Solar Charging details

So, one of the first problems we ran into with the RV was a steady drain on the engine battery. One of the main reasons for getting the RVwhisper all setup was to monitor this data remotely and see if it was real or just typical lead-acid battery flakiness.

After replacing the batteries to eliminate old batteries as a cause, I started collecting data, and noticed the following pretty quickly. Here’s a graph of the House battery (2 6V Batteries in series) and the Engine battery.

(Green in House Battery, Red is Engine Battery, grey bars indicate nighttime so you’re seeing the voltage over about a week).

Nothing was on or running during this period, the RV was just sitting in storage and the RVWhisper & Modem were plugged into a portable battery (another article). You can see pretty clearly that there’s a substantial drain on the engine battery here, dropping about 0.3V in a week compared to the House Battery.

To some, this is a known problem. Lots of things in the RV continue to run when it’s “off”, things like smoke detectors and such. To avoid showing up to a dead battery, many people recommend disconnecting the battery. That seemed a bit extreme to me, so I moved onto the next solution: Solar Suitcase.

Solar Suitcases are popular in the RV world for this very reason: Small portable solar panels with the charge controllers and all integrated, that you can hook up via giant gator clamps to your battery and provide some offset current during the day. After a bit of research, I went over board and bought a big 200 watt/20 amp Renogy panel.

With that hooked up, I’ve never had any more battery problems. But I did notice a new interesting quirk in the charging graph, that looks like this.

Every day there’s a swift uptick in voltage to a peak just under 14.5V, then it falls back to just under 13.5V to spend the rest of the day, before falling back down to 12-and-change at night. What’s going on here?

I suspect that the first 14V spike is the actual charging step. Measure a running car battery with the alternator going and you’ll read something similar. This is when it’s actually pumping a fair bit of juice into the battery to charge it up. Given that I went overboard with a big 200W solar panel, it doesn’t take long. Then it drops to this 13.5V range, where the battery is basically “full” but it’s still absorbing a tiny bit of power to counter whatever drain is coming from the RV. Then it simply quits charging overnight when there’s no sun for the solar.

Seems legit, but can I monitor this more usefully? A bit of python and SQL query later, and I’ve added a new tool to the RVwhisper-Monitor package, `solar.py`. You can run it independently or have it added to the output of the `slope` tool, and get a chart like the following.

This shows the hours of sunlight in the day (actually seconds, I need to fix that sometime) as the faded red bar (Yes, the days really are getting shorter now). It also shows the time spent in that 14V Peak range (in green), and the 13.5V “sustained” range (in blue).

You can see that on days after a trip when I hook the solar panel back up for the first time in a while, there’s a lot of green. That makes sense, as it has to recharge from the few days of running the battery down. As things recharge over the first day or two, you see the blue begin to dominate as it’s now just having to recharge the continuous draw. And given the size of this solar panel, I can also power my Netgear Nighthawk & the RVwhisper module directly from the engine battery.

Hope you find this useful!