About 6 months ago I build my first battery pack in my first DIY. It is a Molicel P42A 12S3P pack that really was not built to the best quality. I was looking to remove the welds and redo the pack when i also realized that it would only cost 70 bucks to add another 12s pack in parallel. Given that the pack only has about 300 miles on it and never saw intense amp draw, would it be safe to remove the old welds and put and extra battery on it?
If you are adding one extra cell to each group, that is absolutely the best way to do it.
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do you think the batteries would be worn enough to not be safe to put with a newer pack? or should it be safe?
If you are mixing them equally like that so that all of the groups are 3 old/1 new, its totally fine as long as they are the same/similarly capable cells.
You’ll likely run into problems tho if you run unequal packs in series, like if 9 of the groups were made of old cells and 3 of the groups were made of new cells, that would be bad as the cells would charge/discharge at different rates.
When you join cells in parallel, they basically become a single cell, so as long as you create all of your parallel groupings with the same ratio of new:old cells, it’s fine as long as the OG cells were safe to begin with.
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This is the way, keep them all in the same grouping and add new cells to every group.
Be careful disassembling the pack, don’t tear the welds off - use a Dremel and grind the welds off carefully on the negative side. It doesn’t take much to dent/deform or even puncture the cells pulling or levering the nickel off of them.
Gotcha, Tysm both of u!
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Sorry for hijacking this post. I’m thinking of doing something similar. In my case I want too upgrade a 10s3p too a 12s3p or even 12s4p. My cells are Molicel P26A in NESE modules and are not welded together. Have been in use on a single drive since May 2023. Don’t have a metr or other tracking gadget so not really sure how much use they have seen. I do have a smart bms installed so i could discharge all the P groups to around 3.5 V or what ever, to match with the new cells. I’m guessing the capacity has doped a bit, maybe also the IR. Can I measure all of this parameters to find good matches for the new P groups? Any tips appreciated!
Good question: short answer no. The age difference between the cells will lead to premature battery pack death
Longer answer: yes BUT it is a lot of work. I did a write up on how to reduce (but not eliminate) the risks and failure points on packs made with mixed cell ages and such. It’s not exhaustive but gets you in the ballpark.
If you’re not turned away after reading the whats and whys i described there, definitely start a project thread so people can help through the process. Lots of helpful people around on here to help you avoid some of the pitfalls, if you can read past some of the coarse formatting 
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If I were in your situation, assuming your 10s3p is healthy and balanced, I would take 24 of your 30 existing cells and arrange them in a 12s2p. Then take 1 or 2 new cells and add them to each group to make 12s3p or 12s4p. This way, every single group will contain the same amount of old cells and new cells and should age consistently.
Definitely do NOT take your 10s3p and just add 2 groups of new cells. Mixing old and new cells in series like that is asking for trouble.
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That is a decent idea, more cells per group is mor better, and keeps the pgroup average characteristics the same
Thank you for the answer. 
I think I’m just better off leaving this pack for my thane build with single drive. And build a new bigger pack with 21700 cells. Probably wouldn’t even save that much money because I would have to buy two additional NESE modules, 12 more cells + another BMS…,. I’ll have to do some quick napkin math 
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How would I go about identifying which cell of the 30 I should eliminate. Can you test or measure easily which cells have lover capacity or bigger Internal resistance? I do have a multi meter just haven’t mesured much more than the voltage with it
If you wanted to be thorough, you could follow the advice outlined in @Pecos thread. Personally for me, if they were all the same voltage i’d just go with whatever ones look the best of course still assuming that the existing pack was healthy to start with.
Oh yeah …I’ll read that than … Well its my first pack so what the hell do I know. But yeah …I would say so. The p groups have all been around 0,02 V difference. Maybe One group has dropped bellow 3 V once or twice. But the whole pack has never been below 31 V or above 41 V and usually around 36 V while not riding.
What is it about the addition of new cells to a pack that would accelerate its aging versus using just the older cells?
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I don’t fully understand it - just my anecdotal experience when not being through mixing cells. Imho smaller pgroups (less than 8) need more effort and attention paid to matching the cells… see long answer
If i had to guess it could be that if there was a big enough difference between a cell in a group in charge or discharge rate or internal resistance combined with the stress of eskate high draw and fast charging - the weakest cell would have different thermal characteristics. The stronger cells would be doing more work comparatively also as lower resistance = higher current. The pgroup would be self balancing- or maybe equalizing is a better term. More activity means more aging.
The anecdotal experience is from using cheap used ebike packs with no name cells and making 3,4,&5 p 10&12s packs when I first started. A sample size of 10ish dead packs on the ladder of experience. Hard to tell also as my pack design was rapidly improving too
Okay, thanks. Too many things to separate out and have a great discussion about but I will toss out that used, no-name cells can act a lot differently than cells from the big manufacturers. Especially in terms of how they age and their self-discharge rate consistency.
Absolutely true, I guess the flavor i want to add is “surface level - there are a lot of things to consider so do your due diligence and post about it so i have something to read please”.
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Looks clean, daly I could never i have trust issues though and strong opinions. Opinions only though
How’s the dv6s doing for you? Just ordered one even though i wasn’t impressed with the d100s i had. Hoping for boring, will accept a slight vesc&bile aftertaste