Custom BMS for Drone Battery Packs: What to Specify and Why It Matters

Not every drone battery needs a custom BMS. A basic protection circuit does the job on simple platforms. But as drones move into professional, commercial, and regulated applications, the gap between a protection circuit and a proper BMS becomes operationally significant. This post explains the difference and helps you decide which your platform actually needs.

PCM vs BMS: the baseline distinction

These two terms are often used interchangeably. They are not the same thing.

PCM (Protection Circuit Module):

• Overvoltage, undervoltage, overcurrent, and short circuit protection
• No active balancing, no SoC measurement, no communication output
• Passive, low-cost, low-weight
• Right for: simple platforms, fixed-wing gliders, development prototypes

BMS (Battery Management System):

• All PCM protection functions, plus: cell balancing, SoC estimation, state of health monitoring, thermal monitoring, and a communication interface
• Active management during charge and discharge
• More complex, more expensive, more weight
• Right for: professional drones, commercial platforms, any application requiring fuel gauge accuracy or flight controller integration

The distinction matters. Engineers who specify “BMS” when they mean PCM end up paying for complexity they do not need. Engineers who specify PCM when they need BMS hit a wall during integration. Being precise upfront reduces scope creep, cost, and re-spins.

What a BMS does in a drone battery pack

Cell balancing

Individual cells in a pack do not have identical capacity. They come from the same batch, but small manufacturing variations create differences that accumulate over cycles.

Without balancing, the weakest cell limits the pack. The BMS cuts off discharge when the lowest cell reaches minimum voltage, leaving usable energy trapped in stronger cells. Over time, imbalance grows and usable capacity drops faster than it should.

Passive balancing bleeds charge from high cells as heat. Active balancing redistributes charge from high cells to low cells. Passive is standard and sufficient for most applications. Active is justified when cycle life requirements are extreme or when pack weight prevents passive balancing resistors.

Result of either approach: more usable capacity per cycle and longer pack life.

State of charge (SoC) estimation

A PCM has no SoC output. It does not know how much energy remains, it only responds to voltage limits.

A BMS estimates SoC using coulomb counting, tracking current in and out of the pack over time, combined with voltage-based correction at known reference points. Good BMS designs achieve ±2% SoC accuracy.

That level of accuracy is what return-to-home triggers require. If SoC is off by 10%, the drone either lands prematurely and aborts the mission, or runs the cells too low and does not make it back. Neither outcome is acceptable in commercial or regulated operations.

Thermal monitoring

Cell temperature directly affects both performance and safety. Capacity drops in cold conditions. Internal resistance rises. At the other end, elevated temperatures accelerate degradation and increase runaway risk.

NMC 811 cells have a lower thermal margin than NMC 622. The higher nickel content improves energy density but tightens the window between operating temperature and thermal instability. A BMS with thermal sensors closes that gap: it reports pack temperature to the flight controller, triggers cutoffs before cells reach dangerous temperatures, and logs thermal data for post-flight analysis.

Recommended minimum: one thermal sensor per cell group. High-cell-count packs need more coverage, a single sensor does not capture spatial variation across a large pack.

For operations above 40°C ambient, or high continuous discharge rates, thermal monitoring is not optional.

Communication interface

A smart BMS outputs real-time pack data to the flight controller via UART, I²C, CAN, or SMBus. The data stream typically includes: cell voltages, pack SoC, charge/discharge current, temperature, cycle count, and fault flags.

That data enables three things:

1. Real-time pack monitoring in the ground control station
2. Automated pre-flight checks, the flight controller can refuse to arm if SoC is below threshold, temperature is outside range, or a fault flag is active
3. Fleet management logging, per-pack data stored on the aircraft and uploaded post-mission

The communication interface also determines integration complexity. UART is simple to implement. CAN is robust in electrically noisy environments and standard in automotive-derived systems. SMBus is common in commercial smart battery implementations. I²C works for short runs with low noise. Specify the interface early, the flight controller firmware team needs to know before they write the battery integration layer.

State of health (SoH) tracking

SoH measures how much of the original capacity remains. A fresh pack at 100% SoH degrades over charge cycles. The rate depends on chemistry, charge rate, discharge depth, and temperature history.

