P377SM-A Charging voltage

[Wacholek]

Hi everyone. 

After five, grat years of usage of my Clevo P377SM-A a battery pack needs replacement. 

I can by a new battery pack but what would be fun of that. 

I disassembled the pack and make some measurement. 

The battery pack contains 8 ICR18650-30B hi voltage cells. Those cells are simply where out. Voltage on all is pretty the same even after complete discharge. +-50mv. But it is going down in 10min .  

And there is a problem. I can exchange those cells for a new high capacity cells like INR18650-35E.

INR18650-35E has max charge voltage of 4.2V. Acording to the service manual the charge voltage should be 16.8V (4.2V/cell). Unfortunately after measurements the charge voltage of the patery pack is 17.60V (4,4V/cell) which is good for ICR18650-30B but not for INR18650-35E. The new pack will last for about a month. I have no idee why this battery pack is charged so high. 

I would like to change the charge voltage to 16.8V. 

Now, the main charge IC is OZ8681 and I have no datasheet for this IC. And there is two possible explanation how this charging circuit works. 

 

1. Charging IC is "talking" with the battery pack IC ( I2C) and receive desired charge max voltage. Since the battery pack IC does not do the current limit ( only current balancing, and cut off) charge current is set in  OZ8681 to 2.8A (according to the manual). 

2. OZ8681 is doing both current and voltage limiting by sensing the actual output voltage through one of the currant detection lines ( ICHP, ICHM) or by detecting output voltage through LX line. 

 

In first case there is  really nothing  to do since the battery pack IC will continuously demand higher voltage. But in the second case the voltage can be manipulated to lower it. 

However the LX signal line is connected before the choke and capacitors that are filtering the PWM distorted current. 

If anyone have an idee how to change the output voltage in this circuit please give me some hint. 

Best regards,

Chris