Question about IMR18650 parallel vs series (discharge rate)

Zeron1

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Hey guys,

This will probably be the first time I attempt to make a pack for an airsoft. From what I read, they need a draw of 20a and 25a for continuous draw. It will be a butterfly pack with at least 1 cell on each side.

It's going to be two pairs of series and then those pairs will be wired parallel. I won't be soldering the cells to each other. Instead, I will be making a plastic cylinder with contact springs on both ends. I will then solder both pairs of series into a parallel butterfly pack.

When I wire them in that fashion, is the 25a draw split between all the cells or do each individual cell have the same draw @ 25a? I've looked around the only 18650 cells I could find is rated at 20a continuous.
 

ChrisGarrett

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Hey guys,

This will probably be the first time I attempt to make a pack for an airsoft. From what I read, they need a draw of 20a and 25a for continuous draw. It will be a butterfly pack with at least 1 cell on each side.

It's going to be two pairs of series and then those pairs will be wired parallel. I won't be soldering the cells to each other. Instead, I will be making a plastic cylinder with contact springs on both ends. I will then solder both pairs of series into a parallel butterfly pack.

When I wire them in that fashion, is the 25a draw split between all the cells or do each individual cell have the same draw @ 25a? I've looked around the only 18650 cells I could find is rated at 20a continuous.

You would want to buy the Sony VTC-5a cells, which are rated for 25A-30A continuous.

I'm just an English Lit./Communications major, but I think that parallel (strings) splits the total amp draw between cells, but series arrangements don't.

Don't trust me on that, but that's how I understand it.

How you would calculate multiple cells in series spread out over parallel strings, I won't venture a guess, but those Sony VTC-5a cells are what you want, but they are flat tops for the most part and you might have to find some with added button tops.

Good luck.

Chris
 

Zeron1

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You would want to buy the Sony VTC-5a cells, which are rated for 25A-30A continuous.

I'm just an English Lit./Communications major, but I think that parallel (strings) splits the total amp draw between cells, but series arrangements don't.

Don't trust me on that, but that's how I understand it.

How you would calculate multiple cells in series spread out over parallel strings, I won't venture a guess, but those Sony VTC-5a cells are what you want, but they are flat tops for the most part and you might have to find some with added button tops.

Good luck.

Chris


Those cells are exactly what I'm looking for. I already have the NiMH packs coming in but they're a measly 1600mah. Although manufacturer recommends 9.6v, reported users recommend 8.4v to prolong the gearbox's life. I'll go with this route and order the cells. If needs be, I can always modify the airsoft to accept 4 cells @ 8.4v battery pack.

Thanks a bunch, Chris.
 

peter yetman

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Think of the cells as pipes. In series the current passes through a single pipe, so both cells see the same current. In parralel the current splits between the pipes and the cells see half the total current.
That how it works in my brain, yours may visualise differently.
P
 

Timothybil

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Think of the cells as pipes. In series the current passes through a single pipe, so both cells see the same current. In parralel the current splits between the pipes and the cells see half the total current.
That how it works in my brain, yours may visualise differently.
P
To put it another way, in series voltage adds and current and capacity stay the same. In parallel, voltage stays the same, and current and capacity add. So yes, you are stacking sets of two cells in series to get the higher voltage, and putting those stacks in parallel to obtain the needed current output. As a by-product you gaining the extra capacity as well. Your arrangement is what we would refer to as 2S2P - two sets of cell wired in series, with the two sets themselves wired in parallel. Two cells in series by themselves, as seen in a lot of lights, would be 2S1P. and two cells wired in parallel would be 1S2P. Hope that helps.

Oh, and by the way, your initial setup right after charging might be 8.4v, but soon after it will drop to more like 7.4, then continue to drop with use. And how are you planning to recharge the cells?
 
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Gauss163

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Think of the cells as pipes. In series the current passes through a single pipe, so both cells see the same current. In parralel the current splits between the pipes and the cells see half the total current [...]

