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A very simple Li-ion battery charger. This circuit I also pulled from the TI datasheet for the LM317.
I'm using a capacitor to try and simulate a battery. The voltage source of 2.8V will help discharge the "battery." Use the switch to start charging.
I say "very simple charger" because it does not seem to use the standard way of charging lithium batteries, which is the Constant Current Constant Voltage (CCCV) technique. Instead, it uses a voltage limit set to the maximum charging voltage. And, it uses a current limit to restrict the maximum charging current to a safe level.
Typically, a 3.7V Li-ion or Lithium Polymer battery is charged to a voltage of 4.2V. So, the 2 resistors 2kΩ and 4.7kΩ) are used to set the max charging voltage.
When the battery first begins charging, it will consume a lot of current. This needs to be limited (usually at or below 1C, which is the capacity of the battery). The series resistor (3.3Ω) is used to set the max charging current.
So basically, it is a constant voltage source part and a constant current source part combined.
So, the full cycle goes like this. At first, when the battery is discharged, the constant current part of this circuit dominates and limits the current. But as the battery charges and the voltage climbs higher, the constant voltage portion of the circuit kicks in and limits the charge voltage to 4.2V, and the charge current goes down.
LM317 Series:
Adjustable Voltage Regulator: https://everycircuit.com/circuit/6138847104335872
Constant Current Source: https://everycircuit.com/circuit/5424946364547072
Adjustable Current Source: https://everycircuit.com/circuit/4902539157569536
Soft Power On: https://everycircuit.com/circuit/6070007032971264
CCCV Li-ion Battery Charger: https://everycircuit.com/circuit/5336266329292800
Linear Power Supply: https://everycircuit.com/circuit/6413550326906880
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