How do I change the Battery Percentage that corresponds to a Voltage in Android My original Li-ion has 3000 mAh capacity. I've modded my device to have a total of 7000 mAh capacity now. 

The enhanced capacity means that my percentage is not displayed accurately anymore.

About 2000 mAh charge displays 75% charge. That means 5000 mAh are located in the top 25% of my battery indicator.

(Charging times confirm: 1 Hour @1800 mA goes from 25% to 75%, but 75 to 83 takes almost the same time and so on.)

I know a lot is possible with a rooted Android device but I'm not too much of a software person.

How can I change the percentages that display? There has to be some kind of lookup table where the Vbatt→"%" conversion is done. 

Edit: If no one can help me, I am going to add a bunch of tp4056 chips to enable actually charging.

The software seems to mess around quite a bit. 

Now my phone is charging the 2nd time, and (I assume because it is calculated that the battery should be “full” by now in terms of mAh) it has reached “100%” despite the Vbatt being 3.95 Volts. Actual charge is ~ 70% at max!

Here you can see it charged more than 2000 mAh after reaching “100%”. I stopped the charge at 4.2 V.



The lower portion reached 75% after 1000mAh and took 900 more for the rest. So this has lost all it's 'linearity'. 

Can I (additionally to changing the display behavior) also change the charging algorithm? 

Do I have to compile a kernel by myself? Is there settings for this? I want higher current first and very low current for the charging above 4.1 V.
Let's look at the challenges you would face in achieving correct reading, rooting is not a magic key that let's you do everything easily:


The battery capacity (mAh) is coded in two places - the ROM and kernel (see Where are the battery capacity files located?) Only OEM would know from where the values are picked up, so you would need to have a custom ROM and kernel that stores this higher value. You would need to do this own and this is definitely not a trivial effort. ROM "chefs" wouldn't be interested in this (not appealing or in demand). You say you aren't a software person, so it's safe to discard this path.
The charging algorithm is also proprietary and not open source given the competition ( I can  charge from xx to yy% in so many minutes is the sales pitch). So that's your next huge challenge since the charger, charging cable and the device need to be in sync with the algorithm. Impossible for you to replicate. See the charging algorithm 
(Source
) of an Anker power bank to get a sense of how challenging it would be to figure out on your own. 
Charging incorrectly is going to damage your new battery and the purpose of acquiring it is partly defeated. This would be the result since you aren't easily going to achieve the first two points. 


It is for these and probably more reasons that third party sellers who provide extended batteries do not attempt to show the correct percentage. Read the reviews of any such product and you will find complaints around this

What you can do


Use a battery bank of desired capacity. 
Experiment at various charging speeds and arrive at a rough correlation between shown and actual voltages. 
Rig up hardware based system as you suggested in your question (off-topic on this site)