• State Resolved
  • Locked Locked
  • Replies 4 replies
  • Answers 1 answer
  • Subscribers 64 subscribers
  • Views 631 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

BQ25504 PV harvester charging issue

Nikolaos Terzopoulos
Prodigy 40 points
Other Parts Discussed in Thread: BQ25504

Hello,

I am using the bq25504 as PV harvester with several PV cells in order to charge a 4.1V, 50uAh solid state rechargeable battery.

The issue I am dealing with is that when using a double PV cell with VIN_DC=1.08V and total cell power @1000 lux   145.0uW the bq25504 harvester is able to charge the battery (over night charge) and keep my system up and running for more than 2 hours in dark conditions.

However, when trying a more efficient single PV cell with VIN_DC=0.63V and total cell power @1000 lux   233.0uW with the same over night charge as above the my system is not running for more than 20minutes in darkness.

1) Is this has to do with the efficiency of the bq25504 ? (since the VIN_DC varies from 1.2V in the double PV cell case to the 0.63V in the single PV cell case)

2) How can I perform bq25504  efficiency calculations? (based on measured data)

Many Thanks

Nik

  • 0
    TI__Guru*** 148885 points

    Using the datasheet efficiency curves to estimate efficiency is the fastest and simplest method.  From d/s figure 7, assuming you are getting the 223uW and 145uW, respectively, the efficieny should only be about 10% different with VIN_DC regulating to 80% of 0.63V and 1.08V.

    If you want to measure efficiency of the bq25504, I suggest reading http://www.ti.com/lit/pdf/slua691 and http://www.ti.com/lit/pdf/slyt558. 

    Can you send your schematic and a scope plot showing LBOOST, VIN_DC, VSTOR and IIN_DC (if you have a current probe) zoomed in to show two or more switching pulses?   I want to confirm that you VIN_DC is regulating to 80% of your open circuit voltage.

  • 0 in reply to Jeff F
    Prodigy 40 points

    Hi Jeff,

    Thanks for your reply.


    My schematic is identical to figure 2 of the d/s, as attachment you can see a scope plot showing VIN_DC, VSTOR and an indirect measurement of Iin.

    My measurement results suggests that the Pin when using the double PV cell is 180uW, while in the single PV cell (but more power efficient) case Pin drops down to 80uW. Can the 10% difference in efficiency justify this ?

    I am using a 3.9V/ 50uAh rechargeable battery connected at Vbat pin and a 60uW load connected at Vstor pin.

    1031.SCOPE PLOT.pdf

  • 0 in reply to Nikolaos Terzopoulos
    TI__Guru*** 148885 points

    What is VR10? 

    I configured an EVM with sourcemeters on the input and output as shown in the attached plots.  The exact test setup is explained here:  http://www.ti.com/lit/pdf/slua691.

     The input sourcemeters were outputing current at their compliance level and regulating to the voltage measured on the screen.  The output meter was configured to 3.9V (VBAT_OV=5.5V) and sinking current as shown on the screen.  As you can see, I get efficiency numbers near 80% for both with the 0.63V configuration giving more output power, as expected. 

     

     

    bq255xx_test.zip
  • 0 in reply to Jeff F
    Expert 2600 points
    Dear Sir,
    I am using BQ25504 as our PV harvester charger. our vin =1.7V , Iin=0.2A, UV=2.7V, OV=4.2V, Vstor=VBAT=4.2V
    so far we have EVT 10kits , five kits function are good which can boost to 4.2V Vstor. Other five kits can't boost to 4.2V. it only show 1.6V vstor. After checking the Vin , it seems to drop to 0.6V. But these 10 kits are all the same condition.
    May you give me some suggestion what exactly happens? Thanks.