Most efficent way to keep battery charged with solar panel ?

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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.

Regards
   Anton Erasmus


Re: Most efficent way to keep battery charged with solar panel ?
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try sci.electronics.design.

Re: Most efficent way to keep battery charged with solar panel ?
snipped-for-privacy@p.o.b.o.x.com says...
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What you are looking for is a power point (or maximum power point)
tracker.  I thought they were fairly common for larger solar
installations.  Try a search on those terms, you should find a fair
amount of material.

Robert

Re: Most efficent way to keep battery charged with solar panel ?

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Thanks,

" power point tracker"  seems to be the magic search term for this
application.

Now if someone could supply me with the magic term in finding ultra
miniature electronic hour meters, it would make my day. (The
"miniature" hout meters I have found are approximately  50mm x 40mm x
20mm ). I can understand that the mechanical ones need to be 50mms
deep, but surely someone has electronic ons that are not more than
20mm deep ?

Regards
   Anton Erasmus


Re: Most efficent way to keep battery charged with solar panel ?
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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.

Rene
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net


Re: Most efficent way to keep battery charged with solar panel ?

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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



Re: Most efficent way to keep battery charged with solar panel ?
On Sat, 17 Jan 2004 18:49:56 -0700, "Michael Anton"

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Do you perhaps have an example circuit diagram ? I am not sure what
you mean when you say you regulate both the input and output voltage.
If one connects a resistive load directly to a solar panel, wouldn't
both current and voltage change depending on the amount of sunlight
received by the panel ?

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 I am in South Africa, so the difference between summer and winter is
fairly small, and the amount of sunlight received fairly high compared
to places like europe. I do not want to have to use more solar panels
than I have too because my charging circuit is less efficient than it
can be. If I can get away with a cheaper less effiicient panel then
having a more complex charging circuit would be worthwhile.

In principal would a buck, boost or buck-boost regulator be the most
appropriate for charging a 12V battery from a (cheap) solar panel ?

Regards
  Anton Erasmus




Re: Most efficent way to keep battery charged with solar panel ?

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it.
voltage

Yes both current and voltage changes into a resistive load.  The idea is
that
you want to maximize the power transfer.  At a given light level, the panel
will output basically the same current (since it looks like a current source
with a limited output voltage), regardless of the output voltage, so you
will
get more power out if you keep the panel voltage high.  But, remember that
your battery does not look like a resistive load.  The battery voltage will
not
start to climb, until it is reaching the end of charge point.

The basic idea is that you assign a priority to the the two control loops.
You
always limit the output voltage to your battery, but you try to regulate the
panel voltage at the same time.  The main feedback loop tries to keep the
input
voltage regulated to say 19V or so, but if the output voltage gets too high,
then this will back off the output, regardless of what happens at the input.

The design that I played with, used a 78S40 regulator that has an extra
uncommited opamp.  The main feedback to control the regulator came off
of the input side which loaded the panel down appropriately to maintain
the set input voltage.  Then the additional opamp was used to buffer
a feedback voltage from the output side and fed through a diode to pull down
on the input side control point, so trick the control into thinking that the
panel
voltage was now too low, which backed the regulator off.  This control
method
is the reverse of what most regulators do.  The 78S40 was nice for this, as
you can pick the direction of the feedback control.

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average
little

You just need a simple step-down regulator.  Or, if there is a
microcontroller
involved, and you are not worried about maximizing efficiency, then you
could just short the panel out with a FET to turn it off (or use a P-FET to
switch the high side) when the micro reads that the battery is at full
charge.

Hope this helps,

Mike Anton

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Re: Most efficent way to keep battery charged with solar panel ?
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Bob Pease had an article worth reading:
http://www.google.com/search?num10%0&hl=en&lr=lang_en&ie=UTF-8&oe=UTF-8&safe=off&q=pease+solar+charging+anyhow&btnG=Google+Search

Re: Most efficent way to keep battery charged with solar panel ?

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Thanks,

Quite an interesting article.  Of coarse I will need quite a lot of
batteries in series if I want to charge a 3.2V Lithium cell.

I found an interesting NASA article where they put more solar cells in
parallel in stead of in series. They then tie a DC-DC converter to the
solar cell array, and the output of the DC-DC (Which is isolated) in
series with the solar array.  The output of the DC-DC converter is
controlled to keep a fixed output voltage.

Regards
   Anton Erasmus

Re: Most efficent way to keep battery charged with solar panel ?
"Be vewy vewy vewy careful" if you are talking about Lithium
batteries.  They have a way of "letting the magic smoke out"
(and all the rest of their insides for that matter) if they are
overcharged.  Lithium batteries can be very dangerous if
overcharged.


