Introduction
Going into the 2023 Robosub competition, our battery protection system was just a simple 60-amp fuse. This simplistic approach has many flaws. Because we are running two LIPO batteries in parallel, if the two batteries are not charged to the exact same voltage when plugged in, they will rapidly discharge into each other which may cause permanent damage or even a fire. There is also no protection for battery undervoltage besides simple monitoring. When lithium batteries are discharged beyond their rated capacity, the cells degrade at a faster rate which can lead to charging issues or decreased battery life. For overcurrent protection fuses are inherently flawed. They are very slow, and manufacturing tolerances can lead to them blowing at drastically different currents. Fast overcurrent protection (FOCP) is a much quicker and more reliable system that is preferred for many modern power systems. FOCP uses a very small resistor usually to measure the current through a desired node which is then measured with a comparator circuit that turns off a high side Mosfet switch to break the circuit. Two PCBs were developed, a V1 and a V2, to Integrate all of these protection features into a single board.
V1 Design Features
- Ideal Diode Functionality: By using the Dual Low Voltage Ideal Diode Controller LTC4353, two batteries are bale to be connected in parallel with no risk of them discharging into each other. This IC measures the voltage of both batteries and emulates diodes between them using two N-type Mosfets.
- Power Fuses: For the first version of this PCB, I decided to simply use fuses for overcurrent protection as I did not want to implement additional circuitry.
- Compactness: The PCB design takes up much less space than our previous system.
- Connector Compatibility: The PCB must be able to be lotted in between the batteries and existing circuitry. This is done by using the same PCB mount xt-90 connectors that are used on the batteries.
V2 Design Features
- Ideal Diode Functionality: Same Ideal diode functionality as in the v1, this time done with two LM74912. This IC has much greater functionality than the LTC4353 used previously
- Fast Overcurrent Protection: The LM74912 supports FOCP with the use of external sense and setpoint resistors. In this case it was set to 60a but this could easily be adjusted by changing out a resistor or adding a jumper.
- Reverse Polarity Protection: RPP protection prevents damage from the voltage source being connected incorrectly. While this should theoretically not be possible with the connectors we are using, it is still nice to have. This is built into the IC and requires no additional components.
- Short Circuit Protection: SCP protection prevents damage in the case of a short on either side of the board. This is built into the IC and requires no additional components.
- Undervoltage Lockout: The LM74912 supports an adjustable undervoltage lockout for use with lipo batteries. This was set to for the 4S lipo batteries we are using.
V1 Design
Schematic:
TODO: Picture of Schematic
Layout:
TODO: Picture of Layout
Assembled Board:
TODO: Picture of Assembled Board
Testing and Validation:
TODO: Testing
Testing:
TODO: Testing
V2 Design
Schematic:
TODO: Picture of Schematic
Layout:
TODO: Picture of Layout
Assembled Board:
TODO: Picture of Assembled Board
Testing and Validation:
TODO: Testing
Conclusion
TODO