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I have included a LTC4054 IC in my project and here is how the circuit is. I purchased it from here

The schematic is uploaded on github.

enter image description here

The chip has 5 pins

Pin 1 CHRG- connected in series to a resistor (1kohm) and a red LED to 5v pin of micro USB connector

Pin 2 is connected to GND

Pin 3 is connected to the Li Ion battery positive terminal.

Pin 4 is connected to the positive +5V pin of the micro USB. A 47uF capacitor is also connected from this pin to GND.

Pin 5 PROG pin is connected to GND through a 5.1kohm resistor. This gives the charger a maximum charging current of 200mA.

According to the datasheet, the CHRG pin will be connected to a low 20uA pulldown once charging completes. I am expecting the LED to turn off, because 20uA shouldn't be enough to make it glow. The open circuit voltage of my Li Ion battery is currently 4.21 volts, which I think has reached the level of charge expected.

Also, my battery has a charge protection circuit built inside it, which prevents over discharge, overcharging and short circuit protection.

My only question is- Why doesnt the LED turn off to denote that charging has completed?

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    \$\begingroup\$ Please provide a schematic for your circuit. \$\endgroup\$ – Chris Fernandez May 13 at 14:28
  • \$\begingroup\$ What is the active current of your circuit attached to the battery. If it is above 20 mA the charge cycle will never terminate. \$\endgroup\$ – sstobbe May 13 at 17:40
  • \$\begingroup\$ @sstobbe The battery is placed in a circuit which is sleeping. The current draw by the circuit is measured to be 0.3uA. \$\endgroup\$ – Rupin May 14 at 8:08
  • \$\begingroup\$ @ChrisFernandez for some reason i could not add an image to the question, it was failing everytime. I end uploading it on Github. You can see the schematic here github.com/rupin/HotORNot/blob/master/battery.JPG \$\endgroup\$ – Rupin May 14 at 8:11
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While it is entirely possible you have a damaged or counterfeit IC, the first step is to measure the charge current near the end of a charge cycle.

The LTC4054 mirrors the battery charge current by 1/1000 out the PROG pin of the IC (plus 3 uA): enter image description here

During the constant-voltage charge phase of the LTC4052, measure the voltage across resistors \$R_{PROG}\$. In order for the LTC4054 to observe an end of charge condition the voltage across \$R_{PROG}\$ should drop below 100 mV.

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  • \$\begingroup\$ Will report the findings by the weekend. \$\endgroup\$ – Rupin May 15 at 14:23
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Most probably, the LED is DIMMING. After the battery has charged, the LED must have been a bit dim comparing the first state when the battery was charging.

And this is most probably because the battery protection circuit inside the battery chassis and LTC4054 have different thresholds.

enter image description here

Probabyly, the protection circuit insde the battery pack is something like above. An its threshold is below 3.7V. After the protection of the battery cuts off the load, the IC charges and hold the C2 capacitor bypassing the battery.

enter image description here

So either fix the protection circuit and make its threshold higher, or remove it. You don't need a protection circuit with this IC. it provides the protection also.

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  • \$\begingroup\$ I will try this with a non protected battery and post the results. I suspect the Battery protection circuitry as well for this odd behaviour. \$\endgroup\$ – Rupin May 15 at 14:22
  • \$\begingroup\$ @Rupin Is there any update in the problem? \$\endgroup\$ – Alper91 Jun 10 at 12:42
  • \$\begingroup\$ The charge protection circuit was leading to this behavior. A Li Ion battery without the circuit did not exhibit the above reported issue. \$\endgroup\$ – Rupin Jun 16 at 6:47

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