From: ham789@netzero.net   
      
   On 7/19/2014 10:08 PM, danny burstein wrote:   
   > In  mike  writes:   
   >   
   >> sorry, but I have more questions than answers.   
   >> Most importantly, why bother???   
   >> Costs you a nickel to charge it.   
   >> say 26 charges/year, that's $1.30/year in electricity.   
   >> How much can you build for $1.30?   
   >   
   > The area being mowed is pretty far from the building and   
   > any outlet. Moving it back and forth is a hefty pain.   
   >   
   > I'd like to just have a cheap shed for weather protection   
   > around it and use solar panels.   
   >   
   > If I figure on needing 200 watt-hours for a charge (full   
   > nameplate is 450...), then 75 watts of solar panels   
   > should be adequate if there isn't too much loss.   
   >   
   > (rule of thumb: solar cell output times four gives   
   > you the nominal daily output).   
   >   
   >> Stabilizing the input is harder than regulating the output.   
   >   
   >> You're gonna need a custom design.   
   >   
   > The panels should, depending on sunlight and   
   > the battery state, give me very roughly 60 volts   
   > of DC output. (four or five 12 Volt panels in series)   
   >   
   >> Solar panels are expensive.  Use all you can get out of them.   
   >   
   > At five dollars/watt, 75 watts worth would be $400.   
   > Throw in another hundred for an MPPT controller, and   
   > we're at very roughly $500.   
   >   
   > Much cheaper than running a power cable the 500 or so feet.   
   >   
   I did a similar installation design for a mountain-top repeater.   
   Also did sun tracking, but you don't need that.   
      
   Basically, you have to examine your local insolation data.   
   At the worst time of year, you only need to accumulate   
   200 Whr of energy between mowings.  That tells you how big   
   your solar panels need to be.  I'm guessing that it's  a lot   
   less than 75W.   
   Start with 12 or 24V panels.  Use a microcontroller   
   to build a boost converter to get up to the 56V you need.   
   Dither the duty cycle keep the charge current at maximum,   
   that's the mppt part.  Stop the charge when the target voltage   
   is reached.  Since it's only code, it's not hard to step the   
   voltage down if you want.   
      
   There are losses everywhere in the process, so you'll need   
   to scale it up to cover those losses.   
      
   The advantage of this technique is that it will provide charge   
   under a very wide range of light levels.   
      
   I have no idea the details of your situation, so I'll make up   
   some numbers.   
   If you mow every 7 days, you need 29Wh/day and using your   
   4x number, that's 7.5W of panel. YMMV...Your Mowage May Vary.   
   Multiply by some number for losses, bad weather etc.   
      
   Or, you could just series  6 12V 2W panels and use a big zener   
   diode to clamp the voltage at 13.6V/battery and be done with it.   
      
   All the fancy stuff is great for maximizing use of solar.   
   You don't seem to need any of that.   
      
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