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BQ24650 problem with solar panel in low light

Massimiliano Sanna
Prodigy 90 points
Other Parts Discussed in Thread: BQ24650

Hello, I'm developing a battery charger with BQ24650. We use lead acid batteries
that are charged with solar panels. I'm testing a real case and observing an
anomalous current consumption with the solar panel in low light. We are observing
a current consumption of about -8mA (from the battery to the charger) when the voltage drop at
the solar panel falls down the maximum power point that is 18V for our application.
The circuit is composed by:

- 90W solar panel
- 12V, 26Ah lead acid battery with 150mA load
- BQ24650 evaluation module (bq24650EVM) with a shunt resistor of 5mOhm to enable 8A of maximum current,
MPPSET at 18V and output resistor divider to allow 14.8V of voltage regulation

The wrong condition, where we have current consumption, is characterized by the following voltages:

solar panel: 17.5V, MPPSET: 1.17V, Vcc = 17.4V, Vcc-Vsrn = 4.6V, Vcc-Vph = 4.4V (current is flowing
back to the charger), battery: 12.9V

In these conditions the battery develop 8mA to the charger and the solar panel deliver 5mA to the charger too
so we have the effect of discharging our battery in low light conditions (such as fog or rainy days)

The values in limiting conditions are as follows:
solar panel: 17.9V, MPPSET: 1.2V, Vcc = 17.6V, Vcc-Vsrn = 4.7V, Vcc-Vph = 4.6V (current is flowing to the battery), battery: 12.95V
In these conditions the current is from the charger to the battery.

Does anyone explain me the causes of this behavior?
Can anyone help me to cancel the current consumption in low light conditions?

Thank you, best regards,

  • 0 in reply to Ning T
    Prodigy 90 points

    The link shows "Group Not Found" "The requested Group cannot be found."

    Please can anyone help me?

  • 0 in reply to Massimiliano Sanna
    TI__Mastermind 46551 points

    Below is the discussion in the link:

    Q:

    Hello team,

     

    My customer is testing with our EVM as per the application note SLVA437.

     

    They measured that with a voltage slightly below the MPT voltage set by the resistor divider, the battery charger is drawing about 6mA  from the battery.

    They want to avoid this so they modified the EVM as shown in attachment, with the goal of having the BQ supplied by the solar panel only, and adding a diode as shown in red.

     

    Is there any drawback  using this configuration?  First tests on their side are ok.

     

    Thank you for  your feedback.

    A: there should not be any issue.

    A: Yes.  Placing a Schottky diode between the existing output capacitor and sense resistors will stop the leakage current.  In order for the battery detect circuitry to function properly you will also need to add additional capacitance, sized as Charles explains below, from the cathode of the diode to ground.

    Q:It looks like you are using charger only for charging the battery. No system load is needed in your application, is that right? Can you tell me where that 6mA go into in the charger?

    A:The added capacitor after the diode, between the battery (+) and ground should have a time constant around 0.65 seconds, with the total feedback divider resistance;  for your schematic use 1uF.

     

  • 0 in reply to Ning T
    Prodigy 90 points

    Thank you. I have found another solution I want to share. In my application there is a microcontroller with a 12bit analog input free. So I have decide to sample the solar panel to measure the actual MPP voltage. If it goes under my tracking threshold I disable the battery charger pulling MPPSET low via a BSS138 FET. I found it works fine.