[SOLVED] Is it safe to connect several IMR cells of different sizes?

[Killovicz]

Is it safe to connect several IMR (lithium manganese rechargeable) cells of different sizes (capacitys), same voltage into a 12V battery?

I have two 18650 IMR 4.2V and two 18500 IMR 4.2V cells. Would it be safe to connect 3 of 'em (or all 4 of them for another matter) in series and make a 12.6V battery? <<--MVQ

-->>if yes, would it also be safe to connect ICR (lithium cobalt rechargeable) to IMR of same voltage and different sizes (capacitys)?

-->>if yes, would it also be safe to connect any kind of lithium cell to another lithium cell regardless of their size (capacity) or voltage (the materials its made of)?

-->>if yes, would it also be safe to connect any kind of cell to another cell regardless of their size (capacity) or voltage (the materials its made of)? ie nickel cadmium to lithium to alkaline etc.

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Thanks in advance, your time and effort will be greatly appreciated

 

[FoxyRick]

First rule of batteries:

Never, ever, connect dissimilar cells together. Ever. Never! ;-)

Seriously, one of them will end up running out of charge before the others, then it will be reverse charged by the current running through them. If your cells are protected (are IMR cells? I can't remember - probably are) then the best case is the cell will be cut off by the protection circuit. The worst case is it will explode with burning lithium. Nasty!

Even similar cells but of different ages (been used to different extents) should not be connected into batteries.

Edit: Just adding - I use a lot of 18650 and other lithium cells, including recycling unprotected cells from old laptop battery packs. I would never take chances with them. They are too dangerous when things go wrong.

 

[Killovicz]

Thank you very much for your time and effort (+1), much appreciated

..that was what I was afraid of :/, I was hoping that somehow they could recharge each other or just draw less current from the smaller once, however case solved

BTW. my IMR's are unprotected :/, they are so called "safe chemistry" IMR's from AW and I was just about to try to connect 2 x 18650 + 1 x 16500 to a drilling machine, lol, so u have probably saved me a lots of pain

Even similar cells but of different ages (been used to different extents) should not be connected into batteries.

On top of everything else this was rather useful info, since my cells are at lest recharged 100 times each not all evenly many times -->> meaning I shouldn't connect em in series at all

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Once again thanks a lot

 

[FoxyRick]

Glad to help.

Just a little more information... in commercial lithium battery packs, like those used in laptops, each cell has its own protection integrated circuit and the pack is kept in balance. Radio control hobbyists use special chargers which have an output for each cell in a battery and this too keeps the cells 'balanced'. Effectively, it monitors them all individually and keeps them at exactly the same final charge voltage to within a few mlilivolts.

I'm a flashaholic (a flashlight enthusiast - a side effect of enjoying caving and night-time hiking) so I use a lot of lithium cells for powering high-power flashlights. I always make sure that any cells that will be used in series in a multi-cell flashlight are charged to exactly the same end point. Cells will age differently so, as they vary throughout their life span I will group them into pairs (or triplets, etc) that are within a few tens of mV and can thus be safely used in series*. Any single ones left after that will only be used in single-cell lights.

I've seen the results of LiIon cells exploding from short-circuits or reverse charging due to a failed cell in a pack. They can burst the aluminium wall of a flashlight and spit burning lithium out like a firework.

The same principles apply to all types of cell chemistry, but for most the results are not as spectacular as with lithium. NiCd etc will just die and sometimes burst.

*To check cells for charge closeness, let them rest for an hour after charging to a final voltage, then measure with a precise voltmeter. I write the voltage on a label on the cell, along with a number of charges tally. I tend to check them every ten charges or so but some people check them every time.