Dilemma about solar charging (BS2)

James Long

Ok, I'm working on a project for my father, and have a little dilemma.

This is a simple on/off system. The system will run a basic stamp (BS2p) mainly because I already have one.

The Voltage of the system is 6 volt (lead-acid, so about 6.2-6.9). The solar cells are 3 watt, and can achieve a unloaded voltage of about 11 volts.

The problem right now, the system will actually take the battery voltage too high for the unit being run. This results in a over voltage shut down of the system.

I want to take the basic stamp, run it on the solar power only (only control when the solar charging voltage is enough). Then sample the battery voltage with a simple ADC. (not sure which one yet). If the voltage is under a certain threshold turn on a mosfet that will allow the battery to charge.

Here are the problems so far. I do not have a regulator on the board for the ADC. I could probably use the regulator on the BS2.

I know there will need to be a diode before or after the mosfet to prevent reverse feed of the BS2. The solar panel already has reverse diode built in.

Can anyone come up with any problems with my idea so far?

James L

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James L
Partner/Designer

Lil Brother SMT Assembly Services

sam_sam_sam

James Long
·
The Voltage of the system is 6 volt (lead-acid, so about 6.2-6.9). The solar cells are 3 watt, and can achieve a unloaded voltage of about 11 volts.




I have used this regulator in the past and work very well·
http://www.national.com/ds/LM/LM2576.pdf

These would be the specs that i would use

• After reaching peak 7.2 VDC, charger automatically switches to 6.6 VDC float voltage.
• When voltage drops below 6.3 VDC, charger resumes charging back to 7.2 VDC.
• Solid state two color LED indicates stage of charger.


This is the web link that I used
http://www.atbatt.com/product/6840.asp

Here is what I would do if I were doing this Project based on what you have here

I want to take the basic stamp, run it on the solar power only (only control when the solar charging voltage is enough). Then sample the battery voltage with a simple ADC. (not sure which one yet). If the voltage is under a certain threshold turn on a mosfet that will allow the battery to charge.

·
I would use a· LM2576-ADJ regulator··set to 7.2 volts
I would· use a ADC with a·(voltage divider) setup to handle··8 volts would be your highest voltage

I would write the routine to look at the battery and once the battery voltage reach est 7.2 volts cut the mosfet
and wait until the voltage drops to 6.2 volts and repeat the same thing again


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··Thanks for any··that you may have and all of your time finding them

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Sam

Post Edited (sam_sam_sam) : 4/25/2009 5:38:46 PM GMT

James Long

Sam,

If I use the regulator, I couldn't set it to 7.2 volts. That would allow the voltage on the battery to get too high. If the battery bus voltage reaches about 7 volts (I'm guessing because I haven't tested) the camera on the bus goes into an over voltage shutdown.

This is the focus of the problem.

James

sam_sam_sam said...












I have used this regulator in the past and work very well

http://www.national.com/ds/LM/LM2576.pdf



These would be the specs that i would use



<FONT color=red>• • When voltage drops below 6.3 VDC, charger resumes charging back to 7.2 VDC.
• Solid state two color LED indicates stage of charger.




This is the web link that I used

http://www.atbatt.com/product/6840.asp



Here is what I would do if I were doing this Project based on what you have here





<FONT color=blue>


I would use a regulator set to 7.2 volts





<FONT color=black>mosfet

<FONT color=black>and wait until the voltage drops to 6.2 volts and repeat the same thing again

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James L
Partner/Designer

Lil Brother SMT Assembly Services

James Long

Also, as a secondary note, the solar cells were designed to charge 6 volt lead acid batteries, so I do not think a regulator is really needed.

I just need to either let the battery charge, or not let the battery charge. Unfortunately, my plan will take some of the charge current away from the battery (BS2 and ADC). The nice thing about the way I am doing it, the BS2 will only be powered when the solar cells are putting out enough power to charge the battery.

I'm building the system with the ability to charge 2 batteries, independently of course.

I think for now I'll continue on with the plan I had, but thanks for the insight.

