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bq24650 - solar charger for SLA

marcelo deoliveira
Intellectual 840 points
Other Parts Discussed in Thread: TPS61200, BQ24650

Hi Chris

I am designing a solar light which has a 6V SLA Battery, in this case it will be 3 cells in series with total of 2500mAh

 http://www.houseofbatteries.com/documents/Cyclon.pdf

I have the BQ24650EVM board. I will drive a LED during the night with 150mAh and it has to last throughout the night. I want if is possible be able to charge this battery at its full during a single sunny bright day, considering a pic solar radiation as of 1000Watts/m^2 . In South Florida solar radiation for a design is to be considered as 5 hours of sun light. My intention is to use a solar panel between 2 to 3 watts monocrystalline. For such low voltage I have read and educated myself that the best approach is a boost conversion. I would like to use this board but it seems that the min Vin is 5 V so since I will be charging a battery which requires around 7.35V, I would like to use a lower voltage solar cell and have its voltage booster to mach the part. To be very honest to you I don't know if I am going in the right direction. Solar cell size is  physically an issue, I don't have to much space to put it and I would like to be very efficient during low light condition ( clouds ) . I also don't believe it will be possible to charge this battery considering fast charge, so please let me know what is the best direction . One last thing, what do you think of using a TPS61200 between solar panel and BQ24650 to bust the solar voltage under low light condition ?

Thanks

 

  • 0
    TI__Guru**** 150640 points

    I split this thread since it's a new topic.

    The minimum Vin to the bq24650 is the maximum battery voltage plus some dropout voltage (around 0.5V but depends on charging current).  So, you need to boost higher than your maximum battery voltage to charge at all.  The bq24650 is buck based.  It can only step down.

    This means you'll need a different boost than the TPS61200.  One that can give 8 or 9V out.

    But the easiest thing to do with the bq24650 is to have your panels put out a high enough voltage that the bq24650 can charge from that. I really recommend this if you can.

  • 0 in reply to Chris Glaser
    Intellectual 840 points

    Hi Chris

    I understand, but don't you think a higher voltage panel something like 10Vdc would have some issues in lower light conditions ? Is there a part to replace the TPS61200 that would boost to a higher voltage ?

    Thanks

     

  • 0 in reply to marcelo deoliveira
    TI__Guru**** 150640 points

    I assume the size of the lower voltage panel would be the same as the higher voltage panel and thus the output powers of each would be equal.  If so, then going directly into the charger and not through a front end boost will dramatically improve the efficiency of the system by bypassing the first conversion stage.

    Also, the bq24650 reduces the charging current to keep the input voltage to a pre-determined level.  So, if the panel can only output so much current, it will charge at a slower rate.

    We have lots of higher voltage boosts. You can find one you like at power.ti.com.  You might be able to use the same TPS61200 MPPT circuit if the boost starts operating at a higher voltage than your op amp of choice.

  • 0 in reply to Chris Glaser
    Intellectual 840 points

    Hi Chris

    Below is the specification of the panel that I am going to use , the battery is a 6V SLA as described before, could you help me to figure out the value for the resistors ?

    The board I have is for 20V power source and a 12 V battery.

     

    Voc  10.8V
    Isc 0.61A
    Vmp 8.8V
    Imp 0.57
    Pmax 5W
  • 0 in reply to marcelo deoliveira
    TI__Guru**** 150640 points

    I'm not sure which resistors you are referring to.  Are you going directly from the panel to the bq24650?  The bq24650 datasheet explains how to size the components.

  • 0 in reply to Chris Glaser
    Intellectual 840 points

    Hi Chris,

    For the paper SLVA437 page 7, what diode do you recommend ?

    Thanks

  • 0 in reply to marcelo deoliveira
    TI__Mastermind 32350 points

    Hi Marcelo,

     

    I assume you are trying to disable the pre-charge phase using the clamper circuit described in the SLVA437 on page 7. I designed the clamper for using the bq24650 as a super cap charger, and the attached file describes my finding using this clamper circuit.

