Hey All,

I'm very new to PCB design so this is probably a very easy question, but I'm making a small piece of wearable tech with a battery, so to save space I've moved the battery charging module, a TP4056, into the charger itself. I'm going to use contact charging to charge the device, but I've connected the charge_in pin to the positive battery pin, and I'm not sure how the charging module is going to take the device running while it's plugged in. Is this even an issue I should be worried about? Is the easiest fix just to shut off the device while charging? I've included a screenshot of my current schematic. Thank you all for your help!

Max

 

There may be an issue with the TP4056 not terminating the charge cycle when the device is running. Take the case where the device draws more than 1/10th the programmed charging current; the TP4056 will never terminate the charge cycle.

TP4056 Datasheet page 1: The TP4056 automatically terminates the charge cycle when the charge current drops to 1/10th the programmed value after the final float voltage is reached.

From the Battery University Charing Lithium Ion article, "A portable device should be turned off during charge. This allows the battery to reach the set voltage threshold and current saturation point unhindered. A parasitic load confuses the charger by depressing the battery voltage and preventing the current in the saturation stage to drop low enough by drawing a leakage current. A battery may be fully charged, but the prevailing conditions will prompt a continued charge, causing stress."

TP4056 https://dlnmh9ip6v2uc.cloudfront.net/datasheets/Prototyping/TP4056.pdf

Charging Lithium Ion https://batteryuniversity.com/article/bu-409-charging-lithium-ion

 

A battery may be fully charged, but the prevailing conditions will prompt a continued charge, causing stress."

If the battery is fully charged that means its terminal voltage has reached the threshold and the TP4056 will terminate the charge.
If there is a load which draws more than 10% of the charge current then the battery will not reach the threshold voltage. The TP4056 will continue to supply current and show that it is still charging but the charging current is not going into the battery, it is going to supply the load. This situation will not stress anything.

[edit] See this from BigClive:

 

Do you turn off your phone to charge it.

 

.

If the battery is fully charged that means its terminal voltage has reached the threshold and the TP4056 will terminate the charge.
If there is a load which draws more than 10% of the charge current then the battery will not reach the threshold voltage. The TP4056 will continue to supply current and show that it is still charging but the charging current is not going into the battery, it is going to supply the load. This situation will not stress anything.

[edit] See this from BigClive:

So connecting the charge pin and the battery pin as shown in the schematic won't cause any issue?

 

So connecting the charge pin and the battery pin as shown in the schematic won't cause any issue?

No.