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Energy Harvesting and high efficiency Gallium Arsenide solar cells

Pentleman
Intellectual 970 points
Other Parts Discussed in Thread: BQ24210, BQ25505, BQ25504, BQ25570, BQ51013B, BQ51013

I have an application that is using an series of AltaDevices high efficiency solar cells, Voc = 1v, Isc = 225 mA.    I am planning on using either 5 in series or two parallel sets of 3 in series in my application which is an ocean camera with sensors.   I am greedy and would like to harvest the tiny bits of energy when the animal is in low light and want full power for charging the lithium batteries (4.2v) when near the surface.

I'd like maximum power point tracking to squeeze the solar.     A processor will prevent the batteries from ever getting fully discharged.  

I would also like to support an external charging circuit path to the lithium battery and I'm using a TI wireless power BQ product.   I do not want this charging circuit to cause battery drain.

I'd rather not use two energy harvesting devices but that is looking like what is required to get maximum solar power to the batteries.   The BQ24210 does not seem a good fit even tho it can supply 800mA.

Thoughts:

1) Use two BQ25505 (or BQ25504?), one for each string of 3 solar cells.    

Questions:  if one string of cells is supplying and the other is shaded, will there be an issue connecting the outputs of the two converters?  Do I need diodes or some cool pass FET circuit between the harvesting converter outputs and the battery?

What is the recommendation for BQ51013B also connected to the battery - I'd like to prevent leakage and I do care a lot about microamps.

2) Use on BQ25504 and live with it's max 100mA and create a processor controlled FET circuit that can directly connect the solar cells to the battery and bypass the BQ during high solar/current availability (using the EN pin to disable the BQ)

I have a space constrained design for the electronics and also want to avoid complexity (sprinkling processor controlled FETs might be challenging for me to implement well).

Thoughts and suggestions would be welcomed.

Thanks in advance,  Thom

  • 0
    TI__Genius 12475 points

    Thom,

    That is an interesting application! 

    Have you looked at the BQ25570 where you have an integrated DC/DC to run the MCU from? to get to lowest leakage, you are right you might have to use the /EN pin. this will save you from a processor controlled FET ;)

    On the muxing portion, I have been able to pull /EN pin to USB voltage and was able to "disable" the charger from charging the battery. You should be able to connect the two outputs together. 

    BQ51013B is a wireless power receiver. Just making sure this is the right device.

    Regards,

    Gautham

  • 0 in reply to Gautham
    Intellectual 970 points

    Yes BQ51013B is what I'm using to charge the batteries in the lab, at sea it is the solar.

    I did look at the BQ25570 and built my first board with that devices when I was using IXYS solar cells.  BQ25570 is good on the lower end but tops out at 100mA.

    When the EN pin on the wireless charger BQ51013 is disabled, my question is about the parasitic leakage - the wireless power will be disabled for 9 months and I'd rather not be draining the battery by a few microamps as that adds up especially if the solar is in the dark.   It's a tradeoff tho between circuit complexity and battery conservation.

    What do you think about using two BQ25570 in parallel to get 200mA?   Diode or'ing needed or not?


    Thanks for the note.


    Thom