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Charging Lithium-ion can be don, but do carefully because it needs some protection.

Lithium-ion protection is necessary, as Lithium-ion will explode when they are shorted circuit or overcharged, and damaged when they are over discharged.

But with a charger module like the TP4056, it's easy to handle one 18650 battery.
If two or more batteries are the case, you need a BMS (Battery Management Systems)
Something like the HX-2S-D20 for two batteries.
For three a 3S like the HX-3S-01 and for four a 4S.
Offcourse some other parts may be handy like a LM2596 stepdown module for charging.

capacity indicator

DC in adapter

LM2596 stepdown cc cv

MP1584en module

One Li-ion 18650 battery

With a TP4056_charger_module.

Charge module- Linear charging.
The input voltage is 4.5V-5.5V but if the input voltage is too high, like 5.2v, the current will be less than 1000mA.
It is a protection function - auto-substract the charging current to avoid burn damage to chip.

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Current- 1A adjustable. Charge precision- 1.5%.
Input voltage- 4.5V-5.5V. Full charge voltage- 4.2V.
Led indicator- red is charging. Blue is full charged.
Input interface- micro USB.
Work temperature- -10? to +85. Inversed polarity- NO.
Size - 25X19X10mm.

2S Li-ion 18650 battery pack

With a HX-2S-JH10 module.

After the batteries are connected, there is no output because it needs to be charged with 8.4V first.
Note: The battery voltage, capacity and internal resistance should be matched well when combined with batteries.

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Upper workinging current: 8A. Upper instantaneous current: 10A.
Quiescent current: < 10uA. Internal resistance: < 300mO.
Charge voltage: 8.4V-9V.
Overcharge voltage range: 4.25-4.35v±0.05v.
Over discharge voltage range: 2.5-3.0v±0.05v.
Working temperature: -40---+50. Storage condition:-40-- +80.
Short-circuit protection: Yes,Charge recovery required.

3S Li-ion 18650 battery pack with capacity indicator

With a HX-3S-01 module.

Display indication:
25% first LED is on (voltage: 10.4 V)
50% second LED is on (voltage: 11 V)
75% third LED is on (voltage: 11.5 V)
100% fourth LED is on (voltage: 12.4 V).
After the batteries are connected, there is no output because it needs to be charged with 12.6V first.
Note: The battery voltage, capacity and internal resistance should be matched well when combined with batteries.

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For nominal voltage 3.6V 3.7V lithium battery.
Charging voltage: 12.6V.
Maximum output current: 20A.
Maximum output power / charging power: 252W.
Display indication: Static power consumption 2.3mA.

4S Li-ion 18650 battery pack

With a T-5016A15 module.
After the batteries are connected, there is no output because it needs to be charged with 16.8V first.
Note: The battery voltage, capacity and internal resistance should be matched well when combined with batteries.

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Protection IC: Seiko S-8254AAK. Protection MOSFET: 4x AO4407A
Overcharged voltage protection 4.35V.
Overdischarged voltage protection 2.7V.
Maximum Continous Curent 8A.
Maximum Peak Curent 15A, <20ms.
Output Short circuit protection YES.
Internal resistance <8mO.
Operating temperature -20 to 70 C.

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