James L

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James L
Partner/Designer

Lil Brother SMT Assembly Services

Beau Schwabe

James Long,

You don't necessarily need a regulator, but you do need something to monitor the batteries voltage and cut the power from the solar to prevent overcharging and drying the batteries out.

It's typical for an unloaded solar panel to put out about twice the voltage of the intended battery to be charged. Think of solar cells as a battery or power source with a high internal resistance. Ideally when solar panel is under load the internal resistance creates a peak power point (The rated power of the solar cell) at the design voltage.· In your case 6V.

Think about it... In order to establish any amount of "charging current" you must have a·voltage differential... A larger voltage differential·if the internal resistance of the power supply is high.

"Unfortunately, my plan will take some of the charge current away from the battery (BS2 and ADC)" - So keep the BS2 in sleep mode most of the time waking up every 5 seconds or so? The ADC could also be powered up from the BS2, so it doesn't need to be on all of the time either.


Here is what I would do if it were me....
Charge the battery full tilt from the solar panels as much as they are willing to give.· Once the batteries reach a specific voltage of 7.2V, apply a regulated voltage adjusted·to 6.6V to keep them at a float.· Use a light bulb rated a 1A or so as a dynamic current regulator between the regulated 6.6V supply and the battery.· If the battery falls·to 5.5V (<-- don't let it go below this)·repeat the cycle and apply the full solar voltage once again.· You could conservatively bump this value up a little bit from 5.5V to lessen the deep cycle and extend your battery life.·

"If I use the regulator, I couldn't set it to 7.2 volts. That would allow the voltage on the battery to get too high. If the battery bus voltage reaches about 7 volts (I'm guessing because I haven't tested) the camera on the bus goes into an over voltage shutdown." - In order for the 6V Lead acid battery to reach a healthy charging level, in really needs to go to 7.2V ... Why not introduce a diode to power the camera? The diode would provide a 0.6V drop, so when the battery is at 7.2, the camera would "see" about 6.6V


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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Post Edited (Beau Schwabe (Parallax)) : 4/26/2009 3:29:47 AM GMT

James Long

Beau,

That is a good idea (the diode), I hadn't thought of that. Ok, an introduction of a diode is in order for the camera. It will change the design a little, but I do not see a big deal with that.

Actually I was going to adjust the sleep mode according to how close to the top charge voltage the battery was. The closer, the shorter the sleep time. This would let the battery have more "juice" when in a low charge state.

As for a diode, what do you recommend. I'm not the best electronics person when it comes to dealing with differences. I have some GF1B's from Vishay. I think their voltage drop may be too much.

Anywho, thanks for the design advice.

James L

Beau Schwabe (Parallax) said...
James Long,

You don't necessarily need a regulator, but you do need something to monitor the batteries voltage and cut the power from the solar to prevent overcharging and drying the batteries out.

It's typical for an unloaded solar panel to put out about twice the voltage of the intended battery to be charged. Think of solar cells as a battery or power source with a high internal resistance. Ideally when solar panel is under load the internal resistance creates a peak power point (The rated power of the solar cell) at the design voltage. In your case 6V.


Think about it... In order to establish any amount of "charging current" you must have a voltage differential... A larger voltage differential if the internal resistance of the power supply is high.

"Unfortunately, my plan will take some of the charge current away from the battery (BS2 and ADC)" - So keep the BS2 in sleep mode most of the time waking up every 5 seconds or so? The ADC could also be powered up from the BS2, so it doesn't need to be on all of the time either.






<SPAN style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: Verdana">Here is what I would do if it were me....<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com[noparse]:office:[/noparse]office" /><o></o>

<SPAN style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: Verdana">Charge the battery full tilt from the solar panels as much as they are willing to give. Once the batteries reach a specific voltage of 7.2V, apply a regulated voltage adjusted to 6.6V to keep them at a float.<SPAN style="mso-spacerun: yes"> Use a light bulb rated a 1A or so as a dynamic current regulator between the regulated 6.6V supply and the battery. If the battery falls to 5.5V (<-- don't let it go below this) repeat the cycle and apply the full solar voltage once again. You could conservatively bump this value up a little bit from 5.5V to lessen the deep cycle and extend your battery life.

