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  • Help with designing a solar charger powering an autonomous robot

    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: http://www.kynix.com/uploadfiles/pdf9675/MAX4193CPA.pdf), however, it could be adjust using a voltage divisor, I configured to aprox. 10V. (Schematic attached: MAX633_schematic.jpg)


    The output of MAX633 is connected to MCP73864, the schematic is also attached (MCP73864_schematic.jpg). 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 that could give me some advice?

    Thanks in advance.

  • #2
    The first problem I see, is you need a big filter cap on the load side of D3, to present a fairly even
    load to the panels. Your converter draws current in big lumps, which the panels cannot efficiently
    provide. Bruce Roe

    Comment


    • #3
      Originally posted by Joshua_S View Post
      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)
      What do you mean in parallel? You cannot measure current in parallel.

      One other thing strikes me is why you are using Boost rather than Buck? You want at least a 20-cell panel to charge 2S LFP, 36-cell and higher is better.
      Last edited by Sunking; 04-25-2018, 11:14 AM.
      MSEE, PE

      Comment


      • #4
        Originally posted by bcroe View Post
        The first problem I see, is you need a big filter cap on the load side of D3, to present a fairly even
        load to the panels. Your converter draws current in big lumps, which the panels cannot efficiently
        provide. Bruce Roe
        Bruce he is using a Boost Mode. Capacitor would be on the load side of L I think.
        Last edited by Sunking; 04-25-2018, 11:18 AM.
        MSEE, PE

        Comment


        • #5
          Originally posted by Sunking View Post
          Bruce he is using a Boost Mode. Capacitor would be on the load side of L I think.
          Appears to me, the panels minus connect to GND, panels plus to SPP_OUT. The panels can only supply a
          continuous current, not the lumpy peak current the converter wants, so smooth it out with a filter at the junction
          of D3, L2. Anyway you do not want that high frequency current element flowing out through the inductance
          of the panel feed, besides killing the ckt, radiates a lot of noise. Output is filtered by COUT1.

          Adding to that, if the devices have an on/off lead, I'd put a V det on that filter cap (with some hysteresis) and
          only turn it on when it charged up to the high side of MPPT voltage. Bruce Roe

          Comment


          • #6
            I don't see what you need D3 for, the input does need a good size capacitor. Given that this is unlikely to see wide temp changes, the boost converter can be run in MPPC mode keeping the panel at a constant voltage. This can easily be done using a TL431 monitoring the panel voltage (even just a FET would work). Just have a pull up resistor on the cathode. Feed that through a diode to the feedback pin. When the panel voltage drops below the set point the resistor pulls up on the feedback pin faking the boost converter into thinking the output is over voltage and shutting down. Without this the panel is pulled into a death spiral and produces little output.

            Upon looking at the data sheet it looks like putting a voltage divider on Ic will monitor the panel voltage and shut down the chip. I didn't look but some chips have a delayed start of a couple ms and having a decent capacitor on the input will be needed. The output may bump on and off. It will still be efficient at charging a battery.
            Last edited by PNPmacnab; 04-25-2018, 02:47 PM.

            Comment


            • #7
              Originally posted by Joshua_S View Post
              Any idea? Someone trying to do some solar cell charger that could give me some advice?
              1) Add some capacitance at the input (as others have suggested.)

              2) Measure panel voltage at the input. That boost converter will operate down to 1.3 volts, so you are likely pulling the panel down to ~1 volt, and the resulting power from the panel (160mW) isn't enough to get the charger fired up, since you have it configured to provide about 5 watts to the battery.

              Also there is something puzzling about your design. If you have a 7.2 volt solar panel (which means you can count on about 5.5 volts when it gets very hot) why are you trying to "boost" it to 5 volts? (The MAX631CPA boosts to 5V) The converter won't work that way. Why not connect it directly?

              Or did you mean to use the MAX632 to boost to 12V instead? If so that's useless with the MCP73861, because that only needs 4.5 volts. Or did you really mean a MAX632 AND an MCP73862? (the 2-cell version of the charger.) If so you have some work to do.

              To fix this:

              -Get rid of the boost converter. You don't need it with the existing schematic parts.

              -If you still want to boost, design and build a circuit that connects to the PROG pin of the charger. Configure it so that if the panel input voltage goes below about 5 volts, the resistance on the PROG pin goes up. (You can probably do this with a 3.9V zener, a resistor and a FET.) That way, if the charger is about to collapse the panel voltage, the charge current is reduced and the panel voltage does not collapse.

