Request comments on simple solar charger

I've got an application where I would like to use 12V 7Ah SLA batteries
such as Powersonic PS-1270, with a solar charger. My device needs 24V
and it is desirable for the system to be able to monitor (coarsely) the
state of individual batteries as well as check the availability of
solar energy.

(Detail: There are actually two sets of batteries and solar cells, and
the system can switch them in and out according to their state of
charge).

Anyway: I've sketched a preliminary idea at
http://www.larwe.com/solardemo.png. The regulator shown is a
placeholder for any old LDO, the idea being to set the output for
around 13.6V and let the battery decide how much current to suck.

This isn't an ideal charger by any means, I realize, but it only has to
keep the batteries within 70% of nominal capacity for a year. After
that, it can die and I don't care.

Does my system look workable? Note that I don't care about discerning
the condition "solar good, battery bad" - I only want to know "battery
good/bad (solar cell offline)" and "solar cell Vout=good". Also not
shown, hysteresis resistors on the comparators, that's a minor detail.

 

[Joerg]

Hello Lewin,

Having trouble zooming png files such as this down so I won't comment on
the overall schematic.

Just one word of caution: Many LDOs are like the princess on the pea.
When the output capacitive load doesn't have just the right ESR range
they can become unstable. The same can happen with some of them if the
source impedance is too high. With solar panels being the source that
can be a serious concern.

Personally I prefer switchers with solar. You could then even do MPPT
but I guess that would be a bit overblown here.

Regards, Joerg

http://www.analogconsultants.com

 

Hi Joerg,

Having trouble zooming png files such as this down so I won't comment on
the overall schematic.

Sorry. If you want to look at the EAGLE schematic, that is also up at
http://www.larwe.com/solardemo.sch - but not that important I guess...

they can become unstable. The same can happen with some of them if the
source impedance is too high. With solar panels being the source that
can be a serious concern.

Personally I prefer switchers with solar. You could then even do MPPT
but I guess that would be a bit overblown here.

That's weird/bad news. I haven't had good results with switchers in
battery applications as the internal resistance of the battery goes up.
For example, a 1 ohm resistor in series with the battery has pushed the
regulator into a Bad Place for me when pulling a lot of current.

 

[Kryten]

Hi.

I also had a look for solar charging circuits, and the one I found on the
web were very crude. They looked like they'd work but waste energy on diode
drops and/or have little or no regulation.

I've bought solar powered gadgets to look inside them, and their circuits
are equally cheap and nasty.

The A of E (see power supply section) points out that the solar panel I/V
curve tends to flatten out at a maximum voltage and is thus a fair match for
lead acid batteries. The latter seem the most robust battery in terms of low
maintenance and management. The British Antarctic Survey use them rather
than Ni-Cd etc.

I think you should be able to get away with fairly simple charging
circuitry, and one diode drop in 24V is minimal. I suspect the battery to
supply rail needs the regulation more. After all, the battery is smoothing
the power input like the main electrolytic cap in a wall-wart (only much
bigger!) before driving the regulator.

I expect you could replace the diode with a switched MOSFET to remove the
diode drop.

Perhaps it could be gradually switched off as the solar voltage got too
high, instead of having the crude shunt regulators.


Such simple circuitry should be okay in sunny climates, but in the UK I
expect solar panels to spend much of their time way below their nominal
voltage. They can still provide energy, but not at a high enough voltage.
Therefore I would want to design a step-up SMPSU on the charging side. This
would then make use of the low-grade sunlight we get here.

In your circuit I don't think you need LDO regs to drive the power rails.
Lead acid batteries are not precisely 12V, in fact they are usually 13V8
when really fresh IIRC. If anything, you might want the LDO between the
batteries and the power rail.

I've been thinking of using a solar panel to charge my bike light.
Since this is only used at night when no charging can happen,
I wondered about an SMPSU that could be switched from regulated charging
to regulated discharging into the light bulb.
Now there's an interesting challenge...

 

Hi,

I've bought solar powered gadgets to look inside them, and their circuits
are equally cheap and nasty.

All the solar gadgets I've inspected were a resistor, a diode and a
NiCd or NiMH battery pack. I guess these things are designed to cost
$5, they work for a while, and when the batteries die you throw them
away.

circuitry, and one diode drop in 24V is minimal. I suspect the battery to
supply rail needs the regulation more. After all, the battery is smoothing

The 24V rail only supplies motors and solenoids, so I can tolerate
variation and noise in it. There is a small MC34063-based switcher
powering the smarts (they pull <300mA max, normally <20mA - the big
guzzlers are the motors).

