External Battery Discharger

Description: The UBA is limited to discharging up to 2.5A per channel. This discharge rate is adequate for testing 99% of the batteries on the market. However, it is insufficient for properly testing batteries that are rapidly discharged in normal use, such as those used in medical applications, UPS systems, and remote control aircraft and cars. For these batteries, it is advisable to discharge them at rates similar to their normal usage. In other scenarios, a constant current load may already be available for use. For instance, a 10A logic-controlled constant current load can be managed by the UBA. This application note outlines the construction and control of an external discharger circuit, which includes two remotely controlled loads. The upper section of the schematic features a standard voltage regulator configured as a constant current load with on/off control. The lower section employs a MOSFET to manage a power resistor.

Within the UBA, there is a 10-pin accessory header that includes two programmable digital input/output lines, configured as outputs for controlling a remote load. Pin 3 corresponds to digital output #0, pin 6 to digital output #1, pin 1 serves as digital ground, and pins 8 and 10 act as analog ground. Access to the accessory header can be achieved by removing the top cover of the UBA or by utilizing an extension connector located on the back of the UBA, which is the preferred method.

Caution: The digital lines on the accessory port connect directly to the digital output integrated circuit within the UBA and lack additional static protection. Therefore, it is essential to implement full static protection measures when handling these connections and during circuit design. The warranty does not cover damage resulting from static discharge or incorrect connections to the accessory connector. Upon powering up, digital control line 0 is set to high and line 1 to low. The active state of the digital signals can be selected (refer to the setup instructions). These digital lines can source and sink currents of up to 2mA. With two digital lines available, it is possible to construct up to two loads. The depicted circuit contains two independent discharging circuits, although it is generally not necessary to build both circuits simultaneously.

The constant current load functions by supplying a fixed voltage to a resistor, Rprog. The load current is calculated as Vo/Rprog, where Vo is the output of the regulator, set at 1.235V for the MIC2941A part utilized in this design. However, this circuit is not practical because the output current is constrained by the current limits of the regulator, typically around 1A, with all power dissipated in the regulator. Consequently, testing an eight-cell battery would restrict the maximum current to approximately 500mA, even with a heat sink on the regulator. Since the UBA can discharge up to 2.5A, this circuit does not offer any advantages. Additionally, this configuration can only discharge down to about 2V, making it unsuitable for testing single cells (NiCd or NiMH). Instead of the voltage regulator used in this scenario, a commercial high-power constant current load or a custom-built alternative would be recommended to fully utilize this technique.

The lower section of the schematic illustrates a switch load, which employs a power resistor as the load (Rload in the schematic). This circuit is simpler than the constant current load. Ideally, a precision power resistor should be used for the load to ensure accurate discharge power dissipation. The use of a resistor simplifies operation compared to an active device. For high-power loads, a similar circuit is employed to control a load bank consisting of light bulbs. Light bulbs are advantageous due to their low cost and relatively constant current characteristics across their voltage range. Consequently, as the battery voltage decreases, the current does not drop as sharply as it would with a purely resistive load. It is crucial for this circuit that the resistance of the load is known precisely, which can be challenging as load resistors heat up and their resistance values change.The UBA is limited to discharging up to 2. 5A per channel. This discharge rate is adequate for testing 99% of the batteries on the market. It is not sufficient to properly test batteries that are rapidly discharged in normal use (for example, some medical applications, UPS batteries and remote control aircraft and cars). For these batteries you sho uld discharge them at a similar rate to the rate they get in normal use. In other situations you might be fortunate to already have a constant current load that you would prefer to use. For instance, if you have a 10A logic controlled constant current load you could use your UBA to control it.

This application note describes how to build and control an external discharger. This circuit contains two remotely controlled loads. The top part of the schematic is a regular off-the-shelf voltage regulator configured as a constant current load with on/off control. The bottom of the schematic is a MOSFET used to control a power resistor. Inside your UBA, there is a 10 pin accessory header. On it are two programmable digital input/output lines which for controlling a remote load are configured as outputs.

Pin 3 is digital output #0, pin 6 is digital output #1, pin 1 is digital ground, and pins 8 and 10 are analog ground. The accessory header is accessible by either removing the top cover of your UBA or by using an extension connector on the back of your UBA (preferred).

Caution The digital lines on the accessory port go directly to the digital output IC on the UBA. There is no extra static protection. You must take full static protection precautions when handling the connections and when designing the circuit. Your warranty does not cover any damage caused by static or mistakes in connecting to the accessory connector.

After power up digital control line 0 is high and line 1 is low. You can select the active state of the digital signal (see Setup). These digital lines can source and sink up to 2mA. Since there are two digital lines you can build up to two loads. The circuit shown here contains two independent circuits for discharging. We built this circuit to test external load discharging, normally you wouldn`t build both circuits together. The constant current load operates by supplying a fixed voltage to a resistor, Rprog. The load current is thus Vo/Rprog, where Vo is the output of the regulator, 1. 235V for the MIC2941A part that we used. This circuit isn`t very practical since the output current is limited by the current limits of the regulator (usually around 1A) and all the power is dissipated in the regulator.

Thus if you want to be able to test an eight cell battery your maximum current is limited to about 500mA with the regulator heat sinked. Since the UBA can discharge up to 2. 5A there isn`t any advantage with this circuit. In addition, this circuit can only discharge down to about 2V, thus it can`t test a single cell (NiCd or NiMH).

Instead of the voltage regulator used here you would probably use a commercial high power constant current load, or build your own to really take advantage of this technique. The bottom part of the schematic is a switch load. This circuit uses a power resistor for the load (Rload in the schematic). This circuit is simpler than the constant current load. Ideally you would use a precision power resistor for the load. Thus the discharge power is dissipated in a resistor, which is simpler to work with than an active device.

For the high power load that we sell, we use a circuit similar to this to control the load which is a bank of light bulbs. Light bulbs have the advantage that they are inexpensive and somewhat constant current over their voltage range.

Thus, as the battery voltage drops the current doesn`t drop as quickly as if using a resistive load. This circuit requires that the resistance of the load be accurately known. This isn`t easy when the load resistors heat up and change their