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--
Mike "mikey" Fields
http://home.comcast.net/~mike.fields /
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Re: Most efficent way to keep battery charged with solar panel ?
On Fri, 06 Feb 2004 23:39:22 GMT, "Mike Fields"

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Yes the voltage must be properly regulated when charging Li-ion.But
one can charge them in series without problems. The voltage must still
be regulated to n x 3.2V for n 3.2V li-ion cells.

Regards
   Anton Erasmus


Re: Most efficent way to keep battery charged with solar panel ?


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These 19V openloop can rise up to 22V or so when it is cold.

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A power tracker, yes, a bit more complex than just a regulator.
Worthwhile for a bigger setup than just a single panel.

Rene


Re: Most efficent way to keep battery charged with solar panel ?
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Take a look at Linear Technology site. They have some switched chargers
that AFAIK work with input voltage lower than battery one. Try Maxim and
TI as well.

HTH.

Elder.


Re: Most efficent way to keep battery charged with solar panel ?

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12 volt battery-charging solar panels complete with controller are an
off-the-shelf item. Try any recreational vehicle store.

--
Al Balmer
Balmer Consulting
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Re: Most efficent way to keep battery charged with solar panel ?

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These are the onse I have seen that only uses a diode to connect to
the 12V battery. This I believe is far from the most efficient way to
use a solar panel, and will also reduce the life of the battery quite
dramatically depending on solar conditions. The ones I have seen with
anything more sofisticated are much more expensive. If you know of
actual manufacturers of oem panels with charging circuits, I would
appreciate it.

Regards
   Anton Erasmus



Re: Most efficent way to keep battery charged with solar panel ?

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The ones I've seen have proper charge controllers. They have to -
typically they're used by "boondockers" with large battery banks who
definitely expect their batteries to last for years. Obviously, proper
controllers cost more than a simple diode, but they're necessary,
since the panels typically produce 16-20 volts.

There are lots of questions. What is your power consumption? Is this a
one-off item, or are you planning to mass produce them? If it's
one-off, where will it be installed (what latitude)? Actually,
latitude is only part of the environmental considerations - you have
to plan sufficient capacity to tide you over stormy days, for example.
What's the budget? Again, if this is a one-off item, the best bet is
probably an RV setup. If you're manufacturing, do a Google search on
"solar photovoltaics manufacturers" (without the quotes) and pick
someone accessible to you. I'm sure they'll be happy to discuss design
considerations.

--
Al Balmer
Balmer Consulting
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Re: Most efficent way to keep battery charged with solar panel ?

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This whole solar panel business is sort of a side line. My product is
a GSM device that can communicate with a central station when certain
alarm conditions occur. When no GSM activity is present the device
consumes less than 1mA at 3.3V this goes up to a peak of 1.8A when
the GSM module is connected to the network and data is being
transmitted. It is possible to reduce the 1mA by using the powerdown
modes of the MCU (ATMega128).
I have had inquiries for an application where the device will be used
where no external power is available, hence the solar panel. I have
been unable to find a solarpanel with the appropriate charging
circuits at a reasonable price. The prices here in South Africa of
a solarpanel without charging circuitry is in the order of 10x cheaper
than one with the circuitry. The latitude is +- 23deg South, with an
average AFAICR of over 90% sunny days per year.
I am currently looking for one or two units for a trial, and if
succesful a potential for quite a few more. The units for the trial
must be the same as the ones that will be used in production if this
ever happens.

Regards
   Anton Erasmus



Re: Most efficent way to keep battery charged with solar panel ?

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If you mean by efficient, getting the most power into a battery it will be
more efficient without a diode, as long as you don't mind the batteries
getting discharged by the panel when it gets dark. But assuming that you do
want the diode then it should be a low forward voltage type like a Schottky.

To protect the batteries, a decent system will use a shunt regulator to dump
excess current. If you don't want to waste the power and make it more
efficient, put more batteries in to cope with the peak charging current.

Peter


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Checked by AVG anti-virus system (http://www.grisoft.com ).
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Re: Most efficent way to keep battery charged with solar panel ?

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do
Schottky.

There are much better ways of increasing efficiency, withou worring about
the diode drop.

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dump

Alas, it is not the peak charging current that is the issue.  The issue is
that
eventually, the batteries will reach full charge, and it is hard on them to
continue charging at this point.  So usually there is a voltage limit put in
place.  The current is already limited, based on the sola panels spec.,
which looks like a constant current source that changes with light
level.

Mike Anton



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