The main purpose of the power distribution board is distributing low-voltage (~12 V) power from the DC-DC converters and the supplemental battery to other low-voltage boards in the electrical system. The Rev 4 board is intended to be tested in 2019 FSGP and distribute power for MS14's electrical system in 2020 ASC.
The main goals for Rev 4 are followings:
- Separate the power distribution board into front and rear power distribution boards
- Charge the supplemental battery with energy from the DC-DC converters
- Collect data on load voltage and current consumption
- Protection against load or harness being shorted to ground
- Add current limiting
Low-Voltage Systems and Current Requirements
Assume all systems are powered by around 12 V. Ideally, the front and the rear power distribution boards should be very similar.
Front Power Distribution Board
| System | Continuous Current (mA) | Peak Current (mA) | Note |
|---|---|---|---|
| Center Console + Pi | 600 | 700 | When the HDMI is on, the Pi draws around 500 mA. Assume the centre console uses around 100 mA. |
| Infotainment Screen | 400 | 600 | A 12" display consumes 4.8 W on average. |
| Left and Right View Screens | 20 | 40 | A 4.3" display draws around 25 mA at 3.3 V. |
| Rear View Screen | 300 | 400 | A mirror monitor (e.g. STSK4531R) draws around 260 mA. |
| HUD Screen | 10 | 20 | A 4.3" display draws around 25 mA at 3.3 V. |
| Fans | 1000 | 1000 | Based on MS12 power budget. |
| Front Lights | 1,500 | 25,000 | The current consumption of the front light on MS12 is ~1 A on average and ~23 A at peak. Assume the DIY light for MS 14 is slightly less efficient. |
| Pedal Board | 10 | 50 | Based on MS12 power budget. |
| Steering Board | 10 | 50 | Based on MS12 power budget. |
| Left and Right View Cameras | 500 | 500 | Based on MS12 power budget. |
| Horn | 0 | 8,000 | Link |
| Protected Spare × 2 | 500 | 500 | |
| Total | 4,850 | 28,860 |
Rear Power Distribution Board
| System | Average Current (A) | Peak Current (A) | Note |
|---|---|---|---|
| Motor Interface + PreCharge | |||
| Rear Lights | |||
| Fans | |||
| BMS Carrier | |||
| BMS AFE × 4 | |||
| Rear Camera | |||
| Telemetry | Rear PWR should be closer to the antennas | ||
| Solar Sense |
| ||
Charging Supplemental Battery | |||
| Protected Spare × 2 | |||
| Unprotected Spare × 2 | Do we really need this? |
Supplemental Battery
MS12 use a 12 V 10-cell? NiMH battery as the supplemental battery. It is used to power the minimal necessary systems to check the battery system and the reset of the car before closing the main battery relays.
The Rev 4 should be able to:
- Safely charge the NiMH battery
- Monitor the voltage and the temperature of the battery
- Measure the current draw from the battery
- Limit the current draw
Charging NiMH Battery
Discharge Curve
Found many different NiMH discharge curve on the internet. The average voltage for a pack of NiMH battery seems to slightly above 12 V. It is not too certain if we should charge the NiMH battery at 12 V or boost it to slightly higher voltage.
We should compare the discharge curves between charging at constant 12 V and charging by an off-the-shelf NiMH charger.
Charging Methods
- Constant Voltage at 12 V
- Simply make the supplemental battery parallel to the DC-DC converters
- Safe
- The battery might not be fully charged (need measurements to prove)
- Constant Voltage > 12 V
- Require a boost converter
- Ensure the battery is fully charged
- Require temperature and current monitors to ensure the safety of the battery
- Constant Current then Constant Voltage
- Safer and more efficient
- Require a more complicated circuit
- Off-the-shelf Changer
- Safe and efficient
- Extra weight
- Expensive
Solution?
- Charge the supplemental battery with 12 V while the car is racing
- Charge the battery to the maximum capacity with an external charger during the nights?
- Requires a circuit to switch from a 13 V battery to a 12 V DC-DC converter
Load Voltage, Current and Power Sensing
Voltage Sensing
Since the loads are parallel, they should operate at the same voltage. The PWR board only needs to measure the output voltage of the supplemental battery and the DC-DC converters
High-Side Current Sensing
The main benefit of high-side current sensing is the ability to detect a short circuit in the load or the harness. There are few solutions to current sensing:
- Hall Effect + (Analog Multiplexer) + ADC
- Resistor + (Analog Multiplexer) + ADC
- Current Sensor IC