solar lead acid battery controller

[walle62]

dear friends

i design a solar lead acid battery controller.

it feautures:

battery nominal voltage: 17 V surge load current: 15A or 30A

it does not need a regulator

it should protect against:
over-, undercharge, short circuit, solar cells reverse current, battery
and load reverse polarity

it must contain a gas gauge (battery charge measurement)

i must have a self explaining display of the battery charge in Ah and
of the charge current in A

the design must be simple and cost effective

my idea is to use bipolar transistors for measurement of the over-,
undervoltage etc. and drive power-mos-fets. or exists a good ic that
features all conditions? perhaps i could also use a multi-op-amp device
as the LM 324 and c-mos-digital-ics for threshold, driving etc.
but i have no idea how to implement the gas gauge. can i determine the
battery charge via battery voltage?

i would be very glad if one of you could help or give me a hint!
thank you very much!

if i find a good design, i will report you here in the forum!

with kind regards

walter nussbaum, zuerich, switzerland

 

[Rene Tschaggelar]

dear friends
[snip]

i would be very glad if one of you could help or give me a hint!
thank you very much!

if i find a good design, i will report you here in the forum!

Can we get a sample each then ?

Rene

 

[walle62]

hi rene

surely! i try to post a schematic, as soon as i have a good circuit.

bye

walti

 

[martin griffith]

hi rene

surely! i try to post a schematic, as soon as i have a good circuit.

bye

walti

what research have you done so far?


martin

 

[Ken Smith]

dear friends

i design a solar lead acid battery controller.
[...]
it should protect against:
over-charge:

For a lead acid, this is basically a voltage regulator with a bit of funny
back lash added. When the charging current it flowing, you allow the
battery voltage to rise a bit above the "float" voltage and then drop back
down to float.

--------- D1
------! LM317 !--+--+/\/\/---+-->!-------battery
--------- ! ! !
! / \ !
! \ / !
! / \ !
! \ / !
! ! ! !
! ! \! !
! ! !-------
! ! /! PNP
+------+--
!
\
/
\
/
!
V
--- D2
!
GND

D1 and D2 are the same sort of diode but very different sizes.

 

[walle62]

hi martin

that´s a good device, the bq2031. perhaps i can use it.
thanks!

walti

 

[Ross Herbert]

hi herbert

thank you very much for your posting!!

Given that you are doing the design based on a client specification
then you have no choice. I therefore agree that you are bound to give
him what he wants despite any recommendations as to the necessity of
certain functions.

It did occur to me that modern deep cycle storage batteries using
Absorbent Glass Mat (AGM) construction does make it impossible (and
unneccessary) to check electrolyte levels and SG, so some other means
of determining the battery condition may be desirable. I still don't
think a great deal of assistance can be gained by having gas gauges
and SoC readings even with AGM batteries. If all batteries in the
string deteriorate at the same rate gauges will likely indicate this
symptom but I think their readings will only tend to confirm what is
already detectable using local knowledge of the system. If only one
battery fails in the string, gauges are not really going to give any
reliable indication of the problem. You still need to get down to
basic checks on each battery in the string in order to find the
culprit.

Good luck!

Ross Herbert

SNIP

 

[walle62]

hi herbert

Given that you are doing the design based on a client specification
then you have no choice. I therefore agree that you are bound to give
him what he wants despite any recommendations as to the necessity of
certain functions.

yes, that's my problem: i'm bound on these specs.

