Today, we will learn how a battery charge cycle works. I have designed a lot of battery charging modules. Most of them are based on Li-ion batteries, so here in this tutorial, with the help of a lab bench power supply, I will demonstrate how a Li-ion can be directly charged without any external charging or protection circuit. If there is no circuit, then you should be there to monitor the charging. More than that, we will replicate the exact behaviour of a well-known battery charging IC, TP4056. Which is basically a 1A constant current/constant voltage battery charger IC. very popular due to its availability and low price. It can charge in constant current (CC) and constant voltage (CV) modes. We will see how these modes are decided. So let's begin with the IC specifications first.
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Battery Charging Modes:- Trickle Charge Mode (Conditioning): If battery voltage is <3V, a low current trickle charge is applied. In this case only 1/10th of the constant current is applied to the battery for safety purposes. So that battery can be pulled back to the state of charge.
- Constant Current (CC) Mode: When battery voltage rises above 3V, the programmed charge current (set by RPROG) is applied. Constant current mode is nothing but just a current limiting phenomena, we are basically limiting the max charging current to the IC. It will be discussed further in more detail.
- Constant Voltage (CV) Mode: As the battery approaches 4.2V, the charging current tapers off. This is basically filling the battery from 90% to 100%, this is important for good battery health but we can also plug out the battery after CC mode is done.
As per the battery voltage, the IC pulled in one of the above given 3 modes. The current is controlled through an external programmable resistor, which just applies a restriction to the max current. IC’s like TP4056 limit the charge current based on die temperature during high power operation or high ambient temperature.
In case of lithium ions the charge voltage is fixed at 4.2V. Automatically terminates the charge cycle when the charge current drops to 1/10th of the programmed value after reaching the final float voltage. Other features of a battery PMIC include cell temperature monitor, under voltage lockout, automatic recharge and two LED status indication pins for charge termination and presence of an input voltage. The basic circuit is given above.
How Batteries Generally Charged:BASIC METHOD: There will be two dominating modes: CC/CV. These two modes are common for LiPo and Li-Ion. CC is nothing but a constant current (limited current) mode, and similarly, CV is a limited voltage mode. Either we can control voltage or current at one time, but both can not be controlled at the same time unless limited by the power supply.
Say if a battery can charge at 10A, then as the voltage increases, it drops down to 9-8-7-6…to zero Ampere. This is an old method and is not used much, because it requires high currents which can not be handled with small-scale integrated circuits.
ADVANCE CHARGING: A discharged battery is charged in CC mode, it gets constant current, and the voltage starts increasing. As the voltage increases, the charging current decreases. But it does not show because the battery is demanding 10A, and we are only giving 1A. So it charges 10x slowly, and when the point reaches where battery voltage is sufficiently high, and it needs less than 1A to charge, then it shifts CC to CV mode.
One more thing to note, during CC mode, the power supply voltage can be shifted a little higher, say 4.2 to 6V, it will not damage the battery because the voltage is not constant, hence drops to the same potential of the battery as it gets connected. I will show this phenomenon in the next section.
But after CV mode, there should be a charging voltage nearly equal to cell voltage, now we can see the current going from 1A to 0A in a linear fashion. We replicate the behavior of the IC, see next sections.
Replicating the Charging IC Behaviour Manually:All we need is a discharged battery, some multimeters, an electronic power supply with current control function and variable electronic load.
To replicate the charging behaviour properly I discharged the battery to some extent using a electronic load, which we made previously see the full build from here.
An electronic load is a device that simulates different load conditions by drawing a controlled current or power from a source, making it ideal for testing power supplies and batteries. Recently, I designed and built my own electronic load PCB to handle precise current control for my lab projects. For manufacturing, I trusted PCBWAY, and the experience was seamless from uploading the Gerber files to receiving a perfectly fabricated PCB with professional quality. If you want to make your design real, then try PCBWAY services, it is a one-stop solution from prototyping to product development for me.
Set The Power Supply:
Step 1: Set the voltage of the power supply to 4.20 volts.
Step 2: Set the maximum current output to 1A. This can be done by directly shorting the pins of the power supply and then turning the current control knob. If you don’t have this feature, use a multimeter and a low-resistance load to set the value to 1A. For this, your power supply must have a current controlling knob. (See the GIF)
Battery Selection and Connection:
Step 3: Now we have 4.2V and amax current limit of 1A. The battery I have here is rated 3C, 2000mAh, with max discharge current and charge current of 6A. But I have limited the max charging current to 1A for safety and to avoid heating issues.
Step 4: Connect the battery to the power supply, and you will see that the current will be limited to 1A. It will charge in CC mode, my battery can take 6A charging current, so it charges 6x slower. The GIF shows how the battery voltage increase while charging.
CC TO CV Shift:
Step 5: Monitor the battery voltage and current, as the battery voltage keeps increasing, lesser current will be drawn. A point came where the battery draws exactly 1A current. This is the point where battery charging shifts from CC to CV mode. And as said earlier, one thing is constant, either voltage or current, so see the GIF, when the power supply is connected to the battery, its voltage drops to 3 volts and the current flows of 980mA.
Charge Completion:
Step 6: Now, in the CV, we have already set the max voltage to 4.20V. It can be set using a power supply or an external voltage regulator. So the battery voltage now maximises to 4.10 to 4.15 and charges in constant voltage mode, in which the current slowly comes down from 1A to 0A. And 0A shows that the battery is fully charged. A GIF shows how the current is decreasing from 1A to 0, with time as the battery charges.
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