Adding new li ion cells in parallel to an oldish pack

[t_tberg]

My 1.5 year old 48v "20ah" battery has always sagged under load, I typically see a 4v drop using a standard kit controller (26 amps I think); now that it's getting cold out it starting to sag a little more. There are probably around 2000 miles on this pack with maybe 80-90 full charge/discharge cycles (never fully discharge and only fully charge half the time) here is a link to a similar looking battery that has pretty much the same specs as mine https://www.aliexpress.com/item/High-Capacity-Ebike-Electric-Bike-Battery-48V-20AH-Electric-Scooter-Lithium-Li-Ion-Battery-Pack-with/32691494220.html?spm=2114.40010308.4.52.IB5OKY
I have recently been able to procure some excess phone chargers/power banks from my university. They are "new" (over a year in storage - from a cheapy looking company) and have the same capacity as the cells in the pack (2200 mah); my question is can I add 2 cells to each parallel group (26 total) without any major issues? I'd imagine that the cells from both the newly acquired power banks and the craptacular 48v battery are pretty close in terms of capacity; the newer cells will obviously have a greater capacity but I'd imagine they're both pretty equal in terms of their shittyness. I know others have added more cells in parallel to their prebuilt packs, how did it go?

 

[MrDude_1]

as long as each parallel group gets the same size addition, so that each "cell" is the same size, it will work great. If the older cells are weaker, the new one will have to work a little harder as it tries to take up the slack... but it will help, at the cost of wearing out slightly faster from that use.

 

[t_tberg]

I do bring the pack in with me most of the time, the pack is still saggy in good conditions. I figured I could drop a few dollars on some copper braid and give the pack a little bit more mustard. I can't see myself starting a battery build with these specific cells for anything else e-bike related, they just dont have the power, I'd have to parallel up more low end cells and have a large inconvenient pack.

 

[MrDude_1]

I do bring the pack in with me most of the time, the pack is still saggy in good conditions. I figured I could drop a few dollars on some copper braid and give the pack a little bit more mustard. I can't see myself starting a battery build with these specific cells for anything else e-bike related, they just dont have the power, I'd have to parallel up more low end cells and have a large inconvenient pack.

theres one other thing you can do with it...
get a new convenient pack that meets your range most of the time, and then connect this pack in parallel whenever you need the range for longer rides.

the new pack wont sag and will give you the punch to climb hills, and accelerate... the old pack will "charge" the new one for the longer range.

 

[dogman dan]

Yes, you can add similar cells in parallel with each group. Use some relatively fat wire to make the connections, not 12g, but at least 16g.

Increasing the size will help, but not as much as doubling the size or at least adding 10 ah more.

I'd say just look for an affordable pack with the same cell count, and put them in parallel. They don't have to be super similar, just same general type, like not lion paralleled with lifepo4.

Charge them full separately,, then when both are full and at nearly the same voltage, connect up in parallel and ride. Magically, they will both reach empty at about the same time. Even if one is 20 ah and the other 10 ah.

I do this all the time, with a 13s 20 ah and a 13s 13 ah. Works great, and very little sag when I run them combined. Worth the cost, to have less than 2v of sag. It's really nice, and adding really significant range is even nicer.