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BQ24074: What's the minimal input current required by these solar chargers?

Will Wang1
Prodigy 250 points
Part Number: BQ24074
Other Parts Discussed in Thread: BQ24210, BQ24250, BQ25985

Hi Sir,

We need some help to choose a solar charger that also works with 5V USB input. 

Input: a single input port that always has a 2W solar panel (Vmp 4V, Imp 0.5A) connected to extend battery life; occasionally, a 5V 1A USB wall adapter can work as well for faster charging.

Output: system draws 6W at peaks from the battery, only happens < 1min per 24 hour period. 

Battery: 1S1P Li-ion 18650 2500mAh cell wit NTC

MPPT: We don't intend to use MCU for real-time MPPT algorithm, it's either fixed or (optionally) adjust a few times a year

Questions:

  1. After sunset, the charger goes UVLO. Do they automatically wake up when there is enough sun on the panels so the Vin > UVLO & Vbatt?
  2. I understand recharge/refresh starts if the Vbat < Vrch, but what's the min input current to start the charge cycle?
  3. What the min input current to keep the charging cycle going until termination? Assuming the Vindpm tries to regulate the input around 4V by reducing the input current.
  4. What's the response if during the charging cycle, the input current is reduced to almost zero to maintain the Vindpm? Eg. not enough power from solar panel

We have a short list of these parts. What's your best pick regardless of the price?

  • BQ24074
  • BQ24210
  • BQ24250
  • BQ25985

I know this seems like a lot, but we really hope to get your expert opinion. 

  • 0
    TI__Mastermind 23220 points

    Hi Will,

    To answer your questions specifically on the BQ24074:

    Will Wang1 said:
    Do they automatically wake up when there is enough sun on the panels so the Vin > UVLO & Vbatt?

    Anytime the device finds that the input voltage is sufficient (VBAT + VIN(DT) < VIN < VOVP) then the the device will be able to charge.

    Will Wang1 said:
    I understand recharge/refresh starts if the Vbat < Vrch, but what's the min input current to start the charge cycle?
    Will Wang1 said:
    What the min input current to keep the charging cycle going until termination? Assuming the Vindpm tries to regulate the input around 4V by reducing the input current.

    The minimum input current to keep charging depends on a few factors:

    • OUT current
      • Generally recommended: Input Current = Output Current + Battery Current
    • OUT voltage (DPPM)
      • As the Output voltage or current drops, the battery charge current will be reduced to prioritize the voltage and the current at the output
    • Input Voltage (if configured for a USB mode)
      • If the input voltage of the device begins to drop, the battery current will be reduced to maintain the input voltage
    Will Wang1 said:
    What's the response if during the charging cycle, the input current is reduced to almost zero to maintain the Vindpm? Eg. not enough power from solar panel

    As previously mentioned, this is dependent on a few factors. This will greatly depend on the input voltage, the set charging current, and the output current. 

    Let me know if there are further questions. 

  • 0
    Prodigy 250 points

    Thanks Anthony, but I don't find answers to all my questions yet. Let me try to explain better.

    1. About the min input current, I understand how those factors affect the input current. Assuming there is no output load current (the system is asleep with 100uA RMS), and the input solar panel is producing 10mA @ 4.5V due to cloudy weather. Would any of the four parts be able to start pre-charge the battery? For sure it's going to be very slow, but we don't use the system often. 

    2. W.R.T the charger behaviour, I understand who Vindpm works. Because the weather condition changes, so the solar panel output will change during the battery charging cycle (pre-charge/CC/CV). What happens to the charging workflow when the input drops to a very low power? Say Vindpm is set to 4.5V, so we only get 10mA at the input of the charger. Does it keep going or exit the cycle? If it exits, when the power increases, does the charging restart from the pre-charge or resumes from where it's left off?

    Thank you!

    Will

  • 0 in reply to Will Wang1
    TI__Mastermind 23220 points

    Hi Will,

    The condition is that VIN needs to be in within the valid operating range (VBAT + VIN(DT) < VIN < VOVP) otherwise the battery voltage will take over. 4.5V should be fine for the device to charge the battery I believe. 

    If the input voltage falls outside of range, the device can switch to battery power to power the load. Once the input voltage falls back into range, charge cycle begins and the device goes into pre-charge, fast charge, or constant voltage mode depending on the battery voltage. 

    Best Regards,

    Anthony Pham

  • 0 in reply to Anthony P Pham
    Prodigy 250 points

    Jeff - Thanks for trying to answer. I think we need a EVB to find out what I'm looking for. It's not the voltage I'm concerned, it's the current. 

  • 0 in reply to Will Wang1
    TI__Mastermind 23220 points

    HI Will,

    Let me know if there are further questions. I'll close this thread.