Whats a good way to test a battery pack? I have a 10s6p i built about a year ago and i wanna make sure its still healthy as i plan to put it in a newer build and turn it to 12s6p.
Ive read online about charging each P-group fully then seeing how much they self discharge over the course of a week. If they drain down below 95-90% theres an issue. Is this correct?
You want to add cells to an existing pack?
If the pack is relatively new its generally fine. But since the pack is already used for a bit, adding new cells to it will give complications down the line.
As far as testing, charge it to full and make sure it has a chance to fully balance, then discharge it and look at the bottom of the charge, make sure no one particular group is discharging faster than the others. Take it all the way down to 3.4v or less and see what it does.
If no one group is dropping significantly faster than the others charge it to full. When it’s full how much balancing does it need to do? Nothing major and staying within .4v then it should be in good health. If anything weird pops up address it as needed.
As far as adding extra cells that’s probably gonna make your life harder later down the road. Make it a 12s5p and you’ll get more life from it. If the new cells have a mismatch in capacity or internal resistance it will stress the p groups and shorten the lifespan
Yeah the only way I could condone this would be if you were to completely tear down the battery to individual cells and rebuild from there.
Using the old 10s6p and adding 2s6p of new cells = no bueno
Tearing down the old 10s6p and reconfiguring into a 12s5p = all good as long as you don’t damage any cells
If you really must have 12s6p, the best way to do it would be to tear down to individual cells and essentially make 12s5p of old cells, and then add a new cell to each p group. Mixing old and new cells in parallel is many times less prone to incident than mixing them in series.
Please read my favourite thread on this forum for more information on mixing cells in parallel
Thank you @glyphiks @Pecos and @Linny i appreciate the detailed responses!!! After reading it all it seems itll be safer and also WAY less work to just make a brand new pack. I appreciate yall 
In my experience, this is pretty much always the case in questions of pack repair/rebuild. Glad you are open to that conclusion. Lits of people have heard this kind of feedback and said “well endlessphere said it was fine so I’m gonna do it anyway” 
Then a few days later they ask why their battery dies fast or doesnt charge🤣