In your circuit I don't think you need LDO regs to drive the power rails.
Lead acid batteries are not precisely 12V, in fact they are usually 13V8
when really fresh IIRC. If anything, you might want the LDO between the
batteries and the power rail.

This is the part I'm having trouble with understanding, probably
because I don't adequately understand the behavior of solar cells. The
batteries I'm using spec a float charge of 13.5~13.8V which is quite a
narrow range. I was planning to build 27 or 28-cell solar panels with a
Voc of 14.58V or 15.12V, and use the LDO to bring this down to ~13.6V.
This would allow for different Voc under varied illumination.

Are you saying that it would be a better policy to use 26 cells for
14.04V, and have just a blocking diode, no regulation?

 

[Kryten]

<larwe@larwe.com> wrote in message
news:1122220714.552702.203640@g43g2000cwa.googlegroups.com...

All the solar gadgets I've inspected were a resistor, a diode and a
NiCd or NiMH battery pack.

Yep, the average consumer can't see how primitive it is, they just sunbathe
it to perk up the cells.

The 24V rail only supplies motors and solenoids, so I can tolerate
variation and noise in it.

Great, so no need for precision regulation.

Sounds like a sun-tracker?

This is the part I'm having trouble with understanding, probably
because I don't adequately understand the behaviour of solar cells.

Roughly speaking, they are big flat diodes.
Illumination causes current to flow 'backwards', and the IV curve is just
the normal diode curve shifted diagonally.

float charge of 13.5~13.8V which is quite a
narrow range.

I think the top voltage is so that it can't force more charge in when the
battery is 'full'.

The lower voltage just ensures there is enough voltage to force a full
charge in.

I suspect that going slightly below that minimum voltage would only meant
the battery never got to 100% charged, which isn't dangerous. It would mean
you might have to change the spec to say it only ran for less than the
maximum capacity of the battery.

I was planning to build 27 or 28-cell solar panels with a
Voc of 14.58V or 15.12V, and use the LDO to bring this down to ~13.6V.
This would allow for different Voc under varied illumination.

Manufacturers write data sheets to show their products at their best.
It may well be true that a panel reaches X volts and Y amps maximum,
but only under Z illumination (sunny day, no cloud).

On sub-ideal lighting, it quickly drops down the IV curve.

A simple resistor load would draw power from the cell for all lighting
conditions.

A diode/battery would not, of course.

Are you saying that it would be a better policy to use 26 cells for
14.04V, and have just a blocking diode, no regulation?

I'd cautiously say yes, because solar cells tend to reach a maximum voltage
(c. 0V45) then flatten out (equivalent to the 0V6 Vf of a diode). So they
won't go 'over voltage' by much. Hence less need for 'dropper' regulators.

The problem is that solar cells seldom run at full voltage unless they are
somewhere seldom cloudy.
Such as on spacecraft, tropical beaches, in deserts, or on top of Mauna Kea
(and other good optical telescope sites).

The crude solution is to have lots more cells so you have enough voltage to
discard even when they are running at less than maximum voltage. In which
case you will need dropper regulators. The snag there is that solar cells
are not cheap.

The ideal solar cell load would follow the maximum power point on the power
curves for all illuminations. If those points joined up to a straight line,
a fixed resistor would do, but they don't, so it won't.

Essentially you need to design a DC to DC converter optimised for a wide
input voltage range.







>

 

[martin griffith]

On 24 Jul 2005 08:58:34 -0700, in sci.electronics.design
larwe@larwe.com wrote:

Hi,

snip
This is the part I'm having trouble with understanding, probably
because I don't adequately understand the behavior of solar cells. The
batteries I'm using spec a float charge of 13.5~13.8V which is quite a
narrow range. I was planning to build 27 or 28-cell solar panels with a
Voc of 14.58V or 15.12V, and use the LDO to bring this down to ~13.6V.
This would allow for different Voc under varied illumination.

Are you saying that it would be a better policy to use 26 cells for
14.04V, and have just a blocking diode, no regulation?

Yep, it is a bit confusing.
the best expalnation I came across was that it's a compromise between
float and boost charging.

Normally a float system is 24/7, and maintaining a the correct cell
voltage is a "good idea".

Since solar power is more like 8/7 to 13/7 it cannot be considered a
true float, so the solar guys tend to overcharge to compensate. It
does mean a reduced battery life, just one of the prices that you have
to pay


martin

 

Hi,

The 24V rail only supplies motors and solenoids, so I can tolerate

Sounds like a sun-tracker?

