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BQ25504: Wind Energy Harvester to 5V supply

Ellis Cave
Prodigy 120 points
Part Number: BQ25504
Other Parts Discussed in Thread: BQ25570, BQ25505,

I have a DC motor/generator driven from a wind source. Output is pulsed DC (filtered) from 1V to 15V at 10ma - 50ma depending on wind speed. I need to charge a Li-Ion  battery, which will then power 5V DC circuitry, which typically pulls around 100ua for bluetooth and sensors, but occasionally needs a pulse of 5-10ma for an actuator. What harvesting circuitry do you recommend to charge the LI-Ion battery, and what battery output regulator do you recommend to provide a 5V regulated DC supply from the LiIon battery for subsequent circuitry?. 

  • 0
    TI__Genius 10385 points

    Ellis,

    Our energy harvesting ICs operate as boost chargers, so the input voltage must remain lower than the output. This is because these products are designed with small solar cells or TEGs in mind.

    Unfortunately your application has too wide of operating range for our energy harvesting family of devices, so we do not have an energy harvesting solution for your circuit.

    Thanks,

    Ricardo 

  • 0 in reply to Ricardo
    Prodigy 120 points

    After more measurements, it looks like the output of the DC generator will stay below 3V if the harvesting circuit loads it down. Does that allow me to use your TI energy harvesting ICs to charge a lithium Ion battery?

  • 0 in reply to Ellis Cave
    TI__Genius 10385 points

    Ellis,

    Our energy harvesting devices sample the open circuit voltage of the energy source on the VOC_SAMP pin. This pin has an absolute maximum voltage rating of 5.5 V. You could use a clamping circuit at the input to limit the open circuit down to a more suitable value, but this will result in power losses in your energy harvesting application. 

    If you are okay with this, I suggest you use bq25505 with a suitable DC-DC converter to regulate your 5V rail. bq25505 has a wider input voltage range than bq25504 and you would not benefit from the integrated buck converter of the bq25570.

    Thanks,

    Ricardo

  • 0 in reply to Ricardo
    Prodigy 120 points

    Ricardo,

    It looks like the output of the wind-driven generator will stay below 3.5 volts, if the bq25505 sinks enough current from the generator when it is charging the LI battery. If the bq25505 has an input impedance of less than 200 ohms when charging the battery, we would be ok. When the battery is fully charged and the bq25505 stops charging, a clamp on the generator output can keep the generator voltage from exceeding 5V into the bq25505.  What is the typical input current and voltage of the bq25505 when it is charging a 3.5V LI battery? Can I adjust the input impedance of the bq25505?

  • 0 in reply to Ellis Cave
    TI__Genius 10385 points

    Ellis,

    You can use the MPPT functionality to modulate the input impedance of bq25505. By design, the MPPT routine will sample the voltage voltage on VOC_SAMP and store the voltage in VREF_SAMP.  The charger will indirectly modulate the input impedance of the boost charger by regulating the input voltage (VIN_DC) to match VREF_SAMP. VOC_SAMP can be driven by a resistor divider as described in section 7.3.1 of the datasheet.

    The input current can roughly be estimated using the power balance equation { VIN*IIN*efficiency = VOUT*IOUT }, but please keep in mind that bq25505 charges discontinuously in pulses. Your IOUT would be your charging current specified by your battery's datasheet. You have provided the nominal battery voltage that would be your VOUT. The VIN would be determined by your energy harvesting device. The efficiency values can be estimated by the efficiency curves in the bq25505 datasheet. Please also keep in mind that bq25505 is a boost charger and VIN must be less than VOUT for charging to occur.

    Thanks,

    Ricardo

  • 0 in reply to Ricardo
    Prodigy 120 points

    Ricardo,

    The title if this thread should probably be changed to "Wind Energy Harvester to LI battery charger" as the subsequent circuitry can be powered directly from the LI battery, no need for a regulator.

    After more testing, it looks like the optimum output voltage of the motor/generator is between 2 & 3 volts. So I will need to set the bq25505 input to hold the input voltage to say 2.5V. The goal is to charge a LI battery, and the optimum LI battery charging voltage is 4.2 volts. So that means that the bq25505 boost converter will be fine. However, if the input to the bq25505 goes to a high impedance (no load) state between the charging pulses, the generator output could go above the bq25505 max input of 5.5 V. If there is a high impedance state between charging pulses, I will have to clamp the bq25505 input voltage so the max won't be exceeded between charge pulses. Am I interpreting the bq25505 functionality correctly? Does the input to the bq25505 go to a high impedance state between charging pulses?

    A second issue is that in some cases, there may be long periods when the generator is not operating, and the LI battery becomes depleted. In that case there is a second, non-rechargeable battery that needs to be switched into the power supply bus, so the subsequent communications/actuator circuitry can run off of the non-rechargeable battery. At this point, the LI battery needs to be switched out of the power bus so it can be at least partially recharged. When the LI battery has been charged to a predefined percentage of full charge (probably between 60 to 80% full charge) the non-rechargeable battery will be switched off of the power bus, and the LI battery switched back in. Do you have a recommended method for performing this power switch? Probably a couple of FETs could be used for the power switch, but is there a control signal from the bq25505 that can directly activate those FETs when the LI battery is depleted? Do you have a suggested circuit using TI FETs that could do the job?

    Also, the motor/generator can put out 50-60ma at 0.3-2V. Can the bq25505 handle that current?

  • 0 in reply to Ellis Cave
    Prodigy 120 points

    Also, the motor/generator can put out 50-60ma at 0.3-2V. Can the bq25505 handle that current?

  • 0 in reply to Ellis Cave
    TI__Genius 10385 points

    Ellis,

    The device does go into a "higher" impedance state when between pulses as the input impedance is modulated due to MPPT, but I would not call it a high impedance state as there is still a quiescent current to run the converter. I would suggest trying it out on the EVM with your wind generator to be sure before proceeding further with your design. If you believe there is the potential to exceed the abs max value I would say you should clamp it.

    You could use a Schottky diode OR between the two batteries and the system. This diode OR will allow the higher of the two battery voltages to be passed to the system. This a simpler, cost effective solution, but it has a few drawbacks compared to a power switch based solution. It has a higher power dissipation and lacks some additional control features. If you are interested in get some advice on a power switch recommendation I would suggest creating a new thread, so the power switches team can direct you.

    The device can handle 50-60mA input current. Additionally the peak power input is 510mW and the even at worse case you are looking at 120 mW at that range, so you should be fine on both fronts.

    Thanks,

    Ricardo