It did occur to me that modern deep cycle storage batteries using
Absorbent Glass Mat (AGM) construction does make it impossible (and
unneccessary) to check electrolyte levels and SG, so some other means
of determining the battery condition may be desirable. I still don't
think a great deal of assistance can be gained by having gas gauges
and SoC readings even with AGM batteries. If all batteries in the
string deteriorate at the same rate gauges will likely indicate this
symptom but I think their readings will only tend to confirm what is
already detectable using local knowledge of the system. If only one
battery fails in the string, gauges are not really going to give any
reliable indication of the problem. You still need to get down to
basic checks on each battery in the string in order to find the
culprit.

that's interesting: so you will say that the charge of cells in a
string can be different despite charging/loading current is the same?

thanks for your good advices?

i wish you a nice week!

walti

 

[mike]

dear friends

i design a solar lead acid battery controller.

it feautures:

battery nominal voltage: 17 V surge load current: 15A or 30A

it does not need a regulator

it should protect against:
over-, undercharge, short circuit, solar cells reverse current, battery
and load reverse polarity

it must contain a gas gauge (battery charge measurement)

i must have a self explaining display of the battery charge in Ah and
of the charge current in A

the design must be simple and cost effective

my idea is to use bipolar transistors for measurement of the over-,
undervoltage etc. and drive power-mos-fets. or exists a good ic that
features all conditions? perhaps i could also use a multi-op-amp device
as the LM 324 and c-mos-digital-ics for threshold, driving etc.
but i have no idea how to implement the gas gauge. can i determine the
battery charge via battery voltage?

i would be very glad if one of you could help or give me a hint!
thank you very much!

if i find a good design, i will report you here in the forum!

with kind regards

walter nussbaum, zuerich, switzerland

Solar charge controller doesn't have to be very quick or precise.
If response time on the order of minutes is too slow, you've got way
too much solar or way too little battery.
Use a PIC (or other processor) with A/D. Use a PWM shunt (charge dump)
regulator. PWM rate can be SLOW, SLOW...no problem with doing it in
software.

For a domestic system, dump the excess energy into a big resistor in the
water heater.

And the PIC can do all your integrating, storage, display.
And it'll be cheaper than trying to do it analog.
And easy to change in firmware when your beta test discovers it's all
wrong.
mike

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[Ken Smith]

walle62 wrote: [...]
For a domestic system, dump the excess energy into a big resistor in the
water heater.

If you simply open the connection between the charger and the battery,
there is no excess energy dumping needed.

If you make your solar pack such that it outputs a little more voltage
than a charged battery, a very simple system will do. There is not much
to be gained by adding a switcher. If you want to get the last bit of
energy, a very simple bucker will do nicely.

And the PIC can do all your integrating, storage, display.

The micropower PICs can work at low clock speeds over a wide voltage
range. This means you can use a very bad regulator to run the PIC.

 

[mike]

walle62 wrote:
[...]

For a domestic system, dump the excess energy into a big resistor in the
water heater.

If you simply open the connection between the charger and the battery,
there is no excess energy dumping needed.

If you make your solar pack such that it outputs a little more voltage
than a charged battery, a very simple system will do. There is not much
to be gained by adding a switcher. If you want to get the last bit of
energy, a very simple bucker will do nicely.

Well....since insolation varies many orders of magnitude
over a 24 hour period, just how do you propose to make a solar pack that
outputs "just a little more voltage than the battery"????

Problem with a buck switcher is that it is lossy ALL the time.
The shunt switcher is lossy only when you have more solar energy than
you can use.

Now, if you were advocating a higher voltage solar pack and a buck
switcher that operates the solar pack
at maximum power output for any level of light input, you might have a
point...
My research indicates you'd get more benefit from sun tracking than
from peak power sensing.
mike

The micropower PICs can work at low clock speeds over a wide voltage
range. This means you can use a very bad regulator to run the PIC.

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[John Popelish]

Well....since insolation varies many orders of magnitude
over a 24 hour period, just how do you propose to make a solar pack that
outputs "just a little more voltage than the battery"????

Problem with a buck switcher is that it is lossy ALL the time.
The shunt switcher is lossy only when you have more solar energy than
you can use.

Now, if you were advocating a higher voltage solar pack and a buck
switcher that operates the solar pack
at maximum power output for any level of light input, you might have a
point...
My research indicates you'd get more benefit from sun tracking than
from peak power sensing.
mike

I think I came to the same conclusion, once.

