Having current bypass USB +5V/USB ground pads mean that you will never get values measured correctly by bq27x00_battery module. Current must pass through charging chip, data pins shorted or not.

Again, shorting data pins by opto or whatsnot is not necessary if you use Pali's BME replacement (no reasons why you shouldn't). Just use the echo'ing thing everytime you want dedicated charging. Also, you can just wire charger to USB +5V/ground pads under battery, it *will* make current go through charger, and bq27x00_battery will get calibrated.

BTW, there is no real-life risk while using pads under battery, unlike what joerg said. They lack common-mode-choke and some external rudimentary second defense line as opposed to going through normal N900's microUSB port, but charging and USB chip still have it's build-in protection, rated few kV (seven, if memory serve... Or was it eleven? Anyway, much more than you will ever need). AFAIK, robbiethe1st uses this method for years, since his USB port died beyond repair, without any problems. I use it too, for two devices, in very EMI-dangerous places, without any special cables/additional shielding, and it works just fine. No reasons, why it shouldn't, BTW, if you check datasheet of USB and charging chip, noting it's internal protection(s).

Woody's trick with opto's was necessary during times of Nokia's BME, but now, he is obsolete.