<SPAN style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: Verdana">

<SPAN style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: Verdana">"If I use the regulator, I couldn't set it to 7.2 volts. That would allow the voltage on the battery to get too high. If the battery bus voltage reaches about 7 volts (I'm guessing because I haven't tested) the camera on the bus goes into an over voltage shutdown." - In order for the 6V Lead acid battery to reach a healthy charging level, in really needs to go to 7.2V ... Why not introduce a diode to power the camera? The diode would provide a 0.6V drop, so when the battery is at 7.2, the camera would "see" about 6.6V

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James L
Partner/Designer

Lil Brother SMT Assembly Services

Ugha

To help out others attempting something like this... can you sketch out the general design/layout of your project?

I'm not clear why you need the BS2 at all in this circuit. Unless there are other applications for it, can't you get by
with other components?

I'm also just curious how the whole thing goes together [noparse]:)[/noparse]

Also... isn't there a kind of diode that is designed to run at constant breakdown while reverse biased in order to limit voltage for projects like this?

James Long

Ugha said...
To help out others attempting something like this... can you sketch out the general design/layout of your project?

I'm not clear why you need the BS2 at all in this circuit. Unless there are other applications for it, can't you get by
with other components?

I'm also just curious how the whole thing goes together [noparse]:)[/noparse]

Also... isn't there a kind of diode that is designed to run at constant breakdown while reverse biased in order to limit voltage for projects like this?

Well, I'm not sure the project requires a stamp. It does require some kind of on/off system. I'm sure I could get by with other components, but I'm not sure I have other components. I do however have a BS2p.

Not sure about the diode you speak of.

James L

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James L
Partner/Designer

Lil Brother SMT Assembly Services

Beau Schwabe

James Long,

Just about any Si diode with a current rating greater than or equal to what the camera demands are.


Ugha,

"Also... isn't there a kind of diode that is designed to run at constant breakdown while reverse biased in order to limit voltage for projects like this?" - A Zener diode, but they usually aren't capable of delivering much current.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Philldapill

James, I've had this problem with a battery charger for a battery bank of mine. These were BIG batteries(12V, about 500AH total). In between draining of the batteries, I would have to charge them back up, but I'd forget to take the charger off sometimes. I'd slap myself in the forehead and run outside, only to find the voltage was creeping up on 15V!

My solution, was a simple dual op-amp, a couple pots, a zener, and a relay. I don't have the schematic in front of me, but here's the gist of it. The first OpAmp would buffer a 5V zener reference. This reference was then compared to the voltage of the batteries(through a potentiometer of course). This second Opamp would compare these two voltages using a hysteresis setup. Once the voltage on the batteries hit the upper limit(adjusted by the first Pot), the opamp would turn a relay off, disconnecting the charger to the batteries. Eventually, the voltage would lower back down to a lower limit, and the relay would re-engage, connecting the charger again. The Upper and Lower Limits were determined by the amount of hysteresis which was then determined by a second pot. This hysteresis curve was centered around a set point, determined by the first pot.

It's a very simple circuit. If you want the schematic, I'll send it later when I'm on my other computer.

James Long

Philldapill said...
James, I've had this problem with a battery charger for a battery bank of mine. These were BIG batteries(12V, about 500AH total). In between draining of the batteries, I would have to charge them back up, but I'd forget to take the charger off sometimes. I'd slap myself in the forehead and run outside, only to find the voltage was creeping up on 15V!

My solution, was a simple dual op-amp, a couple pots, a zener, and a relay. I don't have the schematic in front of me, but here's the gist of it. The first OpAmp would buffer a 5V zener reference. This reference was then compared to the voltage of the batteries(through a potentiometer of course). This second Opamp would compare these two voltages using a hysteresis setup. Once the voltage on the batteries hit the upper limit(adjusted by the first Pot), the opamp would turn a relay off, disconnecting the charger to the batteries. Eventually, the voltage would lower back down to a lower limit, and the relay would re-engage, connecting the charger again. The Upper and Lower Limits were determined by the amount of hysteresis which was then determined by a second pot. This hysteresis curve was centered around a set point, determined by the first pot.

