Date:         Thu, 29 Jun 1995 10:25:10 -0700 (PDT)
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         David Schwarze <des@teleport.com>
Subject:      Re: gas + electric

Cetin Seren writes:
> 
> >From what I understand, you measured the current by attaching an ammeter to
> the ends of the panels -- in this case, all the potential difference being
> generated was being uswed to pump the current through the internal resistance
>of the solar panels themselves...  If I'm not mistaken, you also measured 16.2V
> open-circuit voltage.  This means that, while charging batteries, those
> 16.2 volts will be counteracted by the 12V or so from the battery bank.
> That leaves about 4 volts extra in the circuit tht's usable to pump
> charging current.  The internal resistance of the battery array is negligible,
> so almost all of the leftover 4V will be used to "pump" current through
> the solar panels' internal resistance.  I would expect, if you can measure 
> 8 amps with an ammeter attached to both ends, the charging current with
> a battery actually attached and charging will be (let's see:)
> 
>  4V / 16.2V   ~= .25 
> 
>   8 amps * .25 = 2 Amps...
> 
> So, I would expect the charging current would be more like 2 amps...

I'm not sure what's wrong with this logic, but I regularly get a 4 amp
charge into my batteries from my two solar panels (i.e. half of what
Sami has) with the voltmeter reading 14 volts or higher.  I would bet
every cent I have that if Sami hooked the panels up to a decent battery
they would charge it at 8 amps or more.  

As long as the voltage of the panel is higher, the battery will charge.
The no load voltage of the panels will go up to 20 volts in strong
sunlight - I got the 16.2 volts by multiplying the number of cells in
the panel (36) by the rated voltage of each cell (.45).  16.2 volts is
what the panel should produce under load.

> Not bad, and maybe one can greatly improve this through the use of more
> solar panels and better solar panels... 

The best (most efficient) panels I have seen are on the order of 30%
more efficient than the ones Sami and I have.  Not enough to make a big
difference, IMO, and certainly not worth four times the price.  =)

> This is also a good way of delivering power to the motor.  (that's also why
> we have high-voltage lines delivering power to neigborhoods).  

In the case of delivering power to the 'hood, it's not just a good idea,
but a necessity!  If they tried to supply 110V over a 10 mile cable it
would probably be reduced to less than half that by the time it got to
yer house.  =)

> 																The power
> is the product of the voltage and the current being delivered.  If you can
> keep the voltage high without frying your motor, then you have to deliver
> less current to do the same job -- hence smaller diameter conductors, less
> junction problems, etc...  

Good point.  Why do you suppose they didn't standardize a higher-voltage
battery (say, 24?  36?) for use in cars and things?  I suppose it was
because of the number of cells that would be needed for a singe battery.
Anyone know for sure?

-David

============================================================================
David Schwarze                     '73 VW Safare Custom Camper  (Da Boat)
San Diego, California, USA         '73 Capri GT 2800            (Da Beast)
e-mail: des@teleport.com           '87 Mustang Lx 5.0           (Da Sleeper)
http://www.teleport.com/~des       '93 Weber WG-50              (Da Piano)
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