Electronics > Beginners

Connecting different-sized lithium batteries in parallel.

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ogden:
Resistors or diodes? - No need. Look at Tesla battery pack for example, there's only fuse for each cell in the bus. Before connecting make sure to charge all cells to exactly same voltage or balance them using resistors first, only when every cell has same voltage - replace resistor with fuse. Never ever directly connect discharged cell to charged one in parallel.

Rick Law:

--- Quote from: ogden on November 07, 2019, 08:34:00 am ---Resistors or diodes? - No need. Look at Tesla battery pack for example, there's only fuse for each cell in the bus. Before connecting make sure to charge all cells to exactly same voltage or balance them using resistors first, only when every cell has same voltage - replace resistor with fuse. Never ever directly connect discharged cell to charged one in parallel.

--- End quote ---

Yeah, when they are in at same voltage (and in good shape), resistors (or diodes) are unnecessary.  But the OP asks about connecting different-sized lithium batteries in parallel.  Diode/Resistors can be a way to allow them to be connected at random unbalanced state and provide some limited protection.

ogden:

--- Quote from: Rick Law on November 08, 2019, 01:48:22 am ---Diode/Resistors can be a way to allow them to be connected at random unbalanced state and provide some limited protection.

--- End quote ---
Note that battery pack cells for decades are connected in parallel using just safety elements, usually fuses. All you need to do - charge cells before pack assembly. Adding lossy resistors or diodes to battery pack just because you are lazy, ignorant or just ill-informed is strange solution (to say it politely).

Rick Law:

--- Quote from: ogden on November 08, 2019, 10:14:35 am ---
--- Quote from: Rick Law on November 08, 2019, 01:48:22 am ---Diode/Resistors can be a way to allow them to be connected at random unbalanced state and provide some limited protection.

--- End quote ---
Note that battery pack cells for decades are connected in parallel using just safety elements, usually fuses. All you need to do - charge cells before pack assembly. Adding lossy resistors or diodes to battery pack just because you are lazy, ignorant or just ill-informed is strange solution (to say it politely).

--- End quote ---

Mostly ignorance (which is a superset of ill-informed).  Since we are doing it as hobby, laziness would not be the issue.

For whatever reason, I am fearful a bad battery becoming a dead-short for other batteries to drive current through.  But I said that in my earlier reply already.

Both OP and I were talking about recovered laptop batteries.  In the process of recovery, it was visible how they were connected originally.  We are both finding ways to make the best use of those batteries after we cut it out of the pack.  I am sharing my limited experience in the hope that we can both learn more.  I surely would like to learn a better/safer way of "joining" randomly picked 18650's so I can use whichever fully charged 18650 cell(s) at hand.

Most of us have knowledge limitation.  We just have to work around it until we learn more.  The alternative is to sit on our hands and do nothing until we learn everything about a particular subject - which would be rather impossible.  I bet you by the time we have fully functional giga-bit quantum computer, most of quantum mechanics would still be mysteries even to the creators of the giga-bit quantum computer.

In my view, an ability to work around ones limitation is a good quality to have.

ogden:

--- 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? ;)

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