It's a very simple circuit. If you want the schematic, I'll send it later when I'm on my other computer.

Phil,

That would be great.

James L

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James L
Partner/Designer

Lil Brother SMT Assembly Services

vaclav_sal

Sounds like a job for Maxim chip. Check their on line catalog.

James Long

vaclav_sal said...
Sounds like a job for Maxim chip. Check their on line catalog.

Ha, can you make sense of their website?? I try a few times a year looking for something with Maxim, only to never find what I'm looking for.

I have actually tried to look up a chip I have in hand, to never be able find it on their site.

James L

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James L
Partner/Designer

Lil Brother SMT Assembly Services

vaclav_sal

Here is a link to Maxim search for "battery monitor".


http://search2.maxim-ic.com/search?ie=utf-8&num=20&filter=0&output=xml_no_dtd&client=english&oe=utf-8&site=english&q=battery%20monitor&proxystylesheet=english

James Long

vaclav_sal said...
Here is a link to Maxim search for "battery monitor".





http://search2.maxim-ic.com/search?ie=utf-8&num=20&filter=0&output=xml_no_dtd&client=english&oe=utf-8&site=english&q=battery%20monitor&proxystylesheet=english

Now not to be anal, but how many of those should I sift through to find one that will do a lead acid. I mean, I would rather spend time designing something opposed to going through page after page to find what I'm looking for.

I'm big into micro-controllers (Parallax ones) and make pcbs almost every week. What is one more to the list. I even advertise for Parallax for free. They let me advertise indiscreetly on their forum, so I feel it is warranted.

I've been on the internet too long to sift through a site like Maxim's. It's very unorganized, and non-user friendly. If you know the exact part number you are looking for, you are in business, but otherwise I use Digikey to find IC's.

I may search Digikey to see what is out there.

James L

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James L
Partner/Designer

Lil Brother SMT Assembly Services

mctrivia

try this one: BQ2013HSN-A514

I have not used it or no anything about it but from a quick search seams to fit what you are looking for.

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Need to make your prop design easier or secure? Get a PropMod has crystal, eeprom, and programing header in a 40 pin dip 0.7" pitch module with uSD reader, and RTC options.

Tracy Allen

Hi James,

What is the power requirement of the camera? Voltage min and max limits; also current drain, which may fluctuate different functions are activated.

I'd be inclined to use a low dropout regulator between the battery and the camera, one that has a shutdown pin so that it can be turned on only when there is enough juice to run the camera. For the ADC, an RCtime to monitor the battery voltage. For charging, the solar panel might not overcharge the battery, but one way to be sure is to have a dummy load that can be connected to the solar panel by bringing a stamp pin high. The Stamp could as you suggested go on the solar panel side of the circuit, instead of on the battery side. It would be possible to use a diode to limit the voltage to the camera, instead of a regulator, but a resistor in parallel with the camera might be needed to provide a minimum load.



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Tracy Allen
www.emesystems.com

Post Edited (Tracy Allen) : 4/27/2009 6:45:56 PM GMT

James Long

Tracy Allen said...
Hi James,

What is the power requirement of the camera? Voltage min and max limits; also current drain, which may fluctuate different functions are activated.

I'd be inclined to use a low dropout regulator between the battery and the camera, one that has a shutdown pin so that it can be turned on only when there is enough juice to run the camera. For the ADC, an RCtime to monitor the battery voltage. For charging, the solar panel might not overcharge the battery, but one way to be sure is to have a dummy load that can be connected to the solar panel by bringing a stamp pin high. The Stamp could as you suggested go on the solar panel side of the circuit, instead of on the battery side. It would be possible to use a diode to limit the voltage to the camera, instead of a regulator, but a resistor in parallel with the camera might be needed to provide a minimum load.

Tracy,

The power requirements of the camera are sort of elusive. The camera was designed to run on 6 volt dry cell batteries. I have read the amperage, and it runs at 10ma when in idle state, about 25 ma in active, 250 ma when the flash is charging (only charges at night, naturally). This is a "game" camera (so after a picture it goes into an idle state for a user determined length of time), and it really kills a battery in about 4 days (lead-acid). It mostly takes pictures at night, more animules you know.