     

    Basically the clamper circuit can not disable the pre-charge completely as shown in the test results.

     

    Regarding the values of the resistors, equations are provided in the attachment. For the diode just make sure that the forward voltage does not change a lot with the applied voltage across the diode and include the forward voltage in the equations.   

     

     

    Thanks!

    Tahar

    Pre-charge mode.doc
  • 0 in reply to Tahar Allag
    Intellectual 840 points

    Hi Tahar

    Thank you for your answer, I did put a Bat54 and seems to work. You have to make sure that the voltage after the diode is lower than 2.1V and above 1.55V otherwise it will not work. If is less then 1.55V it will go to pre charge mode, and if goes above 2.1V it will turn off the charger. If I use the values given by the paper it will not work so I did use two 10K resistors. My issue now is that my solar panel MPPT is 7.45V, so therefore when battery is getting close to full charge (close to 7.45) it starts to blink , so it turns ON and OFF the solar panel. Do you have any idea how much higher does the panel voltage has to be reverence the battery ? I am using a 6V SLA, which has to be charged in buck mode with 7.45 V.

    Thanks

     

  • 0 in reply to marcelo deoliveira
    TI__Mastermind 32350 points

    Hi Marcelo,

     

    You are right, equation 01 has a small mistake and it should be:

    (V FB REG + V LOWV ) / 2 = V REF R2 / (R1+R2) – VF 

     

    I used the bq24650 evaluation board tested the proposed circuitry. The feedback voltage regulation is given in the datasheet to be V FB REG: 2.1V and V LOWV: 1.55V. The reference voltage is V REF: 3.3V. In the EVM board, the R3 and R4 values have been given to be R3=499KΩ and R4=100KΩ. They are designed for V BAT (Full Charge): 12.57V.  The diode is chosen to have approximately 0.5V feed forward voltage. Using equation 01, R1 and R2 can be designed. We chose R2=10KΩ to be 10 times smaller than R4 and solving equation 01 for R1 and found to be 5KΩ.

     

    Regarding the solar panel question, see the attachment.  

     

    The V-I characteristic of a typical solar panel is shown in Figure 1. The maximum current can be obtained by shorting the solar panel terminals or at very low resistance across them.  And by increasing the terminal voltage above maximum power voltage (Vmp), the current decreases rapidly. To obtain maximum power, the voltage solar panel needs to be maintained around the maximum power voltage (Vmp) as shown in Figure 1.

    The maximum power point depends on the maximum point voltage and maximum point current. Obviously, the solar panel obtains its power from sun light. Thus, as the sunlight reduces, the maximum current reduces as well. As consequence, that affects the maximum power point. In Figure 1, Sunlight 1 is greater than Sunlight 2; therefore Iamp1 is greater than Iamp2.    

     

    As you said, your MPPT point voltage is around 7.45V. As the battery reaches that level, the charging current decreases and based on figure 01, the solar panel voltage increases allowing more charging room to the battery. The process keeps going on and on till you reach the maximum voltage of the solar panel. I am not sure what do you mean by turning on and off the solar panel.

     

    Thanks!

    Tahar

     

    Figure 1.doc
  • 0 in reply to Tahar Allag
    Intellectual 840 points

    Hi Tahar

    The issue that I am having is that when battery goes above 7V, solar panel voltage starts going all over the place from 6.4V to 9V and current drops from 400mA to 200mA but unstable, so it varies from 250mA to 180 mA. As it goes, current keeps dropping more and more but I have not seem it goes down to zero. LED D4 starts blinking when this happen and as I lower the intensity of my light source voltage of solar panel then becomes stable again around 7.45V and then current gets stable around 200mA. I did do the same thing on the power supply and also using a programmable load and did not see this happening. I did measure Vfb  and when this starts to happening it's voltage is around 1.97V. Doing the simulation with power supplier currents starts to drop when Vfb is around 2.09V which it seems far from 1.97V and also LED does not blink. Any idea ?I

