Electronics > Beginners
Connecting different-sized lithium batteries in parallel.
Gallardo:
What I need to mention is that these batteries are used, the number of times of charging is different, the ESR is different, and the capacity is different. From the perspective of safety and self-loss, even if the diodes are added, they cannot be used in parallel because this is too dangerous.
My suggestion is that each battery is equipped with a booster with charging function and finally connected in parallel on the USB side. Each battery is independently charged and discharged, and there is reason to believe that it will be safer.
digsys:
Theories are fine, but after many years working on EV battery packs, this is what actually happens -
The lowest ESR cell will supply a slightly higher % of the pack energy until it starts to match the pack "average" ESR, at which point they all become app equal -
Even with wildly different ESRs, the same process occurs, but now you need to factor in that the lower ESR cells will supply a larger share for most the capacity -
In this case, you would not let the capacity drop to what you would "normally" set. Also, derate the max C figures to suit worst case possibility for each cell. 10-50%?
IF you keep this in mind, then in nearly ALL cases that I have EVER seen, when a cell goes "bad", it goes high(er) impedance or O/C. They RARELY go short cct - usually only if you have over-driven them or subjected them to higher temps.
We will OFTEN condition / match up a pack by cycling it from ~ 3.5V <> ~ 1.5V (or less) over 2-3 cycles. Cells in parallel do a great job of conditioning each other. YMMV
ogden:
--- Quote from: Gallardo on November 09, 2019, 06:14:20 am ---From the perspective of safety and self-loss, even if the diodes are added, they cannot be used in parallel because this is too dangerous.
--- End quote ---
Not more dangerous than using fresh batteries/cells. Please do not spread misinformation.
radiolistener:
But for safety purposes it's better to have good fire extinguisher during experiment ;D
Rick Law:
--- Quote from: ogden on November 08, 2019, 08:07:54 pm ---
--- Quote from: Rick Law on November 08, 2019, 06:48:21 pm ---I am fearful a bad battery becoming a dead-short for other batteries to drive current through.
--- End quote ---
Oh yes indeed you shall be fearful. 18650 cell can supply enough energy to set everything around on fire. You need fuse for sure. Diode or resistor possibly can protect against fire accident, yet they both are less efficient as fuse - due to voltage drop. What's the point of using diode or resistor as fuse when you can use just fuse? ;)
--- End quote ---
Typically, rather than a fixed "soldered-together" pair of 18650, I "randomly select" out of the charged-battery box a pair of 18650 for use. One may be better charge than another. Worst yet, and this is the mistake I think I would make eventually, I might have a partially (or hardly) charged battery while the other is fully charge.
When they are parallel-connected via diodes, the diodes serves to prevent the lesser charged battery receiving an inrush of power/current from the better charged one. It allows somewhat of an orderly sharing of the power.
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