Using a BMS with a Motor Controller

Here we’re going to talk about the issues that can arise when using a battery management system [BMS] in conjunction with a PWM motor controller.

More and more small electric vehicles are adopting lithium batteries to take advantage of the increased range and lower weight that these offer compared to lead-acid types. But lithium batteries nearly always require a battery management system [BMS] to protect them against over-charging, too high a discharge current, or too low a depth of discharge. Quite often the BMS is built into the battery itself, particularly the LiFePo4 batteries sold as drop-in replacements for lead acid types.

The chemistry of a lithium cell although powerful is actually quite delicate, the BMS protects the battery by sensing the voltage of each individual cell and the current flowing through it. If the cell voltage gets too low or too high, or if the discharge or charging current gets too high the BMS will protect the battery by disconnecting it from the load.

This disconnection can have serious consequences for a motor controller, let’s have a look at the different scenarios…

1. Vehicle moving slowly.

1.a] Cell voltage drops too low. If the motor is under load the controller will be delivering current into the motor and a magnetic flux will exist in the motor windings, if the BMS suddenly disconnects the battery the main capacitors in the controller will discharge, the voltage available to maintain the current will drop, and the motor will stop. Usually no harm is done.

1.b] Cell discharge current exceeds the BMS limit. This situation implies a much larger motor current and associated magnetic flux. If the BMS disconnects the battery the main capacitors will try to discharge but the decaying magnetic flux will try to charge them [did I really just mention flux & capacitor in the same sentence?]. It depends on the absolute current being drawn, and the model of controller fitted but there is the possibility of a significant voltage spike with the potential to damage the controller.

2. Vehicle moving fast.

2.a] Cell voltage drops too low. Now if the BMS disconnects, as well as the magnetic flux there is the kinetic energy of the vehicle to consider, the motor has now turned into a generator and will generate a voltage that appears across the terminals of the controller. The magnitude of the voltage depends on a number of factors such as the speed and gearing, the likelyhood of damage to the controller depends on it’s design rating.

2.b] Cell discharge current exceeds the BMS limit. More or less the same as 2.a] above.

2.c] Cell charge current exceeds the BMS limit. This scenario implies that strong regenerative braking is occuring with the motor generating a high voltage driving a large current back through the controller into the battery. If the BMS disconnects there will be a high voltage generated across the terminals of the controller, the magnitude of the voltage depends on a number of factors such as the speed and gearing, and it will last until the kinetic energy of the vehicle is dissipated, the likelyhood of damage to the controller depends on it’s design rating but will be larger if the battery voltage is close to the maximum rating of the controller.

So what can be done to protect an expensive motor controller against these risks? it is important to understand the specifications of the BMS and the controller, and to make appropriate adjustments to the settings.

• Set the under-voltage cut off point of the controller to be above that of the BMS, that way the controller will ensure a controlled stop before the BMS trips. The Pro-160 / 360 both have a low battery warning level that can be set higher than the BMS cut-off level that will allow a “limp home” mode under reduced power.
• Set the over-voltage limit of the controller to be below the maximum charge voltage of the BMS. The controller will automatically adjust the deceleration ramp rate to keep the regeneration voltage below the BMS cut-off level.
• Set the maximum current limit of the controller to be below that of the BMS, that way the controller will make sure that the BMS does not reach its threshold for tripping.
• Set the regen current limit of the controller to be below that of the maximum charge current of the BMS, that way the controller will make sure that the BMS does not reach its threshold for tripping.

If strong regen braking at high battery voltage is required then consider fitting an external voltage clamp capable of handling the high currents involved. 4QD will be looking at the possibility of producing one of these.