Current in a charging lipo cell

[ppTRN]

Hi everyone,
I designed and built a simple lipo charger (TP4056 based) and I am currently testing it.

- Without battery it outputs 4.27V and STBY led is on. It outputs 70mA since it is supply the circuitry connected tho the battery.
- If I short the output to ground it shuts the voltage off and does not draw any current (integrated short circuit protection I guess)
- If I put a Lipo charged at 3.97V, the STBY led is still on but the CHRG led does not light up. This also happended when the battery was more discharged. The current drown is about 500mA, as per the programming resistor that I choose (2 4.7k in parallel).

The thing that worries me tho is the fact that even at this state of charge the circuit still drows 500mA. When is it supposed to go into into constant voltage mode and stop feeding current to the lipo? I really do not want to find myself with an overloaded battery.

The only suspect that I may have is about a polyfuse in series with the lipo, that cause a 100mV voltage drop across it, and a shunt resistor used to evaluate current consumption, that only drops a few mV

In the schematic attached you can find 2 lipos, but actually only one is connected.

I am still leaving it connected since I guess the lipo should reach the 4.2V programmed, just wanted to check with you guys.

[kjr18]

Standby LED usually green or blue) should be lit when no battery is connected, and charge LED should be flashing briefly. When charging, only charge LED should be lit. And when charge is finished, only standby LED should be lit.

As for CC/CV curve, probably around 4,12V should start lowering current, and keep constant voltage at 4,2V until current drops below 0,1 C, 50mA in your case.

With fuse it would simply start dropping earlier, because TP4056 monitors it's own pin, not battery terminal. In most cases, there is no noticeable difference in voltage, but in your case it will be some (because of mentioned fuse.

[ppTRN]

Thank you.
It actually started falling after a while.
Now I feel more confident about the correctness of the circuit.