Help with designing a solar charger powering an autonomous robot

Sunnyback

Hello,  

Thank you for your time reading my post.  

Background.  

I'm working on a project where I need to design a solar charger (actually, all the control system) for 2-series Lipo batteries that will power an autonomous robot.  

My first try was just connect the solar cells array directly to the input of the Lipo Charger. That approach was not quite good, because the solar cell i-v characteristics.  

Further research on Solar cells pointed out that cell delivers maximum power Pmax when operating at a point on the characteristic where the product IV is maximum, and some techniques had been developed to work a solar cell as close as possible to this point, using a boost converter and a uC to measure I and V of the solar cell and make a switching control with the boost converter circuit.  

I have no time to implement the last solution, therefore I decided to use a simple boost converter between the solar cells and MCP73864. 

Schematic description  

I have designed a prototype Charge_board,in where is included the Solar charger I have mentioned above. The boost converter I use is MAXIM MAX633, It has an internal N-MOSFET driver, its default output voltage is 12V (see datasheet: <a rel="nofollow" href="http://www.kynix.com/uploadfiles/pdf9675/MAX4193CPA.pdf" title="Link: http://www.kynix.com/uploadfiles/pdf9675/MAX4193CPA.pdf">http://www.kynix.com/uploadfiles/pdf9675/MAX4193CPA.pdf</a>), however, it could be adjust using a voltage divisor, I configured to aprox. 10V. 

The output of MAX633 is connected to MCP73864, the schematic is also attached. The solar cells are from POWER FILM cell array is build using (2) MPT3.6-75 serial connected and (2)MPT3.6-150 connected the same way, and then connecting both arrays in parallel directly to MAX633 input. This array should provide (in theory) an output of 7.2V@150mA)  

I must say that I use a schottky diode betweeen cell array and BOOST converter.  

Issues

I have designed and built a prototype PCB, I connect the cell array and the MAX633 output is correct, as well as lipo charger's output.  

The problem is that when I connect Lipo batteries, the charger could not provide current, nothing!, I use a multimeter to measure current (connecting directly in parallel to know the max current that the circuit could provive), I measured cell array output's, and it provide 160mA, in the MAX633 output = 156mA and MCP73864 output = 0mA, nothing.  

It seems like if the lipo charger just do not use the current provided from the BOOST converter output.  If instead of cell array I use a supply (7.2V), the charger works perfectly.  

Any idea? Someone trying to do some solar cell charger could give me some advice?  

Thanks in advance.  

bill von novak

Sunnyback said:

Background.  

I'm working on a project where I need to design a solar charger (actually, all the control system) for 2-series Lipo batteries that will power an autonomous robot.  

My first try was just connect the solar cells array directly to the input of the Lipo Charger. That approach was not quite good, because the solar cell i-v characteristics.  

Further research on Solar cells pointed out that cell delivers maximum power Pmax when operating at a point on the characteristic where the product IV is maximum, and some techniques had been developed to work a solar cell as close as possible to this point, using a boost converter and a uC to measure I and V of the solar cell and make a switching control with the boost converter circuit.

Didn't you post this on another forum?

In any case, you don't need all that stuff.  First choose a panel arrangement that will give you at least 8.4 volts at maximum estimated temperature.  Then use either a regular zener (IN5343 or equivalent) or an accurate zener circuit connected as a shunt regulator (http://www.ti.com/lit/ds/symlink/lm431.pdf) to limit the voltage.  Connect directly to the battery, or via a diode if you are worried about dark current. That's really all you need.

If you really want to use the charger, then use a 9.1 volt shunt zener at the input and connect the panels to the input of the charger.  You will have to limit the current to BELOW what the panels are providing to prevent activation of the protection circuitry inside the charger.  A zener diode driving the base of a transistor connected to the current limit pin will do that for you.

It is very difficult to get switchers to work with small panel arrays due to voltage collapse, so I wouldn't try that.

Sunnyback

Yeah, you got me man, i guess this is the answer i need,  a panel arrangement, zener, battery and a diode...hmmmm all the stuff i need are here,  small panel arrays with switchers is not easy to be done(maybe not necessary either?), i admit that.
Thanks for your help Bill and may you have a nice day!