Regenerative Braking Design Discussion

flux

The generator inputs, bridge rectifiers, TVS clamp, supercapacitor equivalent, bleeder, and protected bus are wired; I’m wiring the charger, battery, and load rails next.

flux

Created the full schematic for the regenerative braking charger and renamed the project to Regenerative Braking Charger. I added/wired the motor inputs, dual bridge rectifiers, TVS clamp, supercapacitor equivalent, BQ25750 charger stage with external MOSFETs, 2S battery connector/protection, 5 V rail, 3.3 V RC522 rail, LCD/MCU/RFID connectors, and support passives.
Open items:
  • BQ25750 bootstrap/compensation/feedback values still need datasheet-final tuning before layout.
  • Supercapacitor bank is represented as an equivalent C1; final cell count, balancing, fuse, ESR, and discharge timing need sizing.
  • I2C is pulled to 3.3 V for IC safety; add level shifting if your LCD module forces 5 V I2C pull-ups.
Suggested next steps:
  1. Finalize BQ25750 component values from the TI datasheet.
  2. Size the supercapacitor bank from expected braking energy.
  3. Move to PCB layout/placement after those values are locked.
  4. Use the new Board Bring-Up Plan for first-power validation.
Finalize charger and layout
Finalize BQ25750 values
Size supercapacitor bank
Add safety protections
Proceed to placement

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Regenerative Braking Power Path: Part Selection and Schematic Outline
Assumptions
  • Two brushed DC motors with gearboxes are used as generators on the rear wheels.
  • Rectified motor output can range from 0 V to >24 V during braking spikes.
  • Energy is buffered in supercapacitors, then used by a buck-boost charger for a 2S Li-ion pack.
  • Battery pack maximum charge voltage is 8.4 V.
  • Loads are a microcontroller, 5 V LCD display, and RC522 RFID reader.
  • RC522 power must be a clean, regulated 3.3 V rail and must not be powered from 5 V.

