So i've installed a bigger battery on my cheap A33 tablet. Previously it had a 1700mAh battery (rated for 3000mAh actually) for a 2x1700mAh setup. It works fine and actually works for longer, but the battery percentage is not correct anymore, since it now drops to 5% and stays  there for some time until it completely discharges. Does need a special calibration (pmic is AXP223) or will a simple charge/discharge calibrate things ?

You have to update the devicetree blob or fex file with the new capacity and internal resistance of your new battery, then reset the AXP to restart the self-learning process. It will take a couple of cycles for the gauge to become correct again.

 

http://linux-sunxi.org/Fex_Guide#pmu_configuration

 

The two most important ones are

 

pmu_battery_rdc: battery internal resistance in mΩ

pmu_battery_cap: battery capacity in mAh

 

There might be other ones you might want to tweak, but you have to keep in mind the overall power budget of the device and the charger/power adapter. Also don't burn down your house :)