              If you want to experiment, connect a variable pot (a 20K should do it) between PROG and ground. Then play with it and see if you can find a setting where the charger will start up and work. I suspect you'll be able to. Of course the pot setting will vary with available light.

              Comment


              • #8
                Wow, so many experts here, thanks to all of you for your response and tips!!!
                You know I had a look at the pdf of the MCP73861/2/3/4: http://ww1.microchip.com/downloads/e...Doc/21893F.pdf
                On Page 12, it shows that it starts, lowest output at about 150mA charge current, when i have the Rprog open, puting a 1.6k resister means that i want it to charge with 500mA, if connect direct to ground 0 Ohm, it is to use the maximum charge it can give - over 1 Amp.
                Looking at my diagram, i am using the soic version as per pin output. And about the schematic, i have pin3 (Vset) tied with VDD1+2, so you can see i have it programmed for an 8.4v. I though haven't using the bypass cap to Vss as recommended on page 13, maybe this is need for when the IC changes states of charging?
                Now, on Page 12 - same diagram FIGURE 2-31, it shows that if i put pin 7 direct to pin 6 Vss (ground) it will want to use more than 1 Amp.
                On page 17, what i got is to use it at 100 mA, so i need to leave it floating, in other words, will NOT connect it to anything, right?
                Now i have check out my schamatics again, guess maybe i should put a small capacitor of at least 4.7uf/16v on pins 4/5 to ground - observing polarity, and let pin 7 (prog) float now that i want it to work with the solar cells - aka 100 mA charge current? Correct me if i am wrong.


                BUT hmmm....maybe get rid of the MAX633 is a better idea, and adding some capacitance at the input as all of you guys have suggested is what i see now......
                and @jflorey2 a MAX632 AND a MCP73862 would be good for my need? If the answer is yes, can i ask for some guidance of you?

                Thanks again for all of you and may all of you have a good day!

                Comment


                • #9
                  Whenever I see a pretty schematic on the internet my first inclination is to run fast and to run far. Everyone wants to jump to being a designer before learning electronics. The data sheet is your friend. Looking again it says " The shutdown mode is entered whenever pin 6 is driven below 0.2V or left floating." Your pin is floating. OK, I'll just assume you forgot to draw that in. It is unusual that it is 0.2V and not the reference voltage. Most other switchers are higher, but you can work with 0.2V as an input voltage monitor.

                  Switchers always need an input capacitor because they draw current in pulses. If the regulator bumps in shutdown that cap should be at least 1,000uF. Output current will always be less than input current. Panel current will stay about the same for the same light level. Try and draw more than that and the panel voltage drops. V times I is power and if V drops you have lost what little power that panel can produce. You must add circuitry to keep the panel voltage at power point. A simple voltage divider down to 0.2V will likely work.

                  Comment


                  • #10
                    Originally posted by Joshua_S View Post
                    BUT hmmm....maybe get rid of the MAX633 is a better idea, and adding some capacitance at the input as all of you guys have suggested is what i see now......
                    Those panels will give you about 8 volts reliably, but you need 9 volts to keep the charger IC running. So I'd go with 3 panels in series. That gives you 10.8 volts reliably.

                    If your batteries are like most lipo batteries the MCP73864 would be more appropriate (4.2VPC charge voltage.) However the MCP73862 will work OK, just won't charge to 100%.

                    Again, you will need to limit the charge current to match the power from the panel. And again, a zener plus a FET will do that for you. (Will also need a resistor or two.)

                    Comment


                    • #11
                      Originally posted by jflorey2 View Post
                      Again, you will need to limit the charge current to match the power from the panel. And again, a zener plus a FET will do that for you. (Will also need a resistor or two.)
                      Agree you are way over thinking this trying to solve a simple problem with way to much automation. You do not need any controller. All you need is a Zener Diode of 7.2 volts and a series resistor. Should not cost more than $1 and will work every time. If i tmakes you fel better call it a custom made Constant Current Constant Voltage charge custom made for a lithium battery. KISS (keep it simple stupid). Size it correctly and it will never fail.
                      Last edited by Sunking; 04-26-2018, 11:37 PM.
                      MSEE, PE

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