 

[Ken Smith]

Ken Smith wrote: [....]

If you make your solar pack such that it outputs a little more voltage
than a charged battery, a very simple system will do. There is not much
to be gained by adding a switcher. If you want to get the last bit of
energy, a very simple bucker will do nicely.

Well....since insolation varies many orders of magnitude
over a 24 hour period, just how do you propose to make a solar pack that
outputs "just a little more voltage than the battery"????

Check out the voltage out vs light in of a solar panel. You will find
that the open circuit voltage is far less than linear with the light
input.

The short circuit current is nearly linear with the light input. As a
result, the optimum voltage (point of greatest power) remains high over a
very wide range of light inputs.

Problem with a buck switcher is that it is lossy ALL the time.
The shunt switcher is lossy only when you have more solar energy than
you can use.

Please explain what "shunt switcher" means in this context. I took it to
be a simple booster driving some large resistor.

If this is what you mean,I think the idea is clever but needlessly
complex. Unless this is a huge installation a few transistors as a simple
shunt linear regulator would be able to take the power.

Now, if you were advocating a higher voltage solar pack and a buck
switcher that operates the solar pack
at maximum power output for any level of light input, you might have a
point...

Yes, this is what I'm suggesting. The bucker could have very low losses
since its output is usually almost exactly its input. The catch diode and
inductor losses only apply to the voltage difference.

My research indicates you'd get more benefit from sun tracking than
from peak power sensing.
mike

Yes, but sun tracking involves moving parts and electric motors etc. This
is sci.electronics.design not sci.mechanical.design. Over there I would
have suggested a system that uses the solar power to aim the panel.

 

[mike]

Ken Smith wrote:
[....]

If you make your solar pack such that it outputs a little more voltage
than a charged battery, a very simple system will do. There is not much
to be gained by adding a switcher. If you want to get the last bit of
energy, a very simple bucker will do nicely.

Well....since insolation varies many orders of magnitude
over a 24 hour period, just how do you propose to make a solar pack that
outputs "just a little more voltage than the battery"????

Check out the voltage out vs light in of a solar panel. You will find
that the open circuit voltage is far less than linear with the light
input.

The short circuit current is nearly linear with the light input. As a
result, the optimum voltage (point of greatest power) remains high over a
very wide range of light inputs.

Sounds like you have more experience than I do. I understnd about the
short-circuit current. Does the same apply to current into a 14V
battery? I did some experiments with a small solar cell. I determined
that the available charge current went down dramatically for small
changes in incident angle. If you don't track the sun, there's a
significant part of the day when the EFECTIVE light input is way down.

Please explain what "shunt switcher" means in this context. I took it to
be a simple booster driving some large resistor.

If this is what you mean,I think the idea is clever but needlessly
complex. Unless this is a huge installation a few transistors as a simple
shunt linear regulator would be able to take the power.

Yep, all you need is a transistor and a big resistor to ground.
Simple. But when you do the math, 15V max voltage 40A max shunt
current, the maximum power in your transistor is 7.5V X 20A = 150W.

What we actually did for the mountain top repeater was to build a
10A switching shunt and switched additional 10A resistors in as needed.
Once we had the pic for voltage measurement, all the other stuff became
trivial. Very low power in the switching FETS and BIG resistors mounted
on the wall to dissipate the heat. Also limits the surge currents in
your batteries without having to add inductors or caps. The pic can
also let you correct voltage points for temperature. BIG temp changes
on a mountain top. Been running about a year now without a hitch.

Oh, there was another complication. This site also had wind power.
Couldn't just disconnect the load with the wind at 60MPH.

Yes, this is what I'm suggesting. The bucker could have very low losses
since its output is usually almost exactly its input. The catch diode and
inductor losses only apply to the voltage difference.

