An elevator system comprising: a battery; a machine having a motor for imparting motion to an elevator car; an inverter for converting DC power from the battery to AC power for the machine in motoring mode and converting AC power from the machine to DC power for the battery in regenerative mode; and a controller to control the inverter, the controller implementing at least one of: detecting an overload at the battery in motoring mode and reducing car speed in response to the overload; detecting an overcharge at the battery in regenerative mode and reducing car speed in response to the overcharge; and detecting motor direct current in a motor field weakening mode and reducing car speed in response to the motor direct current; characterized in that: detecting the overload at the battery in motoring mode and reducing car speed in response to the overload includes: detecting a voltage at the battery and reducing car speed if the voltage at the battery is below a threshold; or detecting a voltage at the battery and reducing car speed in response to a function relating battery voltage and car speed, or in that: detecting the overcharge at the battery in regenerative mode and reducing car speed in response to the overcharge includes: detecting a voltage at the battery and reducing car speed if the voltage at the battery exceeds a threshold; or detecting a voltage at the battery and reducing car speed in response to a function relating battery voltage and car speed. a battery; a machine having a motor for imparting motion to an elevator car; an inverter for converting DC power from the battery to AC power for the machine in motoring mode and converting AC power from the machine to DC power for the battery in regenerative mode; and a controller to control the inverter, the controller implementing at least one of: detecting an overload at the battery in motoring mode and reducing car speed in response to the overload; detecting an overcharge at the battery in regenerative mode and reducing car speed in response to the overcharge; and detecting motor direct current in a motor field weakening mode and reducing car speed in response to the motor direct current; detecting an overload at the battery in motoring mode and reducing car speed in response to the overload; detecting an overcharge at the battery in regenerative mode and reducing car speed in response to the overcharge; and detecting motor direct current in a motor field weakening mode and reducing car speed in response to the motor direct current; characterized in that: detecting the overload at the battery in motoring mode and reducing car speed in response to the overload includes: detecting a voltage at the battery and reducing car speed if the voltage at the battery is below a threshold; or detecting a voltage at the battery and reducing car speed in response to a function relating battery voltage and car speed, or in that: detecting a voltage at the battery and reducing car speed if the voltage at the battery is below a threshold; or detecting a voltage at the battery and reducing car speed in response to a function relating battery voltage and car speed, or in that: detecting the overcharge at the battery in regenerative mode and reducing car speed in response to the overcharge includes: detecting a voltage at the battery and reducing car speed if the voltage at the battery exceeds a threshold; or detecting a voltage at the battery and reducing car speed in response to a function relating battery voltage and car speed. detecting a voltage at the battery and reducing car speed if the voltage at the battery exceeds a threshold; or detecting a voltage at the battery and reducing car speed in response to a function relating battery voltage and car speed.