Objective:
The Aux Battery needs to be also protected, as well as charged by the main battery while the car is operating on the main battery. Aux BMS will be powered by the aux battery itself, and should enter a very low power sleep state while not monitoring the cells.
All protection functions should have the capability to be handled through hardware (with voltage references and comparators) or via software for more advanced monitoring.
This board should also implement a rough State-Of-Charge (SoC) model for the Aux battery using an OCV vs SoC estimation.
Current State:
Currently, this board charges the Aux Battery. It doesn't do protection, that protection is handled by rear power distribution. This will change in the next revision.
Protection:
- Over voltage
- Under voltage
- Over current (charge and discharge)
- Over temperature
Under temperature - is technically required for a complete proper BMS, however it is not in the regs and not applicable to the conditions we will be using the car in
Behaviour w.r.t Power Source:
| # | Main Power Source | Condition | What happens |
|---|
| 1 | Main | Aux battery is getting charged by the main battery |
and nears its full capacity (Overcharge | (or external source) and gets overcharge (Overvoltage condition) | The BMS must disconnect Aux from the main battery. |
| 2 | Aux |
Aux battery is nearing under | Under voltage. | BMS disconnects aux from the rest of the car. |
| 3 | Aux & Main | Too much current gets drawn from Aux. | BMS disconnects aux from the rest of the car. |
| 4 | Aux | Aux temperature gets too high | BMS disconnects aux from the rest of the car. |
Behaviour w.r.t Car State:
Splitting out the different car states should not be done. This board is meant to protect the AUX battery, and the protection limits and actions will not change with respect to car state.
Car state can be:
- Driving: Main battery is used. everything's powered.
- On: Car's turned on without pressing on brakes. Aux is powering the car.
- Off: Car's turned off, but Aux is being used to power some of the boards.
| # | Car State | Event/Condition | What should happen |
|---|
| 1 |
Off| All | Over Voltage: We replace aux with another battery that's over charged. | Soon as Aux Battery gets connected to the Aux BMS, it should start discharging (with a until it reaches the maximum allowed voltage. |
| 2 |
Off| All | Over Current: why would we have this? |
🤔| - To operate the battery within the specified parameters and max discharge current on the specifies on the datasheet. Could happen due to a short on the battery pins, reverse polarity wiring, etc. | BMS disconnects aux from the rest of the car if the over current condition does not recover after a set amount of time, and reconnects when the current returns within acceptable levels |
| 3 |
Off Rest of the car is not powered. | Someone heats the battery up with a heat gun? The car is sitting out in the desert sun all day? | BMS disconnects aux from the rest of the car, and reconnects when the temperature falls back within the allowable range |
| 4 | All | Under Voltage: The battery is self-discharging / replace aux battery with a low voltage one | BMS disconnects aux from the rest of the car if the under voltage condition does not recover after a set amount of time |
| 5 | Driving | Any: The AUX battery should not used to power any parts of the car while in the driving condition. However, it will be getting charged while driving. | If any faults occur, the behaviour should be the same as if the car is off. In this case, since the car is getting powered by the DC-DC, the driver should be notified of the fault event that occurred. |
Questions:
| # | Question | Answer |
|---|
| 1 | When the car is turned off, how can we tell if the Aux Battery is faulty or not? | When the aux battery is not faulty, we power an LED |
. | . We will also be able to monitor the state of the AUX battery with a seperate device (laptop, etc) connected to the CAN network of the car for debugging. |
| 2 | Are we going to have fans to cool aux battery down in case it faults due to |
hotness?| over temperature? | We will never be loading the AUX battery to the point where the cells should rise significantly above ambient temperature, so cooling it will not be an issue. Also, the ambient temperature |
| 3 | Could we have over current when car's off? | Yes - if someone makes incorrect wiring on an input that is not reverse polarity protected, or other similar situation |