No. A robot of sorts. If you think of an electromechanical scarecrow
targeting nocturnal animals, that can drive around the perimeter of a
field and photograph the beasts it's scaring off, you have an idea of
the sorts of functionality in my device though that is not actually the
application.

the battery never got to 100% charged, which isn't dangerous. It would mean
you might have to change the spec to say it only ran for less than the
maximum capacity of the battery.

I've already speced in slack there. The device needs to run a ~1A motor
load for three to four minutes every morning. It then sits around for
the entire day soaking up rays and doing not much else. There is a
scheduled comms session which could pull as much as 750mA for some
time, but mostly it will be <60 seconds and not that much current.
During the night, the device runs various lights, and motors and other
actuators, not on a 100% duty cycle by any means. A 5Ah battery, fully
charged, could handle the required tasks - so I speced in 7Ah.

The project is defined to be successful if it operates for one year
without intervention, with an availability (i.e. ability to do its
nocturnal stuff) of 50%.

The problem is that solar cells seldom run at full voltage unless they are
somewhere seldom cloudy.
Such as on spacecraft, tropical beaches, in deserts, or on top of Mauna Kea
(and other good optical telescope sites).

I can't guarantee no clouds in my usage scenario But I can guarantee
that the device is smart enough not to attempt anything strenuous while
the batteries are depleted.

The crude solution is to have lots more cells so you have enough voltage to
discard even when they are running at less than maximum voltage. In which
case you will need dropper regulators. The snag there is that solar cells
are not cheap.

I won't quite say money is no object, but solar cells are not a major
part of the project budget. I'm looking at products like the 04-1192
cell from this page: http://www.siliconsolar.com/solar_cells.htm (sorry
there are no direct product links). It's a 6x6" cell, 0.54 Voc, 6A Isc,
$12.80 apiece. An extra $50 or so for four more cells is not going to
dampen my enthusiasm. On the other hand if the simple design with diode
only and fewer cells will get me 365 days of operation, I won't waste
money on greater complexity.

 

[Joerg]

Hello Lewin,

Sorry. If you want to look at the EAGLE schematic, that is also up at
http://www.larwe.com/solardemo.sch - but not that important I guess...

Yes, that worked just fine. I don't see any reference in there. Hmmm...
you need one.

If it was me I wouldn't split it up like that. What if Solar1 drifts
into the shade while Solar2 doesn't? Or maybe got dirty, or a large bird
decided to take a long nap on its top bracket? Or that large bird gets
the feeling he shouldn't have eaten that last road kill and then, well,
you know what I mean.

If you really don't want to use a switcher you could connect the panels
and the batteries in series and get rid of almost half the circuitry.

That's weird/bad news. I haven't had good results with switchers in
battery applications as the internal resistance of the battery goes up.
For example, a 1 ohm resistor in series with the battery has pushed the
regulator into a Bad Place for me when pulling a lot of current.

A well designed switcher doesn't mind a high source resistance. You
could, for example, use a switcher based on the LM3478. Then use a
single solar panel (or two if you need the power) and convert up to the
voltage you need. The nice thing is that it will charge even when the
panel voltage drops below nominal.

To measure the status of the battery you need a reference. The LM3478
provide a nice bandgap but only when it's in regulation. Else, a TLV431
comes to mind.

BTW, if you absolutely have to use LDOs there are some that provide a
signal when they go out of regulation. IOW when the input voltage drops
so much that there isn't enough head room.

Regards, Joerg

http://www.analogconsultants.com

 

Yes, that worked just fine. I don't see any reference in there. Hmmm...
you need one.

Reference to what, exactly? The comparator is powered off a 5V
switcher. The various R dividers there are designed to trip the
comparators when the difference between the two sides of one of the
batteries (i.e. the nominal output voltage) falls below a predetermined
lobat threshhold. Now I come to think about it, the low side
comparators should have dividers at their -ve inputs also.

If it was me I wouldn't split it up like that. What if Solar1 drifts
into the shade while Solar2 doesn't? Or maybe got dirty, or a large bird

Well, do recall what I said in an earlier posting, that there are
actually two sets of everything you see here, and the controller is
capable of switching in a good bank to take over from a bad bank.

A well designed switcher doesn't mind a high source resistance. You
could, for example, use a switcher based on the LM3478. Then use a

I'll look at this part, thanks.

