SMBus pull-up resistor value
12k Ohm pull-up, let's the SMBus of a BT70791 operate fine.

I've tried with changing Pull-up voltage from 2.5V all the way to 5.5V

So as a start with only 1 battery you will get ack from SMBus slave with any voltage above 2.5V and 12Kohm pull-up resistance.(In case you are using BrenTronics)
Considering they can sink 700mA or more. you can get away with an even lower Pull-up resistance and the Battery will still be able to pull the line "low". 

(Ohm's law) [Voltage = OHM * Current]

or in this case 
[(Maximum sink current 700mA) >= Voltage/Resistance] "Less or equal to V/R".

Keep in mind the 2 cell batteries from BT has 2x SMBus. With same address (0x16) (Translated this appears as 0x0b on a packet-sniffer if you wish to watch the SCL/SDA lines)

Another TIP
Keep maximum distance from Master to Slave below 50cm! the I2C line and SMBus is designed for short distance. Data Corruption or no "ACK" appears randomly if SDA/SCL lines go past maximum length of 50cm