Battery Discharge Testing Without Using an Electronic Load
Using a DC Regulated Power Supply to Perform Constant Current Discharge Testing
The standard procedure for performing discharge testing on batteries is to use an electronic load. However, when performing constant-current discharge at high current levels, your electronic load needs to have a commensurately high input rating. If, for example, you want to discharge a 12 V lead -acid battery at 100 A, on paper you will need a 1.2 kW electronic load. If using Kikusui devices, this equates to two PLZ1004W units connected in parallel (for a maximum capacity of 2 kW). However, this represents an outlay of nearly a million yen (about $9,550 USD) on electronic loads alone, which might make this solution out of reach for some.
So, we'll go over a method for performing constant-current discharge of a battery using a DC regulated power supply, which is cheaper than using an electronic load. For a discharge current of 100 A at 12 volts, you can use Kikusui's PWX1500L switching power supply (0 - 30 V/0 - 150 A/Maximum Capacity:1.5 kW; List price: ¥300,000 plus tax or about $2,900 USD) coupled with a resistive load (around ¥100,000 or $950 USD) if using vitreous enameled resistors) as shown in Figure 1.
First, connect the DC regulated power supply in series with the battery. Next, add the resistive load (Figure 2). Constant-current discharge can now be performed by selecting an appropriate resistance. Note that if the resistance is too low, back voltage will be induced in the power supply, preventing constant-current discharge from occurring. By the same token, the use of excessive resistance will increase the power consumption of the resistive load.
Formula for Selecting a Resistive Load
R = Vbatt / I
where Vbatt is the maximum battery voltage (the charging voltage), I is the discharge current, and R is the resistive load.
Power Consumption of Resistive Load
P = I2 × R [W]
When performing constant-current discharge of a 12 V battery (charging voltage: 14 V) at 100 A:
Resistive load: R = 14V / 100A = 140mΩ
Voltage setting of power supply: slightly higher than the 14 V charging voltage (around 15 V)
Current setting of power supply: 100A
Once the power supply has been powered up, constant current discharge can be performed by flicking the switch.
There are a few issues to be aware of when using this discharge system.
1. Ensure the Circuit Can be Interrupted
When the power supply is off, back voltage is induced across the output terminals of the supply, causing current to flow through its internal circuitry. Figure 3 shows a circuit equivalent to the output section of the power supply.
Current continues to flow even when the power supply is powered off, that is, when the supply is completely inoperable. In that state, the fan that normally cools the power supply's internal circuitry will not operate. Unable to cool its circuitry, the supply may be damaged if left connected for a long period of time. A switch therefore needs to be provided, so that the circuit can be broken, and the battery isolated from the power supply once constant current discharge is completed. The switch is required irrespective of whether a reverse-polarity protection diode is used.
2. Use a Reverse-Polarity Protection Diode with a Low Forward-Voltage Drop (Vf)
When the output of the power supply is turned off, reverse voltage is induced across its output terminals, causing current to flow through the power supply's internal circuitry. While this is OK as long as the current does not exceed the maximum current rating of the power supply, we recommend connecting a low Vf reverse-polarity protection diode across the supply's output terminals. This will have the effect of reducing the back voltage as much as possible, thereby reducing the toll on the capacity and life expectancy of the capacitors in the power supply's output section.
3. Protect the Battery
Connecting the battery directly to the power supply with its polarity reversed risks damaging transformers, rectifier diodes, coils and other components as a result of excess current flow. In applications where it is possible to connect the battery the wrong way around, a reverse-polarity protection diode needs to be added across the power supply's output terminals and a fuse or similar protection circuit fitted. A fuse is also recommended to avoid a current surge if the resistive load is shorted.
Chief Engineer, SE Section, Solution Business Promotion Department
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The PAT-T Series DC stabilized power supplies