 

[Joerg]

Hello Lewin,

Reference to what, exactly? The comparator is powered off a 5V
switcher. The various R dividers there are designed to trip the
comparators when the difference between the two sides of one of the
batteries (i.e. the nominal output voltage) falls below a predetermined
lobat threshhold. Now I come to think about it, the low side
comparators should have dividers at their -ve inputs also.

For comparing two sides, ok. But what for is the connection at + of BAT1?

Currently you seem to just charge away and there is no reference voltage
against which a battery status of "full" would be measured. Regulator
chips usually only come in discrete varieties such as 9V, 12V, 15V,
neither of which is suitable for lead acid 13.8V limits. You'd need at
least an adjustable regulator and then there has to be a resistor from
ADJ to GND.

Amother thing you need is diodes to block discharge when there isn't
enough light.

Well, do recall what I said in an earlier posting, that there are
actually two sets of everything you see here, and the controller is
capable of switching in a good bank to take over from a bad bank.

Then I would keep them completely autonomous but you have a connection
in there. Also, allowing two panels to work on both batteries would
provide more redundance than splitting it up completely.

Regards, Joerg

http://www.analogconsultants.com

 

[Bob Monsen]

larwe@larwe.com wrote:

I've got an application where I would like to use 12V 7Ah SLA batteries
such as Powersonic PS-1270, with a solar charger. My device needs 24V
and it is desirable for the system to be able to monitor (coarsely) the
state of individual batteries as well as check the availability of
solar energy.

(Detail: There are actually two sets of batteries and solar cells, and
the system can switch them in and out according to their state of
charge).

Anyway: I've sketched a preliminary idea at
http://www.larwe.com/solardemo.png. The regulator shown is a
placeholder for any old LDO, the idea being to set the output for
around 13.6V and let the battery decide how much current to suck.

This isn't an ideal charger by any means, I realize, but it only has to
keep the batteries within 70% of nominal capacity for a year. After
that, it can die and I don't care.

Does my system look workable? Note that I don't care about discerning
the condition "solar good, battery bad" - I only want to know "battery
good/bad (solar cell offline)" and "solar cell Vout=good". Also not
shown, hysteresis resistors on the comparators, that's a minor detail.

A switcher seems like the way to go. Since you are only using 300C a
day, that is 10mA for 8 hours. Thus, you need about 24*10mA / E to run
the thing. Assume E = 50%, then that is about 500mW. Assume 1/3 of the
days have sun, and you need 1.5W (ie, one day will charge enough for 3
usages, on average.) If you use the cheapo 500mA panels you posted, that
means you need 3V to supply this. Since a switcher isn't going to run
very well at 3V, buy a few more (they are cheap) to get it up to 5.4V.
Assume 10. Then, you have 5.4V at 500mA, or 2.5W, which is more than
enough to keep your batteries charged, assuming a bit of self-discharge.

SLA batteries like CCCV charging, which simply means that you don't want
to dump a bunch of current into them initially. I've heard 1/10 C as a
reasonable figure, so assume 700mA is the max charge they'll take.
However, if you charge them at 50mA, you can basically just leave them
on forever. You may want to cut the charge when they get to 27.2V (which
is the float value). Your call. I think that the cells will last longer
if you do, but 50mA is a fairly tiny trickle, so it may not even make a
difference.

So, use 10 panels at 0.54V, 500mA each, a switcher chip to get you 24V
at 50mA from 5V, and stack the two 12V SLA batteries.

An LT1373 from linear could work nicely, or you could use a
microcontroller to build you own with current limiting.

--
Regards,
Bob Monsen

If a little knowledge is dangerous, where is the man who has
so much as to be out of danger?
Thomas Henry Huxley, 1877

 

Hi,

days have sun, and you need 1.5W (ie, one day will charge enough for 3
usages, on average.) If you use the cheapo 500mA panels you posted, that
means you need 3V to supply this. Since a switcher isn't going to run

? The cells I was looking at are the 6000mA ones further down the page.

SLA batteries like CCCV charging, which simply means that you don't want
to dump a bunch of current into them initially. I've heard 1/10 C as a
reasonable figure, so assume 700mA is the max charge they'll take.

These particular batteries are specified for 2100mA charge current.
When I'm testing them on my bench, I generally set my PSU for 1A
current-limit and set the open-circuit voltage to 13.7V, then let the
battery work it all out. When fully charged, the battery continues to
pull about 30-40mA depending on temperature.

Thanks for the advice. I'm going to do some experiments now and see how
it works out. (Luckily I have a large box of sacrificial batteries that
I picked up cheap!).

