I’m a little iffy about this topic. I hate seeing old batteries go to waste but I don’t want to recommend mixing old and new cells unless you do it properly.
Is it possible?
Is it simple?
The main reason why you are not to mix old and new cells is that as cells get cycled, their IR (internal resistance) rises and the capacity they hold diminishes. These two are related. To charge a battery, you apply a voltage (4.2v) to it with a current limit of course (usually 1.5A per cell) while the battery sits at a lower voltage. A low IR means that for the same voltage difference, you will have less energy lost to heat.
For example, new cell A has IR of 20mO, while old cell b has IR of 100mO. Both of them are sitting at 4.1V and you apply 1A to them. Cell A Voltage will rise to 4.12V while cell B will rise to 4.2V, therefore cell B is closer to fully charged than A. A could take 5 times the energy before it reaches the same voltage as B.
Why is this an issue?
Higher IR means your cells charge and discharge faster at the same current (have less capacity) than new cells.
If you have them in series and you don’t have any individual cell protection (like a discharge bms), by the time you hit your low voltage cutoff on the whole pack (lets call it 28V on a 10S) your new cells could be sitting at 3.2V while your old cells are at 2.4V. (I did not choose these numbers at random, between 3.2V and 2.5V, you usually have 10-15 percent left, and if there is a 10-15% capacity difference between your old and new cells then it would be easy to get caught in this situation.) This would mean that you over discharged your old cells and further damaged them. Eventually, you will either overcharge the bad cells, or over discharge them to the point that thermal failure is possible.
If you have the cells in parallel, you have a little more protection in that the old cells will always be at the same voltage as your new cells so you cannot over discharge or charge them that easily. The issue here is, that the new cells get loaded harder than the old ones, because their ir is lower. Not only this but the old cells also run hotter dissipating heat into the new cells. What this means is that the new cells are getting worn down at a much faster pace than normal. The capacity you get from a pack like this will lie somewhere between the capacity of the new cells and the old ones as opposed to being truncated by the old cells capacity in a series pack.
Series vs parallel mix new and old
New cells dont get accelerated wear
Easy to see and monitor difference in voltage and capacity of new and old cells
Easy to separate if necessary
Reduced capacity of whole pack to old cells capacity
Easy to over discharge and over charge
Higher chances of fire failure if unprotected and not constantly monitored
Capacity is as high as can be
Less likely to over discharge and over charge
Requires less monitoring
Reduced life of new cells
Cannot easily separate old cells from bew.
If failure does occur, new cells have to be replaced as well.
Cannot monitor the health status difference between new cells and old cells.
What I would do if I had to mix cells:
Use a charge/discharge BMS for protection
Use conservative ESC settings
Wire new and old cells in series and monitor health status constantly.
Charge with balance charger and keep an eye on capacity differences.