I'm not sure of the min max, I know when getting around the 7.0-7.2 the camera will over volt, and will not work after it hits that state, no matter what happens (requires a reset, off then back on). Some where around 5.5 volts or so the camera will go to low battery, and will not work until reset (off then back on).

My father is technology challenged, so I have to deal with the camera all the time. Charging, changing, and just dealing with batteries is getting the best of me.

My problem with a regulator, is it wasting power.

My father is about to drive me crazy with this thing. If it goes dead, he is mad, and if it goes to over volts he is mad. I can't find a happy medium. I mean, I have other things in life to deal with like a SMT business??!!

Right now we are only using 4.5 Ah batteries, and they will run the camera for about 4-5 days on a single full charge. A dry battery (standard lantern battery) will run about 3 days max.

Basically all I wanted to do is charge the battery when it needed charged, and disconnect the solar panel when it doesn't.

I was going to use a chipfet to control if the panel was charging or not, and use the solar power on the BS2. This will prevent the need of knowing if the solar panel is putting out enough power. If the BS2 is running, there is enough power. I was going to throw a diode in the mix to the load to prevent the over voltage problem when charging (credit to Beau).

Is the dummy load absolutely required?

I have to admit being very stupid about solar power. I understand the basics, but the specifics, I'm clueless.

James L

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James L
Partner/Designer

Lil Brother SMT Assembly Services

mctrivia

I have a schematic for 98% efficient regulator can send to later today

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Need to make your prop design easier or secure? Get a PropMod has crystal, eeprom, and programing header in a 40 pin dip 0.7" pitch module with uSD reader, and RTC options.

James Long

mctrivia said...
I have a schematic for 98% efficient regulator can send to later today

I see you are just trying to complicate my life.

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James L
Partner/Designer

Lil Brother SMT Assembly Services

sam_sam_sam

James L

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··Thanks for any··that you may have and all of your time finding them

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Sam

Post Edited (sam_sam_sam) : 4/28/2009 12:50:18 AM GMT

Tracy Allen

James,

The way I understand it is that your dad wants to run the camera 24/7. Is it triggered by the presence of an animal (PIR), or does it simply take timed shots?

In any case, the simplest solution would be to use a low dropout float charger for the battery, set at 6.8 volts, below the cutoff of the camera. If the camera operates for 4-5 days on 4.5 amp-hours, 100 hours say, that means that the average current drain is around 45 milliamps. The 3 watt, 6 volt 500 mA solar panel with good average summer sun exposure should keep the battery pretty well charged up and your dad happy. Float chargers are quite simple and can be made with less than 0.5 volt of overhead.

The reason for a controller would be to allow fast charge control, and to shut down the camera either if there is not enough power available (save the battery) or to implement a planned schedule or a fallback schedule for the camera.

Here is a pared down schematic. It includes means to turn the camera power on and off, and to measure the battery voltage, and to divert the power from the solar panel. The Stamp is powered from the battery--Remember that the Stamp can SLEEP most of the time at around 50 microamps. On the solar panel side of the Schottky diode, the transistor can be turned on by the Stamp and divert all of the solar panel current to ground. If the pin is low or an input, then all of the solar panel current goes through the diode into the battery and loads. The Stamp would turn on that transistor when the battery reaches full charge. The RCtime circuit is used to measure the battery voltage. And the Stamp can turn on and off the power to the camera (load) using the second FET, for schedule control.



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Tracy Allen
www.emesystems.com

Post Edited (Tracy Allen) : 4/28/2009 11:02:10 PM GMT

James Long

Tracy Allen said...
James,

The way I understand it is that your dad wants to run the camera 24/7. Is it triggered by the presence of an animal (PIR), or does it simply take timed shots?

