Behavior with a low state of charge

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When operating the bike with a battery that has a low state of charge (i.e. it's almost empty), the BMS will take steps to protect the battery from being damaged.

  1. The current that can be drawn will be limited.
    • This will make the bike seem sluggish or "Dogged".
    • If the bike is being ridden very gently, the current draw may be low enough that the limit will not be hit and this effect will not be noticed.
  2. The system will shutdown.
    • This is the action of last resort. The rider should be aware that they are out of energy and should have taken steps to recharge.
    • It will be necessary to recharge the battery and cycle the system (i.e. reboot) to get going again.

Aggressive Riding

When the bike is being ridden very aggressively, i.e. drawing a lot of current from the battery, the battery will appear to have less energy than it actually has. Current limitations will be noticeable. If those are ignored and aggressive riding continued, the system will shut down and the bike will coast to a stop. Giving the battery some time to recover after the shutdown and then riding more gently will allow the bike to travel some more distance before the energy in the battery is so depleted that the bike will shut down until the battery is recharged.

Battery Protection

Allowing the battery on an EV to be completely discharged will severely damage the battery to the point that it may not be able to hold a charge any more. Merely allowing the battery to reach a very low state of charge may significantly lower the useful lifetime of the battery. The Battery Management System (BMS) on Brammo bikes will takes steps to protect the battery from being damaged when the battery voltage drops too low, indicating a low state of charge.

Current draw and Voltage

A battery will attempt to supply current at some voltage across its terminals. Pulling current off the battery causes the voltage to drop as the chemicals in the battery try to keep the current supply up. The lower the state of charge, the more difficult it is for the battery to maintain the current flow and the voltage. If the current draw is low, then it is easier for the battery to maintain the voltage. Being aggressive on the throttle causes more current to be drawn and so drops the voltage of the battery, which appears to the BMS as a very low state of charge, which in turn will cause current limitations and ultimately a shutdown.

References:

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