Re: Most efficent way to keep battery charged with solar panel ?
From: Michael Anton (manton_at_nocompusmart.abspam.ca)
Date: 01/18/04
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Date: Sat, 17 Jan 2004 18:49:56 -0700
"Rene Tschaggelar" <none@none.none> wrote in message
news:6228a8f0a2b9bfbd92e3806ecf77d258@news.teranews.com...
> Anton Erasmus wrote:
> > Hi,
> >
> > I have a device that needs to run 24/7 away from any power source. It
> > can run off a 12V supply and I want to use a 12V battery that will be
> > charged during the day via a number of solar panels. As far as I am
> > aware both the voltage and current of a solar panel may change
> > depending on the sunlight intensity. What is the best way to charge
> > the 12V battery at 13.8V from the minimum number of solar panels ?
> > Some of the circuits I have seen are simply a number of solar panels
> > in series connected to the battery via a diode.
>
> Not recommended.
> When the ambient temperature drops, the open loop voltage of
> the solar panel rises. You can overload a battery this way,
> reducing the battery lifetime considerably.
> Opt for a regulator.
>
As I understand them, solar panels look just like a current source. They
can
have a high open circuit voltage up to around 19V if they are not loaded
down,
so you may want a way to stop charging the battery to avoid overcharging it.
Mostly, it is just the current that changes with light intensity, not the
voltage.
You can maximize solar panel efficiency, by deliberately keeping the voltage
high, for a given load, by using a step down switcher, that is set up to
regulate
both the input voltage, and output voltage. I did this for a design many
years
ago, and it worked quite well.
Some areas of the world do not receive much light at certain times of the
year,
so you will need to size your panel up to 40 times larger than the average
current
draw that you expect to have. Also plan on having at least 2 weeks of
standby
battery supply available, to account for times when there may be very little
light available.
Mike Anton
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