     

  • 0 in reply to marcelo deoliveira
    TI__Mastermind 32350 points

    Hi Marcelo,

     

    Of what I understand, your MPPT point voltage is at around 7Volts. Thus, as the battery gets charged to that level, you have almost the same input and output voltage levels. Since the bq24650 is not a boost converter it can not charge up the battery more than the input level. Therefore, the charging current decreases and the solar voltage increases based on the V-I solar panel characteristics shown in figure 1 in the previous post. As the input voltage increases, the bq24650 is able again to charge up the battery but with lower currents as you described. The process will keep going till the current reaches very low levels.

     

    The LED D4 blinks, because it indicates that the bq24650 in charging and not charging. Please see table 02 (page 17) on the datasheet for more info on this LED.   

     

     

    • Can you adjust the MPPT point based on the voltage of the battery?
    • To how much you want to charge up your battery (Voltage)?

       

     

    Thanks

    Tahar

  • 0 in reply to Tahar Allag
    Intellectual 840 points

    Hi Tahar

    I did change solar panel to a 8.5Vmpp, so it worked well until battery reached 7.45V which is the cutoff, so after that LED  STAR 2 turns ON for a second then STAT1 turns OFF, then STAT 1 turns ON and STAT2 turns of, so they do that all the time . If I measure battery voltage while STAT 1 is ON Vbat is equal to 7.45V but when STAT2 turns ON its drops to 6.5V. Why is this happening ?

    Thanks

     

     

  • 0 in reply to marcelo deoliveira
    TI__Mastermind 32350 points

    Hi Marcelo,

     

    • When STAT1=ON and STAT2=OFF the charging is in progress.
    • When STAT1=OFF and STAT2=ON the charge is complete.

     

    When the charge is complete the charger stops charging the battery, it does not apply any voltage on it, due to battery characteristics, its terminal voltage drops to 6.5V.

     

    Can you set your voltage regulation level to 7.45V?

    What is the desired voltage of your battery to be considered fully charged? You can play with your feedback resistor to avoid complete charging.

     

    Also, can you play with your MPPSET feedback resistors as described in the datasheet on page 13 using equation (2)? Because if the voltage at the MPPSET pin pulls down below the 75mV, the charging is disabled   

     

    Thanks!

    Tahar

  • 0 in reply to Tahar Allag
    Intellectual 840 points

    Hi Tahar

    Understood, but I did set the voltage regulation level to 7.45V When it reaches this voltage Vfb is equal to 2.1V so it then goes to and complete charge mode, but the problem is that at this point since Vbat drops to 6.5V it will try to charge again. Because this battery has a low internal resistance I think that is why it keeps doing that. Do you think it will damage the battery ? Attached is the battery data sheet. Model 0810-0004.

    Thanks

    2330.US-CYC-SG-003_0608.pdf

  • 0 in reply to marcelo deoliveira
    TI__Mastermind 32350 points

    Hi Marcelo,

     

    Yes, I agree with you that this is happening because of the internal resistor of the battery. bq24650 is suited for LI-Ion/Polymer , Lead Acid battery chemistry types. You may ask Cyclon if that is healthy for their batteries.  

     

    Thanks!

    Tahar

  • 0 in reply to Tahar Allag
    Prodigy 10 points

    Hi, 

    I'm starting to developing an USB solar chargers. I'd like to design these charger to have an USB output from a DC/DC converter directly connected to the solar panel. The solar panel thath I have is a 5W solar module with an output of 1.8V at maximum power ( 2.1V Voc ).
    I would know if it's is possible to use bq24650 in boost mode like described in (http://www.ti.com/lit/an/slyt466/slyt466.pdf) whit my solar panel.

    Thanks in advance for collaboration.

    Best regards