrigerator compartment. I’m also going to disconnect the solar charge controller on the portable panels.  By tying the portables into the existing wiring (rather than having them directly connected to the battery as they are now) all of their energy will go through the Tri-Metric shunt correctly and through the BlueSky charge controller.

This should give us an accurate reading and no more trouble with two solar controllers fighting to charge the battery.  I plan to make this mod when we get to Seattle and settle in for a week or two.