Li-Ion battery charger module with adjustable charge current

Main project requirements and background

The popular TP4056-based Li-Ion battery charger module charges at up to 1A, which can be too high for low-capacity batteries under 500mAh, such as batteries from e-cigarettes.
The charger should allow different charge currents depending on the battery: the original 1A mode, plus 500mA and 100mA modes.

Device schematic and finished device

Li-Ion charger module with adjustable charge current schematic

TP4056 Mini-USB module without protection

Li-Ion charger module with adjustable charge current

How it works and components used in the project

The Li-Ion battery charger module based on the TP4056 uses a 1.2 kOhm resistor (code 122, resistor R3 in the photo of the module with a micro-USB connector). This resistor sets the battery charge current.
The resistor that sets the charge current is the only resistor of this value on the module board. Its location may vary between module versions, but it is always located near the TP4056 chip and has a value of 1.2 kOhm (122).
The 1.2 kOhm resistor (122) was removed, and two wires were soldered to its pads and connected to a separate board.
A board matching the size of the TP4056 module was selected and mounted above it using four 0.75mm copper wire supports.
A three-position DIP switch is installed on the board. All three contacts on one side of the switch are soldered together and connected to one pad of the removed 1.2 kOhm (122) resistor on the TP4056 module. Three resistor networks are connected to the three contacts on the other side:
1. The first switch contact is connected to 1 kOhm and 200 Ohm resistors in series, providing the equivalent of the original 1.2 kOhm (122) resistor. This corresponds to the original 1A charge-current mode.
2. The second switch contact is connected to a 2 kOhm resistor. With this resistance, the typical charge current is about 580mA.
3. The third switch contact is connected to a 10 kOhm resistor. With this resistance, the typical charge current is about 130mA.
The free leads of the resistors described above are connected together and wired to the second pad from which the "122" resistor was removed on the TP4056 module.
By selecting one of the three switch positions, the user selects the resistor and therefore the battery charge current.
For easier battery connection, the contacts of the TP4056 module are brought upward through the wire supports and connected to wires with alligator clips.
Be careful to connect only one resistor at a time. If several resistors are selected simultaneously, they are connected in parallel, reducing the equivalent resistance. This may cause the TP4056 to operate incorrectly.

A list of components used in this project, with links, is available in the Google document ("Slow pace Li-ion charger" sheet).