Important safety features that are crucial for testing batteries safely and effectively include safety contactors, a reverse polarity checker, and a pre-charge circuit.
Neglecting to implement these safety features leave your battery test setup vulnerable and can lead to catastrophic events, unit-under-test (UUT) damage, and downtime. Bidirectional power supplies as well as many battery cyclers do not include these features.
Let’s discuss the 3 minimum battery test safety test requirements that will keep your battery test set up free of vulnerabilities.
Figure 1: Battery Cycler Includes Safety Contactor, Polarity Checker, and Pre-Change Circuit.
Safety Contactors: Ensures Isolation and Safe “Off” Condition
Safety contactors create a safe “off” condition by providing isolation. Without the contactor, even in an off-state, power flow could still occur with your DUT. That flow could be caused when enough resistance is created between those terminals so that the power source continues to draw current and drain the battery or cause other safety hazards.
NI uses a built-in hermetically sealed safety contactor that is designed into the battery cycling equipment. When our test systems are off, our instruments do not draw power and the contactor physically breaks the connection between the cycler and the UUT.
Polarity Checker: Protects Operator and Equipment from User Error
A polarity checker is necessary to prevent damage to the battery and the equipment in the case of incorrect wiring by the user, which happens quite frequently. A built-in reverse polarity checker prevents inadvertent damage by disabling output power when a negative voltage is detected at the output terminals. For example, if the instrument were to detect a reverse polarity, the instrument will prevent the safety contactor from closing. This prevents a customer from accidentally reversing a battery, and then trying to charge or discharge it, which is extremely hazardous. Accidents such as this could also damage the instrument and/or cause a fire or explosion.
NI includes a built-in reverse polarity checker in its battery cyclers. This checker helps ensure proper voltage at its output terminals.
Pre-charge Checker Circuit: Reduces Inrush Current and Prevents Early Degradation of Hardware
Lastly, the built-in pre-charge circuit is significant to prevent inrush current and stress onto the system, which is dangerous to the battery cycler and the UUT components equally. This inrush is caused by the output capacitance of the test equipment, which can be detrimental to the UUT when it is connected and not at the same voltage level. A pre-charge circuit matches the internal voltage of the instrument to the battery, preventing arcs and large inrushes of current onto the system.
For example, if you are using a DC source, DC load or bidirectional instrument to test a 600 V battery, the instrument initially starts at 0 V (zero potential). Since these source/load instruments do not contain a pre-charge circuit, the moment the wires from your instrument connect to the battery, you’re completing that path and in turn, applying 600 volts to the instrument. This instantaneous connection results in an inrush current. This large difference in voltage levels is the cause of the inrush as the instrument’s output capacitors are charged, and the inrush usually leads to early degradation of the relays and switches. A pre-charge works together with the safety contactors by matching the instrument’s output voltage to the battery before the safety contactor closes.
NI’s battery test systems have a built-in pre-charge circuit that provides a “soft start” to automatically match the voltage on the DC output to the battery voltage.