I also have encountered such problem when installed CM7.2 after being used stock Froyo ROM for more than an year. I had searched forums for possible fix, but no method has helped.
Finally, I have made some tests with my phone and found interesting points about Defy battery that explain battery percentage drops.
Our Defy have very simple battery controller (it is referred as cpcap by Motorola). The controller does not calculate and provide battery percentage. It provides only a battery voltage and a current. The battd daemon do the battery percentage estimation. It seems some overcomplicated estimation algorithm is used to do that (I've seen the Coulomb counter technique is mentioned in the battd sources).
There are several battery percentage checkpoints at 5%, 20%, 50%, 80%, 100%. It seems the battd daemon have a voltage-percentage mapping table for these checkpoints. When a battery voltage reaches a checkpoint value then the percentage is immediately set to a corresponding value. All other intermediate percentage values are estimated by the battd daemon. The estimation is based on a battery capacity (requested from the battery itself) and the active current. A stock battery reports 1500 capacity. A 1700mAh Chinese battery, I also have, reports only 1200 capacity battd stores the last calculated battery percentage and voltage in the /data/battd/cc_data file. Data from the /data/battd/cc_data file is read on start-up and the percentage from this file is used as base and reported to the system. When battery is drained, the percentage is decremented. When you delete the cc_data and reboot, battd uses the current voltage and looks for the nearest checkpoint voltage value and sets the percentage accordingly.
How have I found the checkpoint values above?
Assume we have a brand new original Defy battery and it shows 25% of charge.
If you delete the cc_data file and then reboot the phone, battd will recreate the file and request the battery controller(?) for the current battery status. The controller will return 50% and battd show 49% to you (not 49% not 50%, due to rounding to the lowest integer). But the real charge is 25%! Now let the battery discharge. It will show to you 48%, 47%, 46%, etc. Great! But at 40-44% you get the instant drop to 19%, since the battery controller had reported real 20% charge at that point and battd has been forced to sync its estimated charge value to the real charge value.
When a battery is wearing its actual capacity is decreased. For example, it may be 1300 mAh after an year of usage. But battery still reports it have 1500 capacity and battd uses this capacity for its calculations. battd should decrement percentage a bit faster for this worn battery, but it still use the same decrement speed as for a new 1500 battery. When battery voltage reaches a check point value, battd updates percentage to corresponding checkpoint value and you register a drop.
battd has some battery information and correction data in the /pds/public/battd folder. This data helps battd to estimate battery charge more correctly. This folder contain 3 files:
batt_offset_data batt_phasing_data batt_tuning_data
For Defy there at least 2 versions of battd. (Defy+ uses a different battd) Let's name them as old and new ones. To find out which version you have, check the size of the /data/battd/cc_data file.
cc_data = 16 bytes - old battd,
cc_data = 24 bytes - new battd.
Early stock Froyo ROMs have the old battd, newer stock ROMs have the new battd. CM7.2 is bundled with the new battd.
Is the battd version makes any difference in the battery readings? YES. The difference is in a user account which is used to run a battd daemon.
The old battd uses the mot_accy user account, The new battd uses the system account. The corresponding owner and group should be set for the /pds/public/battd folder and files inside it. Otherwise battd will not be able to read battery data from the /pds/public/battd folder and defaults will be used, which cause inaccurate estimations.
If you have used stock old Froyo ROM or old custom ROM, you had the old battd. The /pds/public/battd folder has permissions for the mot_accy user account only.
You install the CM7.2 ROM with the new battd and it can not read battery data from the /pds/public/battd folder, since battd is run under the system user account. You have inaccurate battery estimation and percentage drops even on a brand new battery.
That's why installing the old battd (and its companion lib libbattd.so) on CM7.2 resolves battery drops issue for some users.
If you are using CM7.2 or any newer CM you can just change the owner and group for the /pds/public/battd folder and files inside it. Set both owner and group to system and the new battd will be able to read battery data files.
If you still have battery drops then your battery is worn to some degree and battd can not estimate its charge properly between checkpoints.
Is it possible to correct this? Probably yes.
I was able to adjust checkpoint voltage values and eliminate drops on checkpoints. BUT the discharging speed is still the same and I have good discharge without drops from 100% to 10% and phone is turned off on 10% since a battery reaches critical voltage of 3.050V.
To adjust checkpoints it is needed to change the /pds/public/battd/batt_offset_data file. I have changed the last 3 bytes to 0xA0. It offsets voltage value for checkpoints on 50%, 20% and 5%.
The batt_phasing_data file have not helped at all in my tests. It contains correction values for displayed voltage, temperature, current.
The batt_tuning_data file is total mystery atm.
So the goal is to find a way to force battd to decrement percents faster for worn batteries. The simplest way is to patch the battd with hardcoded capacity value (e.g. 1300) instead of reading this value from the battery.
The other (better) option is to create a custom battery driver and use a very simple mapping from current voltage to percentage. Such method is used to display battery status in touch boot menu. Also Quarx started a custom battery driver some time ago in his git repository.
The battery calibration is a myth! The /data/system/batterystats.bin file is not related to battery charge/life/status (exactly as Google devs stated earlier).
In case of our Defy, charge the battery to 100% and keep charging for few hours after that. Then disconnect it battd will display correct 99%. That's all you need when changing ROMs or batteries! Do not waste your time on "battery calibration"!
- Find out which version of battd you are using.
- Check owner and permissions for the /pds/public/battd folder and files inside it. Correct owner/permissions if needed.
- Charge your battery to 100% and keep charging for few hours more.