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'[EE] charging a 12V battery with solar array'
2006\08\31@011538 by

k...looking at using 14 A/hr 12VDC battery.  In theory if I have a 480mA current source at 18V from a solar array, it would take 2 hours to charge back 1 A/hr of
capacity....?  I need to keep a constant 12VDC, so plan on using a boost convertor
and monitor the battery itself to keep it from discharging below around 9 or 10V.

If the load itself is 1.2A, I'm actually thinking that 14A/hr battery actually wont suffice
since that would again in theory, allow a total 1.4A draw for 10 hours, but means it will drop the battery voltage to zero?  Not a good thing to do to a battery.  And I have to have the total draw of about 8 to 10 hours.

So maybe a pair of batteries, or split the load to a set of batteries maybe?

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Alan,
The rated capacity of the battery will be down to a specific voltage -
not 0 volts. Typically between 1.65 and 1.8v/cell. Rather than using
the generic 14AHr capacity rating you'd be better off getting hold of
the data sheets for that type and looking at how it will perform in
The data sheets will also advise on charging requirements and
temperature compensation factors.

If you can't find the data sheet for that specific type, look at a
number of data shhets for similar batteries - this should give ypu an
idea of what to expect.

Richard P

On 31/08/06, alan smith <micro_eng2yahoo.com> wrote:
{Quote hidden}

>
Another thing to consider is that a boost converter has a negative
resistance input (pretty much). As you decrease the input voltage, the
current increases as it works to output the same power. When driving this
with a solar array, it can quickly run the array voltage down to zero.
There are "maximum power point trackers" that adjust the load on the array
to get the maximum output power. I worked on one of these for a solar
powered car project. This car had a bunch of solar panels, each getting
different illumination. We did a pic controlled boost converter on each
panel. The PIC watched the output current (the current into the battery
from this particular converter) and adjusted the switch duty cycle to
maximize that current.

Harold

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> There are "maximum power point trackers" that adjust the load on the
> array
> to get the maximum output power. I worked on one of these for a
> solar
> powered car project. This car had a bunch of solar panels, each
> getting
> different illumination. We did a pic controlled boost converter on
> each
> panel. The PIC watched the output current (the current into the
> battery
> from this particular converter) and adjusted the switch duty cycle
> to
> maximize that current.

Don't confuse panel output and converter output when (if :-) )reading
the following.

It's probably not intuitive that what Harold is saying is correct- but
it essentially is. For a given insolation (= sunlight ) level a panel
has an optimum voltage x current = power point and the overall panel
plus converter has a point where power output will be a maximum. These
may be slightly different points due to converter efficiency shifting
with input voltage. If you are at maximum energy point and increase
the load you may well increase the current BUT the voltage will drop
proportionately more and energy output will fall.

The panel should be operated at the point where maximum energy is
delivered to the load. For practical purposes at any given time
maximum energy is probably at maximum converter current output into
the battery. [[ie battery chemistry may play small games occasionally
but probably well enough ignored]].

With a single converter and single panel, if you want true optimum
power point you have to either

- use a separate insolation (sunlight) level sensor and model the
panel performance or

- load the panel to a given current and measure voltage or vice versa
and hereby determine insolation and then switch to optimum load or

- swing the converter load backwards and forwards across the area
where energy maximum is (believed to be :-)) and select the best
point. You could lock on a given point for a set period and then
occasionally hunt either side of it to see if the optimum has moved.
In full sunlight the changes will be gradual. Clouds or any other
transient obstruction will move the optimum point.

The last of these 3 is easy to do under program control and makes no
SLIGHTLY sub optimum in output as you have to sometimes have the panel
off peak to determine where the peak is. Almost easier to write the
program than to describe it :-)

>Another thing to consider is that a boost converter
>has a negative resistance input (pretty much). As you
>decrease the input voltage, the current increases
>as it works to output the same power.