Table

StageFunctionRecommended partFlux part UIDWhy this part
1Bridge rectifier, one per motorTS25P05Gad6dc3ce-0aa9-40d9-b3b1-3310854f350bSingle-phase bridge rectifier, 600 V through-hole TS-6P package, strong voltage margin for motor spikes.
1 alternateBridge rectifierB250C5000-3300A2501c276-ed2d-0b0c-fc9a-6f6b2651a759600 V, ~5.8 A bridge rectifier; suitable if available.
2Input transient clampSMBJ24A7319d82b-aa7f-48ce-8889-d95e8df2d22424 V working TVS, 600 W peak pulse, ~38.9 V clamp; appropriate for protecting a wide-input charger from motor spikes.
3Supercapacitor bankSupercapacitor cells + balancing resistorsUse library supercapacitors sized by energy/current requirementMust be voltage-rated above clamp/operating voltage. Series cells need balancing.
4Buck-boost 2S Li-ion chargerBQ257500ac5aae6-7196-42be-a572-e388c94dc94eWide-input buck-boost battery charge controller, supports Li-ion/Li-polymer charging and wide 4.2–70 V input range.
4 alternateBuck/boost chargerMP2650GV variants1c45b2b8-bfa0-4d68-971a-4da5569faac6, d1566088-c877-4607-9c7d-c05192bb0a112S–4S buck/boost charger family, but input/source constraints need datasheet verification.
52S pack protection/BMS ICBQ29209DRBR5453eef3-b803-441a-94d3-703aa772513e2-cell Li-ion/Li-polymer protection/management IC with OV/UV/OC protection and balancing.
65 V rail from 2S batteryLTC3388 Modulee464524f-fb7a-4dae-8d57-a9aca387e1b9Buck converter module with selectable 3.3 V/5 V output. Best for modest logic/LCD loads.
6 alternate5 V high-current buckXL40164fd1b4a2-8d64-4c18-b90c-0bb99f02074eCommon high-current buck, but not ideal if the 2S pack can fall near 6 V because dropout/headroom may be insufficient.
63.3 V RC522 railMIC5365-3.3YC52b3b96b3-76de-4307-9b90-42a1f57b48a4Fixed 3.3 V, 150 mA low-noise LDO. Use from the 5 V rail, not directly from the 2S battery.
TVS / Clamp Selection
For a downstream charger that accepts at least 30 V input, the clamp network should prevent sustained or transient input excursions above the charger’s safe operating range while not conducting heavily during normal generator operation.
Recommended starting point:
  • Use a unidirectional TVS diode across the protected rectified bus.
  • Choose VRWM ≈ 24 V so it does not conduct during normal 24 V peaks.
  • Expected clamp for SMBJ24A is around 38.9 V at peak pulse current.
  • Use 600 W minimum peak pulse rating for a small graduation-project prototype.
  • For high-energy braking pulses, upgrade to SMCJ/5KP class TVS or a dedicated braking dump load, because a TVS alone is not meant to absorb continuous generator energy.
Important: if the charger input absolute maximum is only 30 V, SMBJ24A may clamp too high. In that case use either a lower-voltage TVS plus series impedance/current limiting, or select a charger with higher input tolerance such as BQ25750.
Supercapacitor Bank Guidance
  • Place the supercapacitor bank after the rectifiers and TVS clamp.
  • Voltage rating must exceed the maximum allowed bus voltage.
  • If using 2.7 V supercapacitor cells, series cells are required. For a 24–30 V bus, this means many cells in series, so active or resistor balancing is required.
  • Add a fuse or current-limiting element between the rectified motor bus and supercapacitor bank to control inrush/fault current.
  • Add a bleed resistor so the capacitor bank discharges safely after power-off.
Schematic Wiring Outline
Stage 1: Motor rectification
  1. Add two motor input connectors: MOTOR1_A, MOTOR1_B, MOTOR2_A, MOTOR2_B.
  2. Add one bridge rectifier per motor.
  3. Connect each motor’s two generator terminals to the ~ AC inputs of its bridge rectifier.
  4. Tie both rectifier positive outputs together into net VRECT_RAW.
  5. Tie both rectifier negative outputs together into net PGND.
Stage 2: Overvoltage protection
  1. Place the TVS diode directly across VRECT_RAW and PGND, physically close to the rectifier outputs.
  2. For a unidirectional TVS, connect cathode to VRECT_RAW and anode to PGND.
  3. Add optional fuse/PTC or low-value series resistor/current limiter upstream of the protected bus.
  4. Rename the protected bus after the protection node as VGEN_PROT if using a series protection element.
Stage 3: Supercapacitor buffer
  1. Connect supercapacitor bank positive to VGEN_PROT.
  2. Connect supercapacitor bank negative to PGND.
  3. Add balancing resistors across each series supercapacitor cell.
  4. Add a bleed/discharge resistor across the full bank.
  5. Add a voltage divider to allow the microcontroller to monitor VGEN_PROT, if telemetry is required.
Stage 4: Buck-boost battery charger
  1. Connect charger input/VBUS/VIN to VGEN_PROT.
  2. Connect charger power ground to PGND.
  3. Configure charge voltage for 8.4 V for a 2S Li-ion pack.
  4. Set charge current conservatively based on motor energy, supercap current limit, battery capacity, and thermal limits.
  5. Add all datasheet-required power inductor, input capacitors, output capacitors, sense resistor, bootstrap parts, compensation parts, and I²C pull-ups if using BQ25750.
Stage 5: Battery storage and BMS
  1. Connect charger BAT output to the protected 2S battery/BMS pack positive path.
  2. Connect charger ground/BAT negative to pack/BMS negative path as required by the BMS topology.
  3. Expose pack connector pins: B+, BMID, B- for 2S balancing if using cell-level protection.
  4. Do not bypass the BMS/protection path for system loads unless the protection IC datasheet explicitly allows it.
Stage 6: Load rails
  1. From protected battery output VBAT_2S, feed the 5 V buck converter input.
  2. Connect buck output to net +5V.
  3. Power LCD and microcontroller from +5V if those modules are 5 V-compatible.
  4. Feed MIC5365-3.3YC5 input from +5V.
  5. Connect MIC5365 output to net +3V3_RFID.
  6. Connect RC522 VCC only to +3V3_RFID.
  7. Add input/output capacitors for both regulators per datasheets.
  8. If microcontroller logic is 5 V and RC522 logic is 3.3 V-only, add level shifting on SPI lines: SCK, MOSI, MISO, NSS/SDA, and RST as needed.
Safety Notes
  • A TVS is a transient clamp, not a braking resistor. If the motors can produce sustained high energy during braking, add a dump load or active braking controller.
  • Supercapacitors can deliver dangerous fault currents. Use fusing and safe discharge paths.
  • Li-ion charging requires correct cell configuration, charge voltage, charge current, temperature monitoring, and protection circuitry.
  • Verify every charger/BMS connection against the exact datasheet before PCB layout.

  • Assumptions

  • Recommended Flux Library Parts

  • TVS / Clamp Selection

  • Supercapacitor Bank Guidance

  • Schematic Wiring Outline

  • Stage 1: Motor rectification

  • Stage 2: Overvoltage protection

  • Stage 3: Supercapacitor buffer

  • Stage 4: Buck-boost battery charger

  • Stage 5: Battery storage and BMS

  • Stage 6: Load rails

  • Safety Notes

Documents

  • Regenerative Braking Power Path: Part Selection and Schematic Outline

  • Board Bring-Up Plan — Regenerative Braking Charger

Assets

Assets are files uploaded to this project which can be used in various ways.

Regenerative Braking Charger thumbnail
Regenerative braking power generation and charging circuit using two DC motor generators, rectification, TVS protection, supercapacitor buffering, a 2S Li-ion buck-boost charger, BMS protection, and regulated 5 V/3.3 V load rails.

Properties

Properties describe core aspects of the project.

Pricing & Availability

Distributor

Qty 1

Arrow

$4.67–$4.73

Digi-Key

$7.81

HQonline

$1.84

LCSC

$12.04

Mouser

$16.59

TME

$0.38

Verical

$0.53–$23.53

Controls