I've learned A LOT about solar panels and batteries.  Too much to write here.  But basically, my goal has been to put together a power system as cheaply as possible without sacrificing too much in efficiency.

One large expense for most solar setups is the solar charge controller.  Usually you want to use an MPPT charge controller for max efficiency, but these can be quite expensive.  Also, for this particular application, the size of the charge controller becomes important, as it could easily be the largest single electronic component, and I really don't have a lot of room for it.  Where an MPPT charge controller is really essential is when the solar panel voltage is much higher than the battery voltage.  So, at the recommendation of a friend, I am trying to get rid of the charge controller by bringing the battery voltage up to the level of the solar panel voltage.  It just so happens that a five-cell LiFePo4 battery's voltage is just about the same as the Renogy solar panel's maximum power voltage.

The behavior of a solar panel connected directly to a battery is similar to your typical CC-CV lithium battery charger: a lot of current until the battery gets close to fully charged, and then the voltage rises while the current decreases (the solar panel will put out less current as the voltage rises).  TO put numbers to this, the Renogy solar panel's max power voltage is 17.7 volts (depending on temperature) and the 5-cell LiFePo4 battery's nomial voltage is 16 volts and its fully-charged voltage is 18 volts.  So my first power system consisted of nothing more than the solar panels directly connected to the batteries.  But there were two problems.  The first is that, while the solar panel current output drops off drastically as voltage increases, it doesn't drop to zero until the battery voltage is about 21 volts.  That's 4.2 volts per cell, a little too high for these cells.  And that's assuming that all the cells are perfectly balanced, which brings us to the second problem...

The second problem is that with multiple cells in series, there will be one cell that reaches capacity before the others, so its voltage will skyrocket before the other cells reach capacity, damaging that cell.  So instead of all the cells reaching 4.2 volts, you might have four of them reaching 4.0 volts and the fifth reaching 5.0 volts, WAY too high for these cells.  This problem can be minimized by starting off with the cells being "balanced," but they'll never be perfectly balanced.  And even if they start off perfectly balanced, they won't be that way after several charge-discharge cycles.

So it becomes necessary to add a battery balancing circuit.  There's a lot of these circuits out there, and they usually perform other useful functions such as high voltage cutoff, low voltage cutoff, high and low temperature cutoff, high current cutoff, etc.  Unfortunately, it's difficult to find such circuits for 5-cell batteries.  And they usually expect a certain input voltage.  In the case of the circuit that I found, it expects an 18-volt input voltage, not the 21 volts that the solar panel can potentially provide.  So I added a linear voltage regulator with the output voltage set to 18 volts.

The one thing I'm still missing is a blocking diode to prevent the loss of energy from the battery forcing current backward through the solar panels at night.

Up until now, the strut that connects the thruster pod to the hull has been a bit clunky.  It was a rectangular piece of G10 (a type of high-quality fiberglass laminate) with a floppy fiberglass fairing loosely wrapped around it.  So I'm replacing that with two thin, streamlined carbon fiber struts in tandem.  A previous post showed how I made the plug for the struts and the rudder.  Now I'm going to make molds off those plugs.

First I took the plugs and surrounded them with material to put the mold parting line at the right place.  Notice also the little clear plastic stick-on drawer bumpers.  They will make perfect keying features in the molds.

The molds were then laid up over the plugs, starting with a layer of thickened epoxy as a gel-coat and then several layers of fiberglass, then bonded to a sheet of wood to stiffen.

With the molds ready, I had to decide how much carbon I was going to squeeze into the molds.  I aimed to fill the airfoil section of the struts 80% full with carbon.  In other words, I calculated the cross-sectional area of the airfoil section, multiplied by 0.8, then divided by the thickness of the carbon fiber to get the total width of carbon fiber.  Then I did a test-fit of that much carbon fiber to see if I could get it all into the mold.  Well, surprise surprise, no luck.  I ended up getting only 60% or so in there.

I wetted out the carbon with epoxy, cut it into strips of various widths, and laid half of them in one mold half and half in the other.  The first layer was actually a layer of bidirectional carbon cloth, but then everything else was unidirectional.

And here's the finished result.  When I make composite parts, it usually takes at least two or three tries to get a useable part.  But this time I nailed it.  Looks really nice, although it still has quite a few voids on the surface.  Next time I'll brush some raw epoxy on the surface of the mold before laying in the carbon.

The plan is that one of the struts will have wires going down it (power and signal wires to the thruster pod) and one won't.  This is the strut that doesn't.  When I make the next one I'll implant a cable in it.  That'll be interesting.