Not just a boost converter, but every switchmode converter/regulator.
True on the convertors.  The item that may have been lost...but Richard addressed it somewhat...is that the solar panels will be charging the battery during the day, then at night...the battery bank will be switched over to the load to run it, the panels will NOT be running the load at all, just the charger for gel-cell type batteries.  The main question is...given the panels can provide only 480mA MAX at full sunlight....whats the calculation to figure out ,how long it would take to recharge a battery.  I know thats rather ambigious, because the question is how low are the cells?  That will depend on the battery of course, the bigger the capacity the less they are discharged, but lets go back to saying we have a 15 A/hr battery....its drained to lets say a capacity of 12 A/hr after a period of time....given the 480mA current from the cells, can you assume that for a perfect world, it would take about 2 hours to recharge 1A/hr capacity BACK to the battery?

"Alan B. Pearce" <A.B.Pearcerl.ac.uk> wrote:  >Another thing to consider is that a boost converter
>has a negative resistance input (pretty much). As you
>decrease the input voltage, the current increases
>as it works to output the same power.

Not just a boost converter, but every switchmode converter/regulator.
Ken (not on list) said:

> Russell,
>
> Solar panel power optimisers that work by dithering the current,
> measuring
> the voltage, and moving the operating point in the direction of
> maximum
> power can be very effective  - especially those that use various
> forms of
> modelling algorithms to learn the characteristics of the panel.
>
> Unfortunately many of the algorithms and especially the obvious ones
> are the
> subject (of in my opinion dubious) patents.  That's not a problem
> for
> personal "playing" but is an issue for anyone contemplating a
> commecial
> product.

'[EE] charging a 12V battery with solar array'
2006\09\02@103908 by
Alan,

On Thu, 31 Aug 2006 07:49:00 -0700 (PDT), alan smith wrote:

>...
> The main question is...given the panels can provide only 480mA MAX at full sunlight....whats the calculation to figure out ,how long it would take to
recharge a battery.

It depends what colour the string is... :-)

> I know thats rather ambigious, because the question is how low are the cells?  That will depend on the battery of course, the bigger the capacity
the less they are discharged, but lets go back to saying we have a 15 A/hr battery....its drained to lets say a capacity of 12 A/hr after a period of
time....given the 480mA current from the cells, can you assume that for a perfect world, it would take about 2 hours to recharge 1A/hr capacity BACK
to the battery?

There are so many variables that it's not possible to say.  Unless you live next to James, I assume you have clouds.  I've been experimenting with 4 x
"15W" solar panels, arranged as 2x2 to give 24V, and charging batteries to see how it goes.  At midday, with a clear sky and the Sun pretty-much
square-on to the panels, the most I've had from them is 40W (out of the theoretical 60), and that was brief - through most of a sunny day it was in
the mid-30's.  As soon as a cloud comes over, the output plummets far more than you'd think, down to half a dozen watts or less.

The best I got for a whole sunny day was 10Ah (say 240Wh) and most days when there's scattered cloud it's about 3Ah.

Now, I'm not using MPPT so the power transfer is far from optimum (and the cloudy-time output may be much improved if I did) but one thing I have
found is that the panel-makers' claims are, if not actully bogus, certainly optimistic!

So I think you can assume that the best you could ever do in the situation above is that it will take no less than 2.1 hours to top up 1Ah, and probably
twice or more (much more in overcast weather) than that.

Cheers,

Howard Winter
St.Albans, England

Here is a charger idea that is interesting:

http://www.tunecharger.com/

On 2 Sep 2006 at 15:39, Howard Winter wrote:

{Quote hidden}

> --
> Here is a charger idea that is interesting:
>
> http://www.tunecharger.com/

Not overly cheap.
PIC16F675 based boost converter
Circuit on page 10.

http://www.tunecharger.com/2006_t_0001_mtc_3-330.2%20%20reva%20technical%20reference%20manual.pdf.

Russell,

On Sun, 03 Sep 2006 14:38:52 +1200, Russell McMahon wrote:

> > Here is a charger idea that is interesting:
> >
> > http://www.tunecharger.com/
>
> Not overly cheap.
> PIC16F675 based boost converter
> Circuit on page 10.
>
>         http://www.tunecharger.com/2006_t_0001_mtc_3-330.2%20%20reva%20technical%20reference%20manual.pdf.

