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bq25504: Guidelines for choosing solar cell, one cell 0.5V more amps, or 3 cells 1.5V less amps

lloyd konneker
Expert 1790 points
Part Number: BQ25504

Input to the BQ25504 is an IXYS product.  In the same size, it comes in either one cell KXOB22-12X1L or 3 cells.  I have tried the single cell.  It is agonizingly slow at charging a super cap of say 5F to 3.6V, even in full sun when the cell should be producing about 40mA at 0.5V.  I calculate it should charge the capacitor is a few hours.  Would I be better off using the 3 cell solar cell producing 14mA at 1.5V? 

 Maybe my calculations are wrong, or I have programmed BAT_OV wrong.

  • 0
    TI__Mastermind 21590 points
    Hello,
    The more input power given to the energy harvester, the faster the energy storage element will be charged. You can use the 3 cells to get higher input power into the energy harvester.
    Regards,
    Raheem
  • 0 in reply to Raheem
    Expert 1790 points

    Please clarify.  Do you mean a higher voltage (for the same power) the faster it will charge?  Note that the power (W) and area is approximately the same for the two alternative solar panels (both are about 2cm squared, both about 20mW (one is 0.5Vx50mA, the other is 1.5Vx15mA.)  

    I suppose you refer to Figures 1,2, and 3 (Efficiency versus input voltage) from the data sheet.  I can see that at very low voltage and current (say 0.5V and 10uA) the efficiency is very low.  Is that what you mean?  But in full sun, one panel (0.5V and 50mA) has efficiency 80% while the other panel (1.5V and 15mA) has efficiency of 90%.  Which is not much difference in efficiency.

    I suppose you mean that the knee in those curves is at about 0.5V, that efficiency drops precipitously below that voltage, that a small change in voltage around 0.5V makes a large difference in efficiency, that the actual voltage might be less than I think and thus inefficient.

    So a guideline might be stated "use a solar cell having at least 0.7V, to avoid inefficient operating regions." ?

  • 0 in reply to lloyd konneker
    TI__Mastermind 21590 points
    Hello Lloyd,
    Looking at figure 3 for about 10mA input current and about 1.5V at the input, you'll see that the efficiency is about 89%. if you look at figure 6 with 0.5V and 40mA at the input, the efficiency is about 77%. Looking at this data, going with the 3 cell seems to be the better option.

    Regards,
    Raheem