Voltage matching questions

I have 140 of the same 18650 cells. There's about five different groups ranging in different voltages: 3.63V, 3.73V, 3.83V, 3.92, and 4.04V.

This is my first battery build, will be 72V 20s7p. So far what I read is that I have to match the cells that will be in parallel, to within 0.001 volts of each other. How far can they be off from each other?

When it comes to joining them in series, does voltage matching matter?

3rd question: Should I just fully charge all of them before spot welding?


Thanks in advance endless sphere universe!

Grider said:

So far what I read is that I have to match the cells that will be in parallel, to within 0.001 volts of each other. How far can they be off from each other?

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You answered your own question already.

Grider said:

When it comes to joining them in series, does voltage matching matter?

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Only in that you will have to balance them all at some point, you might as well do it before you build it.

If they aren't all matched cells (capacity, internal resistance, etc) to start with, the batteyr isn't going to perform as well as it would if they were. If you get a bunch of cells that are supposedly all the same, they should all *be* the same, and should all be at the same storage voltage. If they're not, if the voltages of the cells all start out different, you may have a bunch of mismatched cells. You should probably test all the cells (per the various threads about doing that) to ensure they are all matched (and don't use the ones that don't match, at least not in teh same pack) so you can expect the pack to perform equally well throughout.

If they're not matched, the pack will age differently in different portions, and begin having balancing problems, the degree of which will likely follow the degree of mismatch.

Yours sound fairly mismatched, just based on voltage, so their other properties would have to be tested cell by cell to ensure you don't have problems with different groups of them.

There's plenty of info about this sort of thing in various battery testing and building threads, if you poke around.

3rd question: Should I just fully charge all of them before spot welding?

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You'd be safer to have htem all mostly discharged, almost empty. THen if you make a mistake and short something, there is less risk of a disastrous fire, especially as you get near to the end of putting the whole pack together.

Ok thanks.

But now can you guide me on this question:
Do you recommend that I buy a 4 wire Internal resistance meter to check the IR (or RI) for each and every cell? And after I do that, do I need to match parallel groups according to IR measurements? I mean...they are all the exact same type of 18650 battery, not new as they were pulled from a powerwall with 0 recharge cycles.

I've "poked" around on different threads and searching, but they don't have the definite answer. Thanks

Present voltage, since they were run in parallels, is no good indicator of anything.
Recommend you run the cells through a basic cell testing methodology.
1. Charge all cells to identical same voltage.
2. Let cells set, for several days, measure residual voltage, precisely. Note voltage loss as "self-discharge".
(Do not use cells with any notable self-discharge! - unless you don't mind a continually unbalanced battery that gets progressively worse?)
3. Test IR of each cell. Use good IR meter, or apply identical discharge to each cell and note sag voltage as indicator of IR.
(EG - 12V car headlamp discharging 1 18650) Discard any cells with drastically worse voltage sag.
4. Combine cells in series, manageable number, 28 cells at a time if no cells eliminated to self-discharge.
28 cells in series, 28 x 4.2V = 117.6V. Discharging through 2 x 60w light bulbs = 120w = ~1000mAh discharge per hour.
Discharge for 2 hours, or until any cell drops near 3.20V+, label each cell with its residual voltage - several minutes after discharge removed. (identical discharge time for every cell)
5. You now have a comparative capacity of every cell.
Arrange cells by capacity. (Unless drastic IR variance. Same brand-model cells seem to, normally, deteriorate IR relevant to capacity)
Sort into 20s by capacity, best to worst, 1,2,3, ... 18,19,20,20,19,18, ... 3,2,1,1,2,3, ... 18,19,20,20,19,18 etc.
Banks should be nearly identical in capacity ... unless any cells of noticeably bad capacity.

Where did these cells come from. Do you have a RC hobby charger ? Make a string of matched parallel cells and bring them to 4.0v then let set. And bring the next group of matched voltage cells put in parallel and charge to 4.0v and so on let for 48hr to find any self discharging cells. In parallel they will act as one cell seperate to find self discharg. That's a start. One weak discharging cell in a string will take down the parallel string.

I parallel bare cells using copper tape and neodymium magnets.
Fold copper tape over magnets spaced for 18650s or cut or fold thin enough, then fasten with magnet power.
6 or 8mm x 2mm recommended, (1mm thick for 18650 nipples, wrap in copper tape for best use.) cheap by the hundred.

Copper tape-magnets strings-connectors

Remember the whole case is neg. only the top of the tip is pos. good to have gaskets on pos end yes extra positive gaskets. Cheap fire safety. Yes the ring at top of cell is also neg and can short if conteched with nickel strip. Break out the volt meter and see ? All battery experiments need an adult to watch over and not walk away. Clue !