A BMS that tracks SoH enables predictive maintenance. Instead of replacing packs on a fixed cycle schedule, operators flag packs when SoH drops below a defined threshold, typically 80%. Packs that have degraded faster than average get caught before they cause a mission failure.

For fleet operators running dozens or hundreds of packs, SoH data is operationally significant. It shifts battery management from reactive to planned.

What “custom” BMS means in practice

Custom BMS does not usually mean designing the PCB from scratch.

For most drone OEM applications, custom means:

• Selecting a BMS platform that fits the pack format and cell count
• Configuring cell thresholds, balancing parameters, and communication protocol for the specific pack design
• Integrating the BMS communication output with the flight controller (MAVLink, DJI SDK, proprietary protocol)
• Validating BMS behaviour against the pack’s actual cell characteristics under real operating conditions

Full clean-sheet BMS hardware design makes sense in a narrow set of cases: very specific form factor constraints, proprietary communication protocol, classified programs, or volumes high enough to justify the NRE. For most drone OEMs, that threshold is not met.

The right answer for most programs is a configurable BMS platform integrated into a custom pack. The integration work, thresholds, protocol mapping, validation, is where most of the engineering value lives.

When a custom BMS is worth it

Yes, if:

• Your flight controller requires fuel gauge data (SoC, voltage, temperature) for flight planning or safety systems
• You operate in high ambient temperatures or at high continuous discharge rates where thermal monitoring is a safety requirement
• You are building a fleet and need per-pack data logging for maintenance and replacement planning
• Your platform requires IEC 62133 or DO-160 compliance, both regimes assume the pack has proper protection and monitoring
• Your pack is 6S or above with NMC 811 cells, cell imbalance and thermal runaway consequences at high cell count are serious

No, if:

• The platform is a development prototype or internal test vehicle
• The pack is 4S or below with a simple, well-understood discharge profile
• No communication with the flight controller is required
• Cycle count is low and the pack will be replaced frequently regardless

If you are on the boundary, 4S to 6S, occasional commercial use, early-stage commercial platform, the communication interface is the deciding factor. If the flight controller cannot use the BMS data, the added cost and weight are hard to justify. If it can, the integration payoff compounds over the program life.

What to specify when requesting a BMS

Incomplete specifications are the primary cause of BMS re-spins. Give your supplier this information upfront.

• Cell count (series x parallel configuration)
• Cell chemistry: NMC 811 or NMC 622, affects charge voltage limits, thermal thresholds, and balancing parameters
• Required SoC accuracy: ±2% is the standard for professional UAV applications
• Communication interface: UART / I²C / CAN / SMBus, include protocol version and baud rate if known
• Thermal sensor count and placement: one per cell group minimum; specify location constraints within the pack envelope
• Balancing type: passive (standard) or active (justify with cycle life or weight requirements)
• Cutoff thresholds: minimum cell voltage, maximum cell temperature, maximum continuous charge and discharge current
• Physical constraints: maximum BMS PCB dimensions, connector type and location, mounting method
• Data logging requirements: what parameters need to be stored, storage capacity, access method (USB, wireless, via flight controller)
• Regulatory requirements: IEC 62133, DO-160 Section 16, UN38.3

Lock the communication interface before the pack design is final. Retrofitting a protocol after integration has started is expensive and often requires firmware changes on the flight controller side.

Decisiones clave: resumen

• A PCM protects the pack. A BMS manages it. Professional drone applications need management, not just protection.
• The three functions that drive the most value for drone OEMs: SoC accuracy at ±2%, thermal monitoring with per-cell-group coverage, and a communication interface the flight controller can consume.
• Custom BMS means configuring and integrating an existing platform. Clean-sheet PCB design is a separate, larger program.
• NMC 811 packs operating at 6S and above need active thermal monitoring. The lower thermal margin leaves less room for error.
• A properly specified BMS extends pack service life, reduces mission risk, and produces maintenance data that pays back over the fleet lifecycle.

Dan-Tech Energy builds custom drone battery packs with integrated BMS, including UART, CAN, and SMBus communication options.

Explore our drone UAV battery packs or contact us with your flight controller platform and pack requirements. We will recommend the right BMS configuration for your application.