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Current sensor installation
Install the load current sensor

The current sensor measures the battery current to any high voltage load connected to it (and back from it, in case of regen), whenever the BMS master is powered through its "Ignition 12 V" input.

The load includes:

  • The motor driver, or course, but also, any of the following:
  • A DC-DC converter
  • A heater
  • An inverter to provide utility AC power

The current meter needs to see all of the current from all of the loads conneted to the pack; otherwise the ability of the BMS to calculate SOC will be reduced.

Note that the charger current is not included in the list above. The BMS master measures the charger current, with a different current sensor, whenever the AC power is present.

The load current sensor is installed anywhere along the path of all the battery current that occurs when the ignition is on:

  • On either battery terminal (make sure the current sensor sees all the load current), or
  • on any bus bar between cells inside the battery pack

If the pack uses cells in parallel or strings in parallel, watch out for a common mistake: putting the current sensor on a bus bar or power cable that carries only part of the battery current. The sensor must "see" all the battery current,

If it is possibible that the ignition is on at the same time that the car is still plugged into the AC power, a problem could occur: if the load current sensor also sees the charger current (such as if it is installed on a bus bar inside the battery), the charger current would be counted twice, once by each sensor, resulting in errors in the SOC. To prevent that, either ensure that the ignition cannot be on when the car is plugged into the AC power, or place the load current sensor in a place that does not see the charger current,

Note that the load current sensor measures the battery current, not the motor current.
With rare exceptions, the battery current is always less than the motor current: the motor current may be 500 A when gunning the car from a stop, but the battery current will be something like 200 A.

The power cables carrying the battery current to the motor controller should be sized to carry the average current, yet handle the occasional peak current.

When looking for the current carrying capability of wire, you will find a variety of numbers. Those numbers depend on the rating agency, on the temperature rating of the wire's insulation, and on whether the wire is bundled (in a cable) or in the open air.

The ultimate rule here is that, under heavy use in the hottest day, these cables' temperature must not exceed the rated temperature of the cable's insulation (typically 105 °C PVC).

In a typical application with a relatively low voltage battery pack, the average current when driving may be 100 A, with a peak of 400 A when hard accelerating ("gunning it").
For such an application, 4 AWG wire is sufficient (20 mm2).
Yet, typically you would see 0 gauge, or 4/0 gauge used, simply due to common wisdom and availability. (0 gauge - 54 mm2 - is good for 245 A average, 4/0 gauge - 107 mm2 - for 380 A average.)

Using larger wire allows the wire to run cooler (more efficient) but the vehicle much carry more weight (less efficient).

The biggest wire that can be squeezed into the opening in the Lite's load current sensor (2 AWG, 33 mm2) can carry far more peak current (~1800 A) than the sensor can detect (900 A). So, if your cable doesn't fit, then:

  • either the application handles more battery current than the Lite's current sensor can handle,
  • or the wire is oversized for the application (e.g.: you're using a 4/0 gauge wire for 380 A average when it only carries 100 A average).

Regardless, if the wire is larger than 1/0 gauge, it will not fit in the opening on the sensor. In that case you can:

  • Use a short length of 2 AWG wire in series with it, and place the current sensor on it
  • Use a short length of bus bar in series with it, and place the current sensor on it
  • Place the current sensor on a bus bars between cells in the battery
Current sensor on a bar
Bar-mounted current sensor

Note that we define discharge current as positive, regen current as negative.

For your reference, current flow is:

  • On positive battery terminal,
    • charging: current flows into the battery
    • discharging: current flows out of the battery
  • On negative battery terminal,
    • charging: current flows out of the battery
    • discharging: current flows into the battery

In a typical installation, the battery positive terminal goes to two places:

  • From the charger: this line has a DC rated fuse, rated for the relatively small charging current, and goes through the battery current sensor built into the master
  • To the motor driver: this line has a DC rated fuse, rated for the high load current, and goes through the load current sensor
BMS master, connection point highlighted
Connection location.
Schematic
Current sensing in a Lithiumate Lite master

Slip the current sensor on a cable or bus bar that carries the entire load current. The charger current should not be included in that path.

You may place it on either the positive end or the negative end of the battery, or even inside the battery.

Note that there is an arrow on the the current sensor body; that arrow is in the direction of the discharging current.

The orientation of the current sensor is critical:

  • If placed on the positive wire, orient the current sensor so that the arrow points away from the battery
  • If placed on negative wire, orient the current sensor so that the arrow points towards the battery
  • If placed inside the battery, orient the current sensor so that the arrow points towards the positive terminal of the battery

However, if later you discover that you installed it in the wrong direction, it may be easier to correct it through the configuration of the BMS, rather than turning the sensor around.

Connect the sensor's connector to the BMS master's "Load Current Sensor" connector.

 

 
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Page published on: Jul 19 2013.      Installation manual