Battery charge current issue

[arg7]

Hello everybody,

I've noticed a nasty behavior of battery charge mode of T-Wristband, it pumps about 300mA of current in a small lipo battery of 80mA , overwhelming it.
If you look at battery voltage output (which is wrong by + 0.25V in my setup) on display, you will see that voltage very rapidly goes up to 4.2V and stays here.
This is due to fact, that internal resistance of the battery is quite high and it can't absorb this level of current.

This behavior kills the battery life because it violates the specs of lipo battery charge process.

Normally the lipo battery should be charged at less than 1C (where C is battery capacity) current and 300mA is too much.

I've investigated this problem and it turns out that the problem is in improper configuration of battery management chip TP4054. This chip charge current is configured via pin 5 PROG by attaching Rprog resistance to the ground.

According to datasheet the formula Rprog for small currents is Rprog = 1000/Icharge.

According to schematics, Rprog value is 3.3kOm, so the Icharge current is 303 mA.

In order to fix this problem, Rprog should be changed for much bigger value, I have replaced it with 20kom, which gives 50mA current.

For my lipo battery with capacity of 130mAh it is perfect. For 80mAh may be too much.

All editions of T-Wristband have this problem, should be fixed ASAP.

If somebody wants to repeat this fix (you will need smd rework station), the Rprog is located as shown on image here

This is MPU9250 version.

Best regards,
AR

[genotix]

Nice work figuring this out!
Would be good if TTGO would fix this.