One popular idea is to add a group of solar panels above the vehicle to capture energy during the day as the vehicle sits idle in a parking lot.  If you have the money, it’s not a real bad idea.  The assumption, of course, is that you will have unobstructed sunshine during the day over the vehicle.

 

Let’s look at some numbers to see how this might work.

 

Searching the Internet, I found BP Solar photovoltaic (PV) panels (BP Solar SX-170) for $800 each.  Each panel is rated as 24 V nominal and produces a peak voltage of 35.4 V and a maximum current of 4.8 A in bright sun – that’s 170 W max.  For these panels, you should use 1 panel for every 4 batteries (4 X 6 V = 24 V nominal).

 

In my case, I have 16 batteries.  So, I need 4 panels.  The cost of each panel is $800 making the total cost for the panels $3200, then add shipping.  Each panel is 62.8” long and 31.1” wide.  Laying them side by side, the total dimensions for the 4-panel array are 62.8” by 124.4” (~10.5 ft.), which will cover the bed and cab of my truck, so there isn’t really any room for more.

 

Because the panels will be wired in series, the peak array current will only be about 4.8 A.  A simple and efficient charging circuit will be needed to control the charge voltage.  The peak voltage of the array, in bright sun, will be about 142 V, so my charger will have to limit the charge voltage to about 120 V max.  In my case, I can actually wire the PV array into my onboard night-time charger and have it do the job for me.

 

I should be able to charge at about a 5 A rate.  Over a 9 hour period, that gives me a charge energy of 120 V X 5 A X 9 hrs = 5.4 kWhrs (in fairly constant sun).  I need about 5 kWhrs of energy to replace what I use getting to work (8.7 miles).  As you can see, in 9 hours of bright sun, I will fully recharge my batteries.  On cloudy or rainy days, it won’t happen.  Also, if I lived even farther from work – well, forget about it.  In the winter, with short days, forget about it.

 

Bummer!  The bottom line for me is – it’s not worth it.  The panels are too expensive and they produce relatively little power compared to what I need.

 

- Mark