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Help with MCP73844-8.4 design charging circuit design

roybon

Hello  a colleague at work designed a small charger based on  MCP73844-8.4  chip to charge lithium-ion  batteries pack 7.4v 2600 mAh . It is a 2 lithium ion battery pack , 18650 package.

I had problem with this design, the charger wasnt  working at first:

- I had to change the resistor Rsens (see datasheet p15)   from 100mOhm to 220mOhm to make it work at fisrt.

- Then charge cycles (current and then voltage) are not working properly as it should be.

During the first cycle of charge the transistor associated to the MCP controller is heating a lot. After 6 hours approximetely the charger stops and the led STAT1 is blinking endlessly.

Then I unplug the power supply and I plug it back again. The STAT1 led is switched on .  The second cycle begins, the transistor do not heat as much as the first cycle (it seems to be in voltage regulation for this cycle). The charge completes normally till the led STAT1  switch off.

So to summarize the charge is occuring "manually" in two steps with an fault at the end of the first cycle.

I cannot determine what is really wrong with the design. Below is the schematic R2 called Rsens has been changed to 220mOhms instead of 100mOhm. I also added a voltage regulator to get 9 V at the input circuitry  to prevent the MCP controller from heating to much.

From the datasheet when STAT1 is blinking it is either due to :

- safety timer fault

-invalid temperature cell (unmanaged in my case because it is the 8 pin package)

I will try to have a look with the timer.

If you have any other idea, Thnks for you help.

Schematic of the design below . Replace R2 by 220mOhm ans a 9v voltage regulator at the input right after the power supply :

image

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  • I only took a quick look at this so check all this carefully.

    Your fast charge current with R2 = 0.22R will be about 0.5A. So the battery will need maybe 4 hours at fast charge. The timer cap you have fitted is 100nF which allows a fast charge max time of 1.5 Hours..

    How are you getting heat out of  Q1 - with a reasonable fast charge current of 2A the dissipate from about 4W to 1.2W during fast charge, at 0,5A it will be less (1W - 0.4W).

    It's a tiny little SO8 package so will fry at 4W.

    The simple thing to do is increase C5 to 470nF, which should give 7 hours fast charge max (or maybe 390nF for 5.85 hours) and keep R2 at 0.22R. Measure the voltage across R2 when charging so you can estimate the current.

    MK

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  • I only took a quick look at this so check all this carefully.

    Your fast charge current with R2 = 0.22R will be about 0.5A. So the battery will need maybe 4 hours at fast charge. The timer cap you have fitted is 100nF which allows a fast charge max time of 1.5 Hours..

    How are you getting heat out of  Q1 - with a reasonable fast charge current of 2A the dissipate from about 4W to 1.2W during fast charge, at 0,5A it will be less (1W - 0.4W).

    It's a tiny little SO8 package so will fry at 4W.

    The simple thing to do is increase C5 to 470nF, which should give 7 hours fast charge max (or maybe 390nF for 5.85 hours) and keep R2 at 0.22R. Measure the voltage across R2 when charging so you can estimate the current.

    MK

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