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I have a device in which I have used a 3.7 V lithium-ion battery as the power supply. For charging this battery I am using a TP4056 module.

When I connect a battery to the TP4056, the battery fully charges and the TP4056 disconnects the power to the battery and the device starts to run from the battery. When the battery drains approx. 30 mV, it again starts charging until full.

My device can run from 3.7 V to 4.2 V.

Now I want to make it more reliable for my device. When a battery has fully charged, it's disconnected by the TP4056 module and it stops giving current to the battery and the device starts to run from the battery and when the battery drains approx. 30 to 50 mV, the charger module wakes up again and starts charging and it will affect the life cycle of the battery.

What I want is this: when my device is connected to the adapter and if the battery is fully charged, then my device should run from the power supply, hence the battery won't drain until the power adapter is disconnected.

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  • \$\begingroup\$ Basically, you can run your device from (almost) the same line that charges your battery, however you must prevent this line from back-feeding the battery's "output" since this might screw up charging. The challenge would be that a passive diode there would also result in a small forward voltage drop during normal battery operation. Check parts and see what your system might tolerate. \$\endgroup\$
    – Abel
    Commented Feb 21, 2021 at 18:18

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What I want is this: when my device is connected to the adapter and if the battery is fully charged, then my device should run from the power supply, hence the battery won't drain until the power adapter is disconnected.

If I read you correctly then maybe use a relay to divert supply from the battery to the adapter output: -

enter image description here

Whenever the adapter is producing power, the relay activates and the load is connected to the adaptor output. If the AC supply fails, then the load is connected to the battery.

If this is what you want then a few other details need to be considered: -

  • Adapter output voltage range
  • Load voltage range (might have to add a regulator)
  • Reverse diode across the relay coil
  • A hold-up capacitor across the load for when the relay switches (10 ms glitch protection).
  • Possibly use a MOSFET power OR-gate instead of relay
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  • \$\begingroup\$ I want to replace the relay with MOSFET or any other transistor. How can replace? \$\endgroup\$
    – Aarav Soni
    Commented Jul 5, 2018 at 6:52
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    \$\begingroup\$ Maybe something like this? It is an ideal diode power OR gate - whichever supply is highest gets to supply the load. Or maybe look for hot-swap-controllers? \$\endgroup\$
    – Andy aka
    Commented Jul 5, 2018 at 9:36
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If you manage to get your hands on some 5V relay, something like that could work, Pin6 of TP4056 gets pulled low when charging completes, you can use that to work out some load transfer logic (circuit is for reference, may or may not work)

schematic

simulate this circuit – Schematic created using CircuitLab

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