High discharge current from standard 18650 lithium battery

[najrao]

The first two photos show a cordless driver battery,  before and after a modest top-up charge. Fortunately,  I had moved it  to a safe place. I had bodged five 2.8Ah pouch cells from a RC drone battery, matched for exact voltage but not protected by a BMC.

The next pic shows a 20+Ah, 11.1V pack, 3S-10P. I just scored this, and want to use it for a high drain load.

It has but a modest BMC, next pic. The two 4663 mosfets in series opposition are only good for a few (<5)A, as also so are the pcb tracks and lead wires fitted. I stripped the outer shrink plastic jacket to take pics.

The 10P 18650's should be good for some 40A. The inter cell connections are properly welded.

I propose to fit an external mosfet disconnect switch, to be tripped by the BMC itself if it detects an abnormal condition. A 12V coil relay can also be used, if the coil is not energized in standby condition. That way, the BMC  will not see the high drain current. Charging at <5A can still be done via the BMC. All wiring will be made appropriate. A 40A auto fuse may be enough, but I am looking at better current limiting/ tripping.

Any further safety issues? Please explain with reasoning. Thanks for your time.

Please overlook the wrong order of attachments.

[timpattinson]

As you are using the cells as a high drain load you should really set up some kind of over temp protection.

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[najrao]

Given the flat layout of the 30 cells in the pack, sensing temperature of just the one or two overheating cells is not straightforward at all. Suggestions please.

I do have a high current 3S BMS, from RC battery, to 50A.  I remember reading that there is a right way of connecting the battery wires, all other ways being wrong/unsafe. Do the taps go in later, or what? Help please!

[timpattinson]

Brute force approach to temp sensing:
Connect lots of diode based temp sensors (such as LM335 from ST) in parallel, they will report the highest temperature of them all. In TO-92 package you can arrange one to touch two cells, so you don't need as many.