 

[Bob Monsen]

larwe@larwe.com wrote:

Hi,


days have sun, and you need 1.5W (ie, one day will charge enough for 3
usages, on average.) If you use the cheapo 500mA panels you posted, that
means you need 3V to supply this. Since a switcher isn't going to run


? The cells I was looking at are the 6000mA ones further down the page.


SLA batteries like CCCV charging, which simply means that you don't want
to dump a bunch of current into them initially. I've heard 1/10 C as a
reasonable figure, so assume 700mA is the max charge they'll take.


These particular batteries are specified for 2100mA charge current.
When I'm testing them on my bench, I generally set my PSU for 1A
current-limit and set the open-circuit voltage to 13.7V, then let the
battery work it all out. When fully charged, the battery continues to
pull about 30-40mA depending on temperature.

Thanks for the advice. I'm going to do some experiments now and see how
it works out. (Luckily I have a large box of sacrificial batteries that
I picked up cheap!).

What I was trying to say is that if you buy those 6A 0.54V panels, you
are wasting your money. The cheaper ones will work just as well,
assuming you are right about your project using 1A only 5 minutes a day.
You certainly don't need to buy 28V worth of them! That will cost you
more than $600. Using a switcher, the entire power circuit will probably
cost you less than $50 for the electronics, panels, and battery, and
it'll be far smaller, only taking up 30 sq in. Using your 6A, 0.54V
panels, you'll need 36 * 51 = 12.75 sq feet of panels! Also, you can't
even charge your battery at more than 2.1A, so why do you need 6A?

A switcher like this is a relatively simple circuit. You can download a
copy of LTSpice from http://www.linear.com/software, which has a
built-in designer for switchers (using their parts, of course.) Or, you
can just buy a DC-DC converter that takes 5V and outputs 28V, and use a
SLA battery charging chip. Again, you don't need much current to keep
your battery charged, assuming at least a bit of sunlight.

Anyway, good luck.

--
Regards,
Bob Monsen

If a little knowledge is dangerous, where is the man who has
so much as to be out of danger?
Thomas Henry Huxley, 1877

 

assuming you are right about your project using 1A only 5 minutes a day.

Ah! Now I understand where you're coming from - but I guess you missed
the part where I said:

During the night, the device runs various lights, and motors and other
actuators, not on a 100% duty cycle by any means. A 5Ah battery, fully
charged, could handle the required tasks - so I speced in 7Ah.

So the device does more than just run that 1A motor and do the comms
session.

panels, you'll need 36 * 51 = 12.75 sq feet of panels! Also, you can't
even charge your battery at more than 2.1A, so why do you need 6A?

This is part of why I was asking the question in here - the cells are
speced under ideal solar conditions, so I don't know how far to derate
them for real life. Considering the cost of the device, adding another
$300 to $600 in solar cells is money well spent if it significantly
improves the chance that the device will last for its intended
lifetime.

Thanks for your comments - I'm back to the drawing board.

 

[budgie]

On 25 Jul 2005 08:33:36 -0700, larwe@larwe.com wrote:

assuming you are right about your project using 1A only 5 minutes a day.

Ah! Now I understand where you're coming from - but I guess you missed
the part where I said:

During the night, the device runs various lights, and motors and other
actuators, not on a 100% duty cycle by any means. A 5Ah battery, fully
charged, could handle the required tasks - so I speced in 7Ah.

So the device does more than just run that 1A motor and do the comms
session.

panels, you'll need 36 * 51 = 12.75 sq feet of panels! Also, you can't
even charge your battery at more than 2.1A, so why do you need 6A?

This is part of why I was asking the question in here - the cells are
speced under ideal solar conditions, so I don't know how far to derate
them for real life. Considering the cost of the device, adding another
$300 to $600 in solar cells is money well spent if it significantly
improves the chance that the device will last for its intended
lifetime.

Thanks for your comments - I'm back to the drawing board.

If you do go the DC-DC step-up and SLA reg, I highly recommend the UC3906 in the
latter role. If you have trouble locating the Unitrode/TI data sheet just post
back here - it is found under some arcane name.

 

[Mark]

that is the OPEN CIRCUIT volatge of the solar cells. Under load the
volatge is less.

Think about connecting the solar cells directly (through a blocking
diode) to the battery.

You need to take the load line of the cells and the load line of the
batteries and see where they intersect to see how muvh current the
cells will drive through the battery at what voltage.

Mark
..