Can someone clarify how to charge a battery without an off-the-shelf battery charger?

Question

I am a college sophomore studying EE, I am currently working on making a simple battery charger circuit for an integrated device. The device has 3 Li-Ion batteries in series for my motors, and a single cell by itself for the circuitry. I have been doing some research on charging techniques. I have seen a couple charge controller ICs and some simple schematics. I am trying to square away all aspects of charging a battery. I currently understand that a battery should be charged at its rated voltage (say 4.2V for a single cell) and that you should try to limit the current so that the battery doesn't overheat.

I have some uncertainties about how exactly to charge batteries. I was thinking that I could try plugging in the a battery to my voltage supply, setting it to 4.2V, and limiting the current to something manageable, then when the battery gets to 4.2V there will be no potential difference and the battery will stop being charged.

I was wondering if all of that is true. If not, what is the best method to stop charging a battery?

Answer 1

You're on the right track. Lithium batteries take what is called a "Constant Current, Constant Voltage" or CC-CV charge profile.

Constant Current: When the battery is discharged, you begin charging it by applying a fixed current (at the battery's voltage). Typically this current is somewhere in the neighborhood of 1C (the current required to completely discharge the battery from full over the course of 1 hour - i.e., for a 1000mAh capacity battery, 1C would be 1 Amp). Check the specifications for your battery, however, as the best charge current may be more or less than 1C. Because this phase of charging is performed at a single current, it is called "Constant Current".

Constant Voltage: As the battery charges, its voltage will rise to the maximum battery voltage. Once the battery reaches this voltage, it is not completely charged, but continuing to charge it at the same constant current will cause its voltage to exceed the maximum voltage. Therefore, the charge current must now be tapered off to maintain a fixed voltage on the battery. After some amount of time, the charge current will become very small, and this indicates that the battery is completely charged. Because the objective of this phase of charging is to maintain a constant voltage on the battery, it is called "Constant Voltage".

The charge algorithm can be summarized as follows:

1. Apply a constant charge current to the battery (Constant Current mode)
2. When the voltage hits the max battery voltage, transition to Constant Voltage mode, applying a continuously decreasing current to maintain the constant voltage on the terminals.
3. At some small current, turn off the charger altogether and the battery is charged.

This photo has a nice visualization of the voltage and current throughout the charge cycle:

Your idea to use a power supply set at the max voltage and with a current limit set to the max charge rate of the battery would work just fine. You would have to manually turn the supply off at the end of charging.