By a strange coincidence I found the above recently (have you been hacking my gargoyle [tm] searches? :-)

It is interesting, but it's too puny for what I need: 24V battery system (so open-circuit voltage of solar array will be up in the 30s), at least 10A
charging current - preferably more.  I did wonder about scaling this up, but a number of the components are something of a mystery to me -
especially the MOSFET driver chip - not sure how its characteristics affect the specs for the whole unit.  Obviously the 7805 is the first thing that
won't stand 30V+ across it, but which components result in the 10W power limit I don't know.  It's all a but non-digital, an area in which I am never
comfortable! :-)

Cheers,

Howard Winter
St.Albans, England

On 4 Sep 2006 at 14:44, Howard Winter wrote:

{Quote hidden}

There is this Yahoo group for discussing the project:

<*> To visit your group on the web, go to:
http://groups.yahoo.com/group/JMPulseCharger/

<*> To subscribe from this group, send an email to:
JMPulseCharger-subscribeyahoogroups.com

>> > http://www.tunecharger.com/
>>
>> Not overly cheap.
>> PIC16F675 based boost converter
>> Circuit on page 10.
>>
>>
>> http://www.tunecharger.com/2006_t_0001_mtc_3-330.2%20%20reva%20technical%20reference%20manual.pdf.
>
> By a strange coincidence I found the above recently (have you been
> hacking my gargoyle [tm] searches? :-)

The manual is dangerously naive when discussing battery charging
needs - both lead acid, which it is mainly intended for, and NiCd and
NimH. It is dangerously underspecified for any of these tasks. As a
panel to battery energy transferer it may do a good job.

Their comments on multiple sources in parallel are correct but
generally do not represent means of using more than one source at once
which may confuse the technically inexperienced. .

I am uncertain of the merits of their two stage charging process and
it seems to reduce efficiency. They first use a boost converter to
charge a capacitor to above battery voltage and then discharge the
capacitor into the battery using a separate FET switch. I don't see
that not using the boost converter directly is an advantage.
Discharging a capacitor into a lower voltage load will necessarily
cause I^2R losses (even if it's not evident where the R is it still
exists). They would probably be better off using the second FET as a
synchronous flyback diode to get better efficiency.

To use this circuit in a larger current / power system you would need
to ensure diodes and FETs are adequately sized, which is a relatively
trivial task, and you would need to ensure that the inductor had
adequate current handling ability. This is liable to be the hardest
requirement. Many manufacturers of prewound inductors or of cores
provide sizing information.

I'd suggest that a straight boost converter would work fine in this
application. Then add software which allows battery voltage and
perhaps charging current to be measured.

Russell

{Quote hidden}

I'm wondering if the resulting pulsed current delivery offers simmilar benefits to those claimed by reflex or "burp" chargers.  The designer of this circuit seems to claim that it can revive dead batteries....

Regards

Mike

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>
> I'm wondering if the resulting pulsed current delivery offers simmilar
> benefits to those claimed by reflex or "burp" chargers.  The designer of
> this circuit seems to claim that it can revive dead batteries....

I'm pretty sure that pulsed current is a good thing.
I implemented it in an NIMH charger, sort of accidentally.  I was doing 1C
charging of a battery that was connected across a significant load, and
stopping once a second to make battery voltage measurements. Turned out that
this current load just happened to match up well with the numbers
recommended by the pulse charging docs I'd read.

It makes sense to me that it would help avoid the smooth plating effects
common with lower current charging.  Gas buildup, I'm not sure.  I've seen
docs where they did manometer monitoring of single cells, and their data
suggests an improvement.

My take on it is that since it's just a bit of firmware, why not?  :)
Well...final word on this...convinced the client that it was a bad thing to try and charge his batteries with a solar array...not enough space to put a decent size array AND the fact it would be accessable by everyone walking by...damage......  they decided to take my suggestion of removing the entire control section including batteries...and drop into a charging unit...just by off the shelf for that.

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