I'll need more explanation of this. I always considered the inductor
loss to be mostly due to it's resistance and therefore the current.
For the diode, a diode drop is a diode drop. Yes, there are also
dynamic losses. What's the voltage swing at the input to the inductor
when the input and output voltages are close?

mike

Yes, but sun tracking involves moving parts and electric motors etc. This
is sci.electronics.design not sci.mechanical.design. Over there I would
have suggested a system that uses the solar power to aim the panel.

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[Ken Smith]

Ken Smith wrote: [...]
I understnd about the
short-circuit current. Does the same apply to current into a 14V
battery?

I'm not sure I understand the question but I'll take a shot at it.

A fully discharged 12V lead acid battery is right about 10.5V. A fully
charged one is about 13.7V.

To be able to fully charge the battery, the solar cells need to be able to
make something like 13.7V.

In a simple system, enough solar cells are connected in series to charge
the battery and connected nearly directly to it until the battery is
charged. In the short term, the terminal voltage of the lead acid battery
is nearly independant of the charging current (ie: it has a low
impedance). This will work to your advantage if you are making a switcher
for the charging circuit. If you use a PIC, you can dither the PWMing up
and down to find the point where the greatest current is flowing into the
battery.

I did some experiments with a small solar cell. I determined
that the available charge current went down dramatically for small
changes in incident angle. If you don't track the sun, there's a
significant part of the day when the EFECTIVE light input is way down.

Yes it falls of with angle faster than a cos() curve. The sun hitting the
face of the panel follows a cos() curve. The percentage that refects back
away from the surface increases with increasing angle too.

If you point the panel towards the sun, there will be about 1KW of power
falling on a square meter of panel. 10% of the power can end up as
electrical power. The rest ends up as heat.

It comes down to a question of cost whether tracking makes sense or not.
Any money you spend on gears and motors could also be spent on another
panel. You need to price the situation both ways to see which is better.
At one time the motors and gears where always the lower cost way to go. I
don't know about today.

[...]

What we actually did for the mountain top repeater was to build a
10A switching shunt and switched additional 10A resistors in as needed.
Once we had the pic for voltage measurement, all the other stuff became
trivial. Very low power in the switching FETS and BIG resistors mounted
on the wall to dissipate the heat.

I'd be inclined to use many TO-220 resistors for the power resistor. At
20W each, they would spread the heat around nicely.

[....]

Oh, there was another complication. This site also had wind power.
Couldn't just disconnect the load with the wind at 60MPH.

The "real world" is never as nice as the lab bench.

[...]

I'll need more explanation of this. I always considered the inductor
loss to be mostly due to it's resistance and therefore the current.

The AC resistance of an inductor is always more than the DC resistance and
the more the difference between the input and output, the more inductance
you need.

This means that the losses in the design, end up determined by the input
to output difference. (If the difference was zero, the inductance would
be zero and so could the resistance)

For the diode, a diode drop is a diode drop.

Well not quite. If you don't use a very fast diode, the storage effect
adds to the losses. Also remember that the diode only conducts for a
small fraction of the total cycle.

Yes, there are also
dynamic losses. What's the voltage swing at the input to the inductor
when the input and output voltages are close?

Until they are almost exactly equal, the peak swing remains the input
voltage. The RMS of the AC part of the waveform decreases as you near
equal. There is a bit of a bummer in that the high harmonics decrease
slower. The losses at the higher harmonics are proportionally larger.

 

[Ross Herbert]

hi herbert
yes, that's my problem: i'm bound on these specs.
that's interesting: so you will say that the charge of cells in a
string can be different despite charging/loading current is the same?

Yes. In large battery installations such as in telephone exchanges the
most common cell used was a 2V, 1000 or 2000Ah flooded lead acid type
configured as 24 cells in series. Despite the fact that all cells were
from the same manufacturing batch when installed it was not uncommon
for a single cell to lose capacity after a few years of service which
then had to be replaced. The only way to detect the weak cell was to
perform periodic testing of individual cells.