Charging Batteries with a Power Supply

In this post we will look at how to charge a battery using an instrument grade power supply. The most common algorithm suggested by battery manufacturers to charge a battery, regardless of the technology, is for a charger to supply the discharged battery with a constant current level until the voltage level of the battery reaches a specified value. Once the specified voltage level is reached the charger should remain at that voltage level and continue to charge the battery until the current draw of the battery drops to approximately zero or to some specified low level value. Both the constant current level at the start of the charge and the constant voltage level at the end of the charge are typically specified by the manufacturer in the battery's data sheet. Also the data sheet will specify any charging conditions that could damage the battery or present a safety risk. The figure below shows a typical charge cycle for a Li-ion battery rated at 3.6 V and 1.9 Ah.

Figure 1. Li-ion charge cycle

From the figure, notice the first stage of charging consists of a constant current level set to 1295 mA. After about 1.25 hours the voltage across the cell reaches the 4.2 V specified charging level. At that point the charger switches from constant current mode to constant voltage mode (note the CVCC at the top stands for constant voltage constant current). The charger regulates its voltage at 4.2V until the current reaches about 100 mA. At this point the cell is charged and the charger can be shut off.

When charging a battery with a power supply ensure that any and all current sinking capabilities (if the supply can sink current) have been disabled. For instance if the power supply is 2 quadrant (source and sink current) or if it has down-programming capability (can sink a limited amount of current to pull voltage level down) disable these capabilities so the power supply will only source current to charge the battery and not sink current and drain it. This may be as easy as setting the power supply's low level current limit to 0 A so it disables the output if the current goes negative (sinking current from battery). Check the batteries data sheet to see if any voltage or current levels could damage or degrade the batteries performance. Use the power supply's over voltage or over current protection features to ensure the power supply output shuts off if any unwanted voltage or current levels are reached.

Setting up the power supply to charge a battery:

1. Connect the power supply to the battery. Positive terminal of supply to positive terminal of the battery and the negative / reference low terminal of the supply to the negative terminal of the battery. Use quality connectors and short wiring for making the connection. The lower the resistance between the battery and power supply the better charged the battery will be.
2. Set the power supply voltage level to the specified charging level.
3. Set the current limit on the power supply to the specified constant current charging value.

When you turn the power supply on the voltage and current will rise until it hits the set current limit, which is the charging current for the battery. The battery will then begin to charge at the constant current level. In the meantime the voltage across the battery from the supply will slowly begin to rise. It will rise until it hits the set voltage amount of the supply, which is the specified charging voltage. From there the battery will continue to charge at a slower rate with the voltage remaining constant and the current slowly dropping until it hits the cutoff point. The difficult part of the test is what to do when the battery is done discharging. If you are monitoring it manually you can simply shut off the output. Or if it is a programmable supply with measurement capabilities you can write a simple program that polls the power supply making current measurements until the cutoff current is reached and then the program simply shuts off the output of the supply. In a lot of cases the battery technology is robust enough to be charged and just remain at the constant voltage value with little or no current flow until it is convenient for you to shut off the power supply. In this case you just need to ensure that the power supply will not begin sinking power from the battery (discharging it) and the battery will not be damaged. 

Lets walk through an example of the charging process using an example Lithium ion battery CGA103450A from Panasonic and a N6783A power supply module in a N6705B DC power analyzer from Agilent. The CGA103450A battery's charge specifications from the data sheet are shown above in figure 1. The following explains how the CGA103450A battery was charged with the N6783A:

1. The sink capability on the N6783A was disabled so it could only source power
2. It was properly connected to the battery with the output off
3. In constant voltage mode the N6783A was set to 4.2 V as per the data sheet and the current limit was set to 1295 mA as per the data sheet
4. The output was turned on and the charge cycle was captured and shown below in figure 2
5. The output of the supply was switched off once the current reached ~ 100 mA

Figure 2. Charge cycle captured with N6705B screen shot

The N6783A inside the N6705B mainframe features a datalog function that was used to capture the charge cycle (figure 2 shows screen shot from N6705B). From figure 2 you can see the voltage ramp up to the 4.2 V level in about 40 min. During that time the charge current remained constant at 1295 mA. Once the voltage level reached 4.2 V the charge current dropped to 100 mA in just over 1.3 hours leaving the battery fully charged.

In this post we looked at the most common algorithm used for charging batteries of various technologies and power ranges.We then went over how to apply that charging algorithm to charge a battery using an instrument grade power supply. If you have anything to add to this post please use the comments section below. And if you have any questions please feel free to email me.

For more information on the N6783A click here