In any case, the simplest solution would be to use a low dropout float charger for the battery, set at 6.8 volts, below the cutoff of the camera. If the camera operates for 4-5 days on 4.5 amp-hours, 100 hours say, that means that the average current drain is around 45 milliamps. The 3 watt, 6 volt 500 mA solar panel with good average summer sun exposure should keep the battery pretty well charged up and your dad happy. Float chargers are quite simple and can be made with less than 0.5 volt of overhead.

The reason for a controller would be to allow fast charge control, and to shut down the camera either if there is not enough power available (save the battery) or to implement a planned schedule or a fallback schedule for the camera.

Here is a pared down schematic. It includes means to turn the camera power on and off, and to measure the battery voltage, and to divert the power from the solar panel. The Stamp is powered from the battery--Remember that the Stamp can SLEEP most of the time at around 50 microamps. On the solar panel side of the Schottky diode, the transistor can be turned on by the Stamp and divert all of the solar panel current to ground. If the pin is low or an input, then all of the solar panel current goes through the diode into the battery and loads. The Stamp would turn on that transistor when the battery reaches full charge. The RCtime circuit is used to measure the battery voltage. And the Stamp can turn on and off the power to the camera (load) using the second FET, for schedule control.

Tracy,

This camera uses a PIR but does have a "sleep time" built in for timing between shots. Yes, he wants it to run night and day, everyday.

Will diverting the solar power to ground be a problem?? I figure not, because you know what can and can not be done safely.

That looks to be a pretty simple circuit, and that is what I'm looking for. Of course nothing in the schematic do I have in house. I'll have to run to the local radio shack and see if they have any of it (yea right!!).

I need to place an order with Digikey anyway.

I do appreciate your help. I wish I knew it all, but don't. Let me know if you ever need a small project assembled, I may give a freebie.

James L

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James L
Partner/Designer

Lil Brother SMT Assembly Services

Carl Hayes

I would use an LM317 voltage regulator, adjusted (by a resistor or pot) to precisely 6.75 volts, which is a correct float voltage for a lead-acid battery nominally of six volts (three cells).· This will allow the battery to receive the full output current of the solar array until the battery reaches 6.75 volts, and then no additional charging current current will flow.· Actually you can go as high as 6.85 volts, but on no account should you exceed 6.9 volts.

Put a diode before the regulator (that is, between the solar array and the regulator) to prevent any possibility of backfeeding the array.· This wouldn't hurt the array, I think, but would waste energy from the battery during darkness.

Floating at 6.75 volts, a three-cell lead-acid battery will stay fully charged and will never overcharge.

Don't worry about the efficiency of the regulator.· You need the available output current of the solar array, and you will get it -- ·all of it.· You manifestly don't need, and cannot stand, the full available output voltage of the solar array.· Let the regulator throw away some energy; you couldn't have used that energy anyway.· Solar arrays act much more like current sources than like voltage sources.

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· -- Carl, nn5i@arrl.net

Post Edited (Carl Hayes) : 4/29/2009 6:25:53 AM GMT

Tracy Allen

Diverting 1/2 amp of solar panel current to ground is not a problem. That circuit with the IRL530 is a form of parallel regulator, and control is by switching either all on or all off, with near zero power in either state. The Stamp could PWM the MOSFET thru a resistor, and bring the voltage down gradually, but then the MOSFET would have to dissipate up to 3 watts. The diode prevents back current from the battery to the MOSFET.

I think your original idea was to put the power MOSFET in series between the solar panel and the battery. That could also work, and would be just as simple as the parallel approach if the MOSFET is placed in the negative leg of the solar panel so that the Stamp can control it directly without a level shifting transistor.

If you go into Rat Shack for parts, don't be tempted to purchase an IRF530 or anything in the "IRF" family. Go for something in the IRL family, where the "L" stands for "Low or Logic" threshold, low enough for the Stamp to turn it on fully. The transistor could alternatively be a generic NPN power transistor or darlington, capable of carrying the full 1/2 amp plus, but that would of course need a resistor between the stamp pin and the base. Maybe you already have one of those in a TO220 case. The diode does not have to be a Schottky. It could be a plain 1N4001, because you need to drop a little voltage anyway. The second diode in series with the camera may not be needed if the 1st diode and the Stamp as regulator can keep the battery voltage at less than the camera critical point. Power control for the camera with the second transistor may not be necessary, and it might not even work if the camera has a momentary push button for power-on.

