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bq24618: Dual charger input: AC + Solar, Different supplied current

Ted Fryberger
Intellectual 820 points
Part Number: BQ24618
Other Parts Discussed in Thread: BQ25505, BQ25570, BQ24210

Charging a single Li-Ion cell 3.7V at 2200 mAh

AC adapter input = 16V at 4A max, 440 mA fast charge, adapter connected as needed

Solar cell input = 16V at 44 mA, slower charge, always connected, always voltage out unless dark

AC adapter and solar cells wired in parallel

Questions

1. Is this IC smart enough to deal with these two inputs?

2. There is a 10:1 difference in output current from the two inputs, can the IC deal with that?

  • 0
    TI__Mastermind 20570 points

    The IC itself cannot differentiate what input power is present. You will have to build a external power source selector like a ORing.  Current sense feedback is by sensing the voltage across the sense resistor. For example, if using 20mohm sense resistor, 44mA would have 0.88mV drop at the sense resistor. If you look at the current regulation accuracy spec on page 8, the accuracy for 1.5mV VIREG_CHG is +/-40%. The regulation for 44mA would be really off. 

    What is the application? For single cell li-ion, normally we see customers use 6V panel. Is there a reason that you need 16V input?

  • 0 in reply to Jing Zou
    Intellectual 820 points

    Thanks for your reply.

    Application is a battery powered instrument + LED flashlight in same case. I wanted both AC and solar output V to be the same so I can connect them in parallel. That may not be possible given the difference in current output. I have space for 1-4 solar cells in the case.

    I can add a simple switch to manually select AC adapter when desired, leave in solar mode by default. IC should not care if the input voltages are different.

    Sounds like the current output difference is more of a problem. Looks like higher Vireg_chg is better. I don't want to have to switch in/out different resistors for either the AC adapter or solar cell inputs. You are referring to RSR, not RAC?

    I will look for different solar cells with a higher voltage out, which would allow me to connect them in parallel and get higher current capacity.

    Any comments?

  • 0 in reply to Ted Fryberger
    TI__Mastermind 20570 points

    Both RSR and RAC accuracy is based on the sense voltage at the pin. 

    Place cells in parallel would provide higher current. Not sure if that can be an option for you.

  • 0 in reply to Jing Zou
    Intellectual 820 points

    Thanks for your reply.

    AC charger out current is 300 to 440ma @ 12 or 16V, the best I can do with a solar cell is 178 to 200mA @ 5 or 6V. Is it even possible to accept either of these inputs with this charger IC? Only one charger input will be connected at a time.

    Would I have to switch in/out different RAC, RSR, and other resistors to keep the Vireg_chg voltage in the 20-40mV range?

    Would I be better off using a different charger IC, one that is more suitable to lower voltage solar cell input? Must support Dynamic Power Management.

  • 0 in reply to Ted Fryberger
    TI__Mastermind 20570 points

    If the two input source has different power capability, you will probably need to have two settings for RAC. If you only have RAC setting for the AC charger, then when the solar panel is connected, the solar panel is going to crash.

    Currently we only have bq25570, bq25505 or bq24210 for lower  power charge IC for solar panel. 

  • 0 in reply to Jing Zou
    Intellectual 820 points
    Thanks for your help Jing. I've changed the design to use removable batteries; solar charges while in device, external AC adapter charges removed batteries. Evaluating using a different IC.