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Solar Charging with BQ25505-EVM

Adam Rahman
Prodigy 40 points
Other Parts Discussed in Thread: BQ25505, BQ25570, BQ24210

Hello, I am trying to use the BQ25505 EVM to charge a supercapacitor with a small solar panel for testing purposes. I am a little unsure as to how I am supposed to be connecting both the solar panel and the super capacitor. I am not sure of the configuration in which the super capacitor and the solar panel should be connected to the EVM in order to ensure optimal charging. the current setup is the solar panel is connected to J2 terminal block, and the SC is connected to J5 VSTOR block. It is charging very slowly even when the SC is precharged to 2 V. Im having a little bit of trouble understanding how the EVM works and which configuration would optimize the charging and the use of  MPPT. The panel produces about 5.5 V and roughly 500-700 uA of current. An indication of what type of output I should expect is also very helpful. Your help is greatly appreciated. Thanks! 

  • 0
    TI__Genius 12485 points
    Adam,

    You would have to look at VIN, VSTOR and VBAT_SEC to understand the board itself. Also, how big is the super cap? You might have to modify your VBAT voltage to match the super-cap's. There is an excel sheet to compute the resistor changes. www.ti.com/.../toolssoftware

    Also you might try conneting a DC supply say at 2V to VIN and then try charging the capacitor first. Then you can connect the panel to check back on the result.

    Regards,
    Gautham
  • 0 in reply to Gautham
    Prodigy 40 points
    Thanks for the quick reply,

    The super caps I am testing are a 6800uF. 5v and a 1.2F 4.5V. So the 4.2V output from the EVM should be ok. I have tried precharging past the 1.8V so that it would no longer be in coldstart but I did not notice a difference in charging rate. If the results I am getting are actually true then I am better off charging through a diode as it actually charges faster. But I find that hard to believe and I am sure that I am doing something incorrect. What configuration would you recommend I hook up the super cap to the EVM?

    Thanks
  • 0 in reply to Adam Rahman
    TI__Guru**** 166896 points
    Did you say your panel provides 5.5V? If so, this is too high for the boost based charger. It expects a voltage lower than VBAT_OV setting. When higher, it actively tries to pull down the input voltage below VBAT_OV.
  • 0 in reply to Jeff F
    Prodigy 40 points
    5.5V is the Voc of the panel, but the evm is outputting around 4.2v just like it is designed to. Excuse my ignorance as this really isnt my area but would this cause any kind of problem when trying to charge? Im only trying to charge the SC up to 4V. Im really just trying to figure out why my SC is being charged so slowly. As it currently stands it is being charged way slower through the EVM than if I just connected the panel straight to the SC which tells me something is wrong. Shouldnt this chip charge the SC more efficiently?

    Thanks,
    Adam
  • 0 in reply to Adam Rahman
    TI__Guru**** 166896 points
    Assuming 80% MPPT setting, the boost charger will pull the output of the solar panel down to 0.8*5.5V = 4.4V. This is greater than our VBAT_OV = 4V setting. So the IC is likely pulling current to ground through VIN_DC. That extra pull down plus the diode from VSTOR to VBAT is giving 4.2V on the output. The bq25570 is not regulating when VIN_DC>VBAT_OV. I do not recommend using it with this panel. If you get a panel with a lower VOC, you can use the bq25505. If not, I suggest looking at the bq24210 but it regulates only to 4.2V.
  • 0 in reply to Jeff F
    Prodigy 40 points

    I have looked into the Bq25570 and it will be considered but the bq25505 is recommended by the manufacturer of this panel and by the SC manufacturer.  I have the evm connected to a 3.5Voc  400uA panel right now and after 12 hrs the SC is at 0.1v and thats after I initially charged it with a battery up to 2.5V so that it would not be stuck in cold start. For some reason it discharged. Where as the same super cap under the same conditions charged up to full without the chip in half the time. I have tried it now with a variety of panels a with varying Voc. I have tried precharging the SC I have tried different SC's with no luck. My main question is now , if you had the BQ25505-EVM and you were going to use it to charge a SC up to 4.2V (According to the technical documents that is the output of the EVM). What is the setup you would use. What terminals should be connected where? I have different types of panels I can test (DSSC and Si) and with varying Voc from 1.5V - 5.5V and I have tried them all with no success even when VIN_DC < VBAT_OV.

  • 0 in reply to Adam Rahman
    TI__Guru**** 166896 points
    I meant to say bq25505 in my previous post. If you use either the 05 or 70 with a supercap at BAT, both chargers stay in cold start for a long time. One method to avoid that delay using the bq25570 with supercap at VOUT instead of BAT is explained here: www.edn.com/.../Fast-charging-a-supercapacitor-from-energy-harvesters. Another method is to place current limit resistor from BAT to the supercap in order to limit its charging current until the voltage gets close above 1.8V cold start threshold and then short out that resistor. Keep in mind that supercaps tend to have higher self-discharge currents.