Again I agree that a simple float charger set at 6.8 volts may be the simplest and most reliable alternative.

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Tracy Allen
www.emesystems.com

James Long

Wow, you guys are great.

I asked for simplicity, and you provided it.

I'm now not sure which method I'm going to use. I was looking forward to some stamp programming (it's been a while), but wasting a BS2p just to control battery voltage is like driving a nail with a jackhammer.

I may just order us a LM317. They are cheap enough.

Like I said, thanks for all the help.

James L

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James L
Partner/Designer

Lil Brother SMT Assembly Services

Tracy Allen

Here is another type of solar charger that uses the parallel switch concept, where a mosfet running cool, either fully on or fully off, diverts the solar panel current when the battery is fully charged. This uses a common 4.2 volt reset IC, a 3-leg chip that usually monitors power and generates a reset\ (output low) when the power supply voltage falls below 4.2 volts. But in this circuit, it senses the battery voltage and with the feedback keeps it in the range of 6.6 to 7 volts. I cribbed this circuit from a Motorola data sheet. The ratio of resistor R1 to R2 set the threshold, and R3 adds more hysteresis, above what the chip itself offers. C1 is needed for stability. This is like the Stamp circuit above, but dumbed down, no Stamp!



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Tracy Allen
www.emesystems.com

Post Edited (Tracy Allen) : 4/29/2009 8:45:25 PM GMT

Philldapill

Wow, Tracy, that IS simple... I've just reworked my own circuit and finished the PCB. It works wonderfully. If you don't mind shorting the solar panel or whatever power source, Tracy's will work great!

However, if your power source(solar, alternator on a car, etc.) doesn't like to be shunted... Try the attached circuit. It disconnects the Grounds of the power source, and your battery... Once the battery is charged, of course. It has some hysterisis that is adjustable using R7. You can adjust the "float" voltage by turning R6. For this particular circuit, you would need to use a 5V zener because this is intended for a 12V battery. It would still work fine for any battery down to 5V, but would need a lower value zener.

Here's the basics of how it works. The main chip is a run-of-the-mill quad op-amp. It doesn't have to be super fast, but it does need to be able to withstand the voltage of your power source(my solar panels are 32V... and that's pushing it). The first stage takes the Zener voltage refernce, and buffers it through a voltage follower. The second stage is a differential amplifier, and takes the voltage on the battery, then runs this through a voltage divider(R6). The output of the divider is run into another voltage follower to buffer it. The last op-amp takes the 5V reference, and the divided battery voltage, and compares them - with some hysterisis added by R7. If the battery voltage is below the value established by the hysterisis and voltage divider(R6), the output of the op-amp goes high. This turns on the MOSFET, connecting the Battery Ground, and the Power Source Ground, allowing current to flow into the battery. Once the battery voltage falls below the other threshold, the opamp goes low and the mosfet turns off.

Granted, this circuit would be a little more "standard" if a P-Channel fet were used, or a high-side driver were used for the current mosfet, but frankly, I didn't feel like messing with all that when I made this. [noparse]:)[/noparse]

James Long

Well....I decided to go ahead with the Stamp Solar Charge controller. This was a good exercise to learn a few things (mainly electrical, not programming).

I figured it would be a good base for someone wanting to take the control further.

I have attached the Schematic, and will post the final project one the PCBs are assembled and tested.

James L

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James L
Partner/Designer
Lil Brother SMT Assembly Services

Are you addicted to technology or Micro-controllers..... then checkout the forums at Savage Circuits. Learn to build your own Gizmos!

Cluso99

If you are into quick charging of large batteries, like AGM 510AH on my boat, search for the Outback MX60 charger. It inverts the input from the solar panel and has a 5 stage regulator to charge. Extracts an extra 20% from the solar panels compared to a normal regulator. I am talking 400-600W solar panels here. We charge initialy at 14.2V, reducing to 13.1V for float.

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