BQ51050B: IC is not taking power after 1 hour

Hello

Can you tell if the board in epoxy resin is damaged ?

The BQ51050B is available in a QFN and BGA version, the BGA version may not work well if encapsulated due to stress on solder joints.

It is QFN version what we have choosen. board in epoxy is not transparent so cant see it. But it works for 1 hour and stops after that

Hello

It is possible the epoxy is damaging the IC or components as it heats up during operation.  The epoxy, PCB and comports will expand at different rates when heated.  This will stress the solder joints and smaller packages leading to failure.  

Have you operated unencapsulated units for longer than 1 hour?

Yes, the unencapsulated unis run for longer than an hour. 

The device worked fine when we tested for 1st time, took the charging for 3hours and discharged for 3 hours for our application.

After that it doesn't run at all more than 1 hour

Have you looked at Silicone Rubber Encapsulation products?  The flexibility will reduce stress on components and solder joints.

we tried that, the question here is when the distance is increased between Transmitter and receiver around 8-10mm it doesn't work anymore. max voltage at battery pin is 3.8V

The coil-to-coil distance is typically 5mm or less. As distance increases the coupling factor goes down and not enough voltage is generated to start the unit.

Check the voltage at RECT pin, it needs to be above 2.7V, UVLO to start.

A larger coil will help start up at larger distance.

there is more than 2.7V at RECT pin when measured. We try to make it around 5mm and see if this works. even with 10mm there is volt at RECT pin but starts at 3.8Vbat

2772.TX Start up and Comm Packets Guide.pdf

See attached --- During start-up the initial ping needs to be above 2.7V.  This applies to the first 3 or so pulses.  The TX at this point is operating in at a low power condition.

But after the three pulses RX and TX are adjusting operating point to increase power.  

Problems with low coupling at start up and 2.7V, no way to adjust operating point.  But after the RX and TX start communicating it can adjust operating point to overcome low coupling at distance.