[MLB-WIRELESS] Cray---zeeee idea.

[Hamish Moffatt]

On Sun, Jan 20, 2002 at 08:42:48PM +1100, Julian Featherston wrote:
> So here's my attempt to cost a solar power supply for 12V 290mA.

I've got just a few comments on your calculations.

> All power would be drawn from cheap deep cycle
> batteries so one would charge to Max~13.6V.
> Average charge would be ~13V say we use cheap
> Vreg for 12V supply, average drain would be
> 	>13V x .29A = 3.77W/hr

Power (V*I) is just in watts, so this figure is 3.77 W (rather than
watts per hour; watts itself is Joules per second).

General purpose voltage regulators will not produce 12V out from
a ~13V input. They have a minimum voltage drop of around 2-2.5V.
You will need a special low drop out regulator in this case.
Those are harder to get and more expensive.

And as the battery discharges, the terminal voltage will drop
and the output of the regulator will start to drop too.

> It's easier to work in days so
> 	>3.77W/hr x 24hr = 90.48W/hr per day

90.48 Watt-hours (like kilowatt-hours used by the electricity companies).

> The people from Murdoch reckon a cell will average in Aus. winter about
> twice
> its rated power output in watt hours per day.
> 	>131.129 x .5 = 65.564W

Unfortunately this doesn't leave any spare capacity for a few cloudy
days in a row. It might work if every Winter day is 50% cloudy,
but not if 50% of winter days are 100% cloudy, and they come in
groups of 3-4 days, etc.

Some radio amateurs have done calculations similar to this to
work out solar power requirements for remote repeaters (very
similar to what you're trying to do). Have a look around the
web and you should be able to find their calculations, with
some more detail about average sunlight etc.


regards

Hamish
-- 
Hamish Moffatt VK3SB <hamish at debian.org> <hamish at cloud.net.au>

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