Cells with different capacities in parallel/series ok?

[YoshiMoshi]

I have a couple of questions with regards to using different model cells of different capacities within a single battery pack, but the same chemistry and at the same voltage. Note I am NOT suggesting that anybody does this or does anything unsafe. I'm simply asking a question to understand a concept, the technical reasons for why it is or why not ok to do, and nothing else.

Is it safe to use two different model bateries (same chemistry, say Lithium ion) within the same battery pack? So say for example two different model 18650 lithium ion cells. In either parallel or series. Assuming that they are fully charged prior to assembling them in the pack.

Batteries of different models often have different rated capacities. Even batteries of the same model have differen capacities within a few hundred mAh depending at the discharge rate.

I have seen videos on YouTube were people use different model batteries in their battery pack just fine.

I have asked this question elsewhere before, and people just say it's a bad idea, but not why? Can someone please explain why this is a bad idea (if it is)?

If I build a battery pack every single cell has to be purchased brand new and of the same manufacturer and model (say Samsung 50S)? Otherwise it can be unsafe?
If I had a bad battery pack and find that one cell has died, I couldn't even replace the one cell with a newly purchased cell, same manufacturer and model (say Samsung 50S), because I would have one brand new cell, and the rest of the cells in the battery pack would be worn out more and likely have much lower capacity?

Thanks for any help in understanding this concept!

 

[sunshine_eggo]

I have a couple of questions with regards to using different model cells of different capacities within a single battery pack, but the same chemistry and at the same voltage. Note I am NOT suggesting that anybody does this or does anything unsafe. I'm simply asking a question to understand a concept, the technical reasons for why it is or why not ok to do, and nothing else.

Is it safe to use two different model bateries (same chemistry, say Lithium ion) within the same battery pack? So say for example two different model 18650 lithium ion cells. In either parallel or series. Assuming that they are fully charged prior to assembling them in the pack.

Batteries of different models often have different rated capacities. Even batteries of the same model have differen capacities within a few hundred mAh depending at the discharge rate.

I have seen videos on YouTube were people use different model batteries in their battery pack just fine.

I have asked this question elsewhere before, and people just say it's a bad idea, but not why? Can someone please explain why this is a bad idea (if it is)?

If I build a battery pack every single cell has to be purchased brand new and of the same manufacturer and model (say Samsung 50S)? Otherwise it can be unsafe?
If I had a bad battery pack and find that one cell has died, I couldn't even replace the one cell with a newly purchased cell, same manufacturer and model (say Samsung 50S), because I would have one brand new cell, and the rest of the cells in the battery pack would be worn out more and likely have much lower capacity?

Thanks for any help in understanding this concept!

All series elements of a battery or battery bank should have highly similar capacity as the capacity of the entire battery will be limited to the capacity of the weakest element. Other characteristics like internal resistance and self-discharge should be as identical as possible.

The less similar your elements are, the more likely you are to have operational issues. Mixing old cells with new means you're going to have cells pretty much guaranteed to fail sooner than the others. If you're okay with that, that's fine.

If you have the choice, all the same (brand, capacity, age, internal resistance) is best. It's not mandatory. You just need to be aware that you're making compromises from step one. All new cells of same capacity, specs, but different brand? Barely a compromise at all if they're equivalent in quality.

The more characteristics you can measure and match, with the three primary being capacity, internal resistance and self-discharge, the better your battery and experience will be.

Breaking it down to a spectrum:

One end: all new identical cells = lowest probability of failure or operational issues.
Other end: all cells the same shape and hold a charge with no other testing than confirming their resting voltage is in the acceptable range = highest probability of failure and operational issues.

You decide where you want to be on that spectrum.