BMS FAQ

A safety and performance overview for riders using custom or high-output battery systems.


Charging Protection: Why the BMS is Essential

Every lithium-ion battery must use a Battery Management System (BMS) for charging, no exceptions.

During charging, the BMS performs two vital functions for safety and longevity:

  • Overcharge Protection: Prevents individual cell groups from exceeding their safe voltage limit. While a charger stops at the total pack voltage, only the BMS monitors and protects each individual cell group.

  • Cell Balancing: The BMS equalizes cell voltages, typically near the end of the charge cycle. This ensures all cells reach full capacity together, which is crucial for maximizing battery lifespan and safety.

Whether the discharge function is bypassed or not, the BMS must be connected for all charging cycles.



Why Discharge Is Often Bypassed in Performance Builds

In high-performance PEV setups like upgraded surrons, high-power e-skates, or custom e-bikes, bypassing the BMS on discharge is standard practice, and often preferred. This is done primarily for performance, reliability, and simplicity.

1. To Eliminate a Current Bottleneck

Most BMS units aren’t built to handle the full discharge capability that your cells can actually deliver. The cells may support much higher current, but the BMS often becomes the limiting component long before the pack does. Pushing that load through the BMS — even a good one — can introduce:

  • extra voltage drop

  • unnecessary heat

  • sudden cutoffs under heavy acceleration or peak demand

Bypassing the BMS on discharge removes that restriction and lets the controller pull current directly from the pack without the BMS acting as a bottleneck.

For devices like Onewheels and electric skateboards, avoiding discharge cutouts is especially important since sudden power loss can directly affect rider safety and stability.


2. Balancing Only Happens During Charging

Balancing only occurs during charging.

There’s no advantage to running discharge through the BMS because it isn’t performing any balancing while the pack is in use. Any minor cell drift that develops during riding gets corrected the next time the battery is charged.

 

What About Undervoltage Protection?

When your battery uses a BMS-bypassed discharge setup, the responsibility for high-current safety and pack-level low-voltage cutoff (LVC) shifts from the BMS to your motor controller. This is an essential feature for performance packs because the controller is a superior tool for managing continuous discharge.

  • Eliminates Mid-Ride Safety Shutoffs: Your controller manages the current flow, preventing the BMS from triggering an abrupt and potentially dangerous hard cutoff mid-ride. This ensures uninterrupted, predictable power under demanding loads.
  • Maximizes Power Output: By bypassing the BMS for discharge, your battery is free to deliver its maximum rated current directly to the controller, ensuring full, uninterrupted power for the smoothest, most predictable performance, even in high-output setups.

Understanding the 0% Battery Display Reading

Most battery displays are calibrated to show 0% when the pack reaches its Low-Voltage Cutoff (LVC) of around 3.0 V per cell. This intentionally leaves a small energy reserve as a safety buffer.

When the BMS is bypassed, the controller handles this LVC function reliably and safely. While a tiny amount of energy technically remains past 0%, pushing your cells this far is not worth the accelerated degradation. To maximize your battery's lifespan, we strongly recommend treating 3.0 V (0% on the display) as the true empty limit.

 

Final Thoughts

Bypassing the BMS on discharge is common in high-performance builds and works reliably when the system is designed correctly. Under normal use with proper fusing or a circuit breaker in place, there’s nothing to worry about. The protection hardware catches faults before they become a problem.