Using protected 18650 cells in a pack? And other questions...

[CentiZen]

Hey there guys, absolutely wonderful resource you provide to the world with this community.

I'm in the process of selecting cells and protection schemes for a custom made battery pack; not for a light but for a similar constant high drain situation. At the moment I have a couple options for cells and I'm trying to figure out how feasible each type are.

The cell is made up of two 18650's discharging at 7.4. It's used in a portable vaporizer called the Arizer Solo. This device uses a lot of power and gets rather hot so I want to make sure I have everything taken care of on the protection side to prevent any issues. It's using a standard tenergy pack right now with two 2200 mah cells. I want to improve on the performance it's getting right now with a better set of batteries.

I'm looking at:

callies kustoms 3100 mah unprotected 18650
AW IMR 2200 mah 18650
AW 3100 mah protected 18650

What I would like to do is use the unprotected 18650 callies kustoms along with a PCB to make the cell. Unfortunately they use USPS and they won't carry LiIon cells. I'm hoping they will FedEx the batteries to me but because of that I want to look into the AW cells.

I'd be buying genuine AW cells off of AW's thread in the CPF Marketplace so I'm not worried about that; but I'm wondering how safe it is to use independently protected cells in a pack together. And if I do this; is it going to cause issues if I add another protection circuit to it? Or is that necessary to ensure safety?

If it turns out it is not a good idea to use the protected cells, and I cannot get Callie's cells, I will then probably end up using the IMR's. In this case, will a protection circuit be necessary? I know the cells on their own don't need PCB's but what is the deal in series?

Sorry to drop so many questions in one post, and sorry if I seem like I'm rambling. I really do love this forum!

Thanks,

Michael

 

[45/70]

Hi Michael and welcome to CPF!:thumbsup: I'll try to help with a couple of your questions.

Whenever Li-Ion cells are used in series, it's best to use some kind of protection circuitry. For LiCo/ICR cells, it's pretty much mandatory, as these cells can be dangerous if discharged in series. The LiMn/IMR cells can be also, but are "safer".

It's not really correct to say IMR (also LiFe/IFR, for that matter) cells don't need protection circuits, as the cells themselves can be damaged just as easily as LiCo/ICR cells. It's just that no distributors that I am aware of supply these type cells with protection circuitry because they are less dangerous and not as likely to "vent with flame"/explode if abused. The cells however, are just as easily damaged due to overcharge/over discharge.

The problem with individually protected cells when used in series is not when the "pack" is discharged, but when charging the cells together in series. When discharging, the added protection circuits should work, and one of the PCBs will trip at the low threshold voltage, thus shutting down the entire pack.

When you attempt to recharge the pack however, you can run into problems. If for example, one cell is in a considerably higher state of charge than the other, it is possible for the voltage across this cell to "trip" it's protection circuit first. This, as when discharging, will effectively shut down the entire "pack", leaving the cell that was not fully charged still needing more charge. One way around this of course, is to charge the cells individually, rather than in series.

I can't help much with cell selection, as I have no idea of your current requirements. Generally, IMR cells are used where high discharge rates are involved. Also, many high capacity cells are better suited to low to medium discharge rates, and often, somewhat lower capacity cells will actually work out better at high rates of discharge.

Dave