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Bq24650 used as MPPT Solar Charger for Lead Acid Battery

Mahesh T.N
Prodigy 50 points
Other Parts Discussed in Thread: BQ24650

We have designed Solar MPPT lead acid battery charger using BQ24650 for the following specifications:

Charge Current = 10A

Battery Voltage = 12 V

Input Voltage = 18 V

Current Sense Resistor = 4 milliohms

Inductor 3.6 micro Henries

The following values are observed on various test points;

TS = 1.94 V

MPPSET = 1.2V

VFB = 1.74 V at battery voltage of 12.10 V

Input current = 0.45 Amps

Output Current = 0.554 Amps

The problem we observed is even though the Vlowv threshold is greater than 1.56 V the charger is not changing state from pre-charging mode to fast charging mode.

The charging current is always constant at 0.54 Amps.

Also the voltage across current sense resistor is 2 mV.

The HIDRV is switching. But LODRV  is not switching

We followed every suggestion in application notes, datasheets and in EVM use guide for PCB layout component selection etc.

Can someone please suggest the solution to get charging current of 10 Amps?

 

  • 0
    TI__Guru 56650 points

    There are 3 loops to regulate the output: output voltage (VFB); output current (Vsrp-srn) and input voltage (MPPT).

    From your test results: The VFB is lower than the regulation point, the Vsrp-srn is lower than the regulation point. But, the input voltage equals the regulation point.

    So, the MPPT loop may regulate the charging current. Please increase the input voltage a little bit.

    In real solar panel system, the input voltage will be higher than the MPPT voltage setting if the solar panel can provide more current than the charger demand.

  • 0 in reply to Wang5577
    Prodigy 50 points

    Hello Wang,

     

    Thanks for the reply.

    As you suggested I tried to increase the input voltage up to 25 volts. In testing I connected DC power source at solar input terminals of Bq24650 board.

    The power supply can support 5 Amps current.

    Event at higher input voltage the input current marginally increases to 0.6 Amps @ 25 V from 0.44 Amps @ 18 V input.

    The duty cycle is approximately at 70%, that is on time of high side Mosfet.

    On power on reset according to Fig 3 on page no 7 of SLUSA75 document, both High side and low side Mosfets should switch on. But in our case it is not the case.

    Why the low side Mosfet is not switching on?

    Thanks

  • 0 in reply to Mahesh T.N
    TI__Guru 56650 points

    Three factors can affect the low side FET switching.(see page 5th)

    1. BATTERY SHORTED COMPARATOR (BATSHORT) can force charger in non-synchronous mode (low side FET is always off when BAT<2V). 

     2. CHARGE UNDER-CURRENT COMPARATOR (CYCLE-BY-CYCLE) can turn the low side FET off early when the sense resistor voltage is lower than 5mV (typical).

    3.LOW CHARGE CURRENT COMPARATOR can hold the low side FET off when the average sense resistor voltage is lower than 1.25mV.

    Please measure the inductor current. And then, you may find the root cause.

  • 0 in reply to Wang5577
    Prodigy 50 points

    Is it better to use input bulk capacitors when solar panels are connected ?

     

    Please suggest me the recommended input capacitor for 10 Amp charger.

     

    There is 10uF capacitor close to the upper Mosfet installed in the board.

     

  • 0 in reply to Mahesh T.N
    TI__Guru 56650 points

    It is good to have a 10uF capacitor close to the upper Mosfet. If input voltage is stable and doesn't have big ripple, we don't need more input capacitors. But, if not, it is better to add more input capacitors.

    There is a calculation tool is available online: http://www.ti.com/litv/zip/sluc175d. You can use that to check the Iripple current with different input, output voltage and inductor value. For 10A output current, you probably want to use a 3.3uH inductor.