Autie retro tech geek!

Mobile Solar: Pickup Mounted 300w Off-Grid System

I’ve been putting some form of solar on my truck for many years now.  It started with a 25w module on my 1997 Tacoma, which expanded to 150w.  Then that truck broke and I had this shiny new 2015 Tacoma with no solar.  It felt incomplete.  The first iteration was a full-size SolarWorld 285w 60 cell module with lead acid batteries.  But that setup was HEAVY.  The next version was to be as light as possible – 3 100w flex modules and lithium ion battery.  This blog details the most recent design.

Here we see the three 100w modules.  They are frameless, with no glass, and are somewhat flexible, thus extremely lightweight.  They are wired up in parallel, with each module having a series diode to prevent a shaded module from becoming a load.  You can score these on Amazon or eBay, but be prepared to pay a premium per watt compared to your standard rooftop module.

This is my “imported straight from China” 100aH 12v lithium battery pack.  It’s a Demuda (DMD) pack which I unboxed and explored in detail HERE. It’s so Chinese, it has Chinese writing on it.  You can also see here my solar combiner, top left.  This has the three modules and series diodes.  To the left middle you see the output load center, thru which all the loads are fused.

As of July 2018, the previous charge controller, a Blue Sky unit, got destroyed because of water penetration.  It had rained so much here that water finally wicked its way down and into the controller.  So, today, July 24th, I decided to roll my own from stuff I had laying around.  This DC-DC converter takes the solar input and regulates it to a perfect 12.5v, which is 0.1v less than the lithium battery’s max voltage.  I’d rather not risk a lithium fire.  The meter is on the charger side and meters the amount of energy going from controller into the battery.  I scored this stuff on Amazon for about $35 (much cheaper than a $300 Blue Sky controller).  I’ve got a diode on the output side (with the red heatsink) so the converter doesn’t become a load at night.

My reliable Xantrex Prowatt 1000w inverter, a true sine model I scored on Amazon for around $200.  That taped up mess at the top is the custom super-capacitor bank I assembled so that the inverter can handle surges.  Otherwise the lithium battery can’t maintain high enough voltage and the inverter shuts off.

The cab load center consists of a switch box like you would find in an old police car.  The 12v from the battery feeds this box, and from this box the loads are connected, each individually fused.  On top of the box with the green numbers is my cab volt meter, and on the left side is a 5-port USB power supply for tablets and phones.

Under the tonneau cover I mounted a pair of LED light bars, which makes seeing into the bed easy at night.

Also inside the cab I installed a 120v receptacle.

This is just a pic of my DIY charge controller being bench tested.  The solar was being simulated by the Makita power tool battery, while the RC car batteries acted as the lithium battery in my truck.

It figures that right after I build my DIY charge controller, I stumble across this gem on eBay.  The Chinese have been very busy making new controllers in the past year or so.  Many of them aren’t so great, and many that claim to be MPPT are just PWM.  PWM controllers are less efficient than MPPT controllers especially if the battery and PV Vmppt voltages are far apart.  However, my truck array and the lithium pack are very close, so I wanted to try a PWM controller.  Until now, I have never found a PWM controller that could have the voltage setpoints programmed.  This one can be programmed to perfectly match my lithium battery’s needs!!  And better yet, it was $46.

Another advantage of PWM over MPPT controllers is for very small systems, because MPPT controllers’ internal processor draws much more power at night, whereas PWM controllers are pretty dumb and use almost nothing.  Lastly, if you kill your battery flat, a PWM controller can often self recover better whereas an MPPT controller’s computer needs enough voltage just to fire up and start charging.