fool device that lipo battery is connected
A Simple Success

I just cobbled together some parts on my bench and fooled a TP4056 LiPo charger into thinking there was a battery connected:



On a TP4056 LiPo charger IC (cheap in China), I connected two LEDs (D1, red, Vf=1.76V and D2, yellow, Vf=1.82V) in series with R2, a 22&Omega; current limiting resistor.  R1 = 12k&Omega; programs the charge current to 100mA.

This made the charge IC happy; it is sitting on my bench and its output "charge" voltage is at 4.15V.  It is working fine without (additional) load, as well as with ~100mA load (36&Omega; through 3.3V buck converter).

Caution

This will probably not work for all charge ICs.  Many ICs will include
protection logic which might get upset with your fake battery.

For example, some other hardware here uses a LTC4081 combined LiPo
charger and buck converter IC.  Even without a connected battery, all seems to be fine initially.  After 4 hours the LTC4081 decides that no rising battery voltage can only mean defective battery, and disconnects power.