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Date:         Thu, 29 Jun 95 12:28:06 EDT
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         cetin@kirk.bellcore.com (Cetin Seren)
Subject:      Re: gas + electric

There's David Schwarze writes: [snip....] > > We measured over 8 amps from Sami's panels in St. Louis, and they are > not anywhere near the most efficient out there (they are the cheapest, > though!). It's possible you could fit 4 more on the roof of a Westy if > you covered the whole roof, which would yield over 16 amps! But maybe > you got the 5 amps by averaging over the whole day? >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...

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

> Do you know what voltage the motor runs off of? > > I think some of the motors run on high DC like 72 volts. This lets them > run a bunch of 6V batteries in series and not have to worry about banks > of batteries in parallel draining each other. If this is standard, you > might not be able to add just a few batteries to increase range.

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). 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... About the extra array, I had a much cruder method in mind -- like manually throwing a BIG double-throw switch to go from one bank to the other...

Cetin


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