Cell Monitor

status
- Early R&D stages
who
- @Chenxuan Yuan @Sam Zhang @Arsalan Nasrifar engineer(s)
what
- system to monitor the cell voltages of a battery in flight
- temperature monitoring with thermistors in the pack
- could have a pre-charge controller on it, but maybe too complicated
  - Cell Balancing? (might be too complex to implement esp if active balancing)
  - Passive vs Active Cell Balancing?
- current sense could be nice, but also don’t wanna tap the main power connection if not required
- CAN functionality
why
- more detailed battery performance metrics
- designed around custom batter pack
context
- MNS has custom battery pack modules we could look at for reference, but weight is much more of a concern for us
BMS/Cell Monitor ICs:

Edit: 2/26/2025, @Chenxuan Yuan

Weight Optimization

Goal is to balance weight, size, and stability
A lightweight BMS should still provide all essential safety features
A drone BMS can weigh as little as 15g 35mm x 35mm x 5mm (ex. https://www.mokoenergy.com/battery-management-system/drone-bce-04/#:~:text=Board%20dimensions )
Our custom BMS board can be very optimized for our requirements

Passive

Most small UAV BMS units use passive balancing where excess charge is bled off from higher voltage cells as heat using resistors to equalize cells during charging
Lightweight because it’s just resistors and transistors per cell
Inefficient because energy is wasted as heat and charging is relatively slow but acceptable if done only during ground charging

Active

If we want continuous balancing during in-flight use — probably not
Redistributes energy from higher voltage cells to lower ones using inductors+capacitors
Energy efficient and allows for in-flight balancing but has much more complex circuitry and additional weight — overkill

For highly accurate State-of-Charge (SoC) and State-of-Health (SoH)
Do we really need? Estimating SoC and SoH from voltage/current/temp data from BMS should be enough for us

Top Contenders:

MP2791 — more integrated, less work (integrated control logic, charging&protection, so reduced need for MCU to handle low level battery management decisions)
https://www.ti.com/product/BQ76940 — more modular

Other:

ST/Infineon options often designed for automotive or industrial applications, might be over-integrated for heavy duty applications and more complex