The current I will be equally split between the paralleled strings only if they both have equal internal resistance R. Otherwise it splits with current ratio inversely proportional to resistance ratio, i.e. IR = V = ir so I/i = r/R, e.g. if one string has n times the resistance of the other then it will supply 1/n 'th of the current of the other. So if you're pushing them near their limits then in order to not exceed the limits it is important that they have very close R (and are of same model/batch) to maximize the chance that the R stays closely matched as they age.
 
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Zeron1

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Oh, and by the way, your initial setup right after charging might be 8.4v, but soon after it will drop to more like 7.4, then continue to drop with use. And how are you planning to recharge the cells?

My plan is carving two pipes out of pcv with end caps to allow me to drop the cells in. It'll be similar to those battery holders you'd see on multi-cell flashlights where you can remove the cells and recharge with a standard lithium battery charger. Instead, it'll be paired length-wise and not side-by-side. On both ends, there will be a contact spring and base that'll be soldered to a wire.

I guess the project can wait till I visit my hometown to get my hands on a lathe and a milling machine.

Thanks for the great help! The information and answers provided by everyone is very thorough and adequate.

Will drop by with photos after I finish the project.
 

Zeron1

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Think of the cells as pipes. In series the current passes through a single pipe, so both cells see the same current. In parralel the current splits between the pipes and the cells see half the total current.
That how it works in my brain, yours may visualise differently.
P

I like your analogy of current and how they're shared. Simple and straight to the point.

The current I will be equally split between the paralleled strings only if they both have equal internal resistance R. Otherwise it splits with current ratio inversely proportional to resistance ratio, i.e. IR = V = ir so I/i = r/R, e.g. if one string has n times the resistance of the other then it will supply 1/n 'th of the current of the other. So if you're pushing them near their limits then in order to not exceed the limits it is important that they have very close R (and are of same model/batch) to maximize the chance that the R stays closely matched as they age.

So this refers to the internal resistance within the cells? I plan on ordering 4xSony VTC5A cells from Illumination Supply and hope that they are similarly matched.
 

ChrisGarrett

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So this refers to the internal resistance within the cells? I plan on ordering 4xSony VTC5A cells from Illumination Supply and hope that they are similarly matched.

Gauss is just suggesting that you get matched cells, if running any of them in series, which you will be.

This means buying them from the same dealer and from the same batch code.

If you have a good way to measure I.R., then you could buy say 8 cells and cull them down to 4, using them in your Airsoft.

Most of the consumer chargers we use, that allow for I.R. measurements, do so in a spotty and inconsistent manner, so at best, we're fumbling around in the dark, a bit.

Absent a good way to measure I.R., we can always take the cells and run discharge tests, getting an idea if they're behaving properly, but this poses a problem for tabbed cells, or cells that aren't easily removed.

Buying from a reputable dealer, even asking if they have a way to match before you buy, is your best bet.

Chris
 

Gauss163

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Gauss is just suggesting that you get matched cells, if running any of them in series, which you will be [...]

Parallel, not series. But it's also important to have very good matching for cells in series since every cell/pack in a series string is effectively limited to the capacity of the weakest one (by BMS under/overvoltage protection). So they should all be very close in capacity (and IR) if you want to use them to their max potential. Best to buy them from the same vendor (hopefully from the same lot) and keep them married from day one so they age the same.
 

Zeron1

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Parallel, not series. But it's also important to have very good matching for cells in series since every cell/pack in a series string is effectively limited to the capacity of the weakest one (by BMS under/overvoltage protection). So they should all be very close in capacity (and IR) if you want to use them to their max potential. Best to buy them from the same vendor (hopefully from the same lot) and keep them married from day one so they age the same.

Thank you, Gauss for the in-dept information. I just received the cells from Craig @ Illumination Supply and literally all 4 cells have the exact same lot number. These 4 will be the cells for my little project. I'm currently waiting on a carbon tube 18mm ID and 20mm OD to arrive for the next part of the pack.

Just one more question regarding longevity. Does magnets affect the cell's resistance or lumen in any way? I plan on using them to connect the cells in series.
 

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