18650 parallel

kalel332

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Is it safe to charge 2 18650 (recycled from a laptop battery) in parallel on an single cell charger (this case an olight omni charger)
 

Lynx_Arc

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Should be ok, however I do recommend keeping an eye on any recycled batteries when charging them the first time just in case as I've had an occasional one with problems charging even though they measured seemed ok.
 

InvisibleFrodo

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Why do you believe those conditions are necessary?

Because otherwise the cells are unbalanced. Running unbalanced cells can lead to one cell being discharged lower then the other cells in the pack. Or one cell being charged up to too high a voltage. Having one or more cells getting excessively low or high in voltage is dangerous and can lead to a fire.
Also when one cell is bad or less healthy then the other cells it will place extra loaf on the good cells.
 

Gauss163

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Because otherwise the cells are unbalanced. Running unbalanced cells can lead to one cell being discharged lower then the other cells in the pack. Or one cell being charged up to too high a voltage. Having one or more cells getting excessively low or high in voltage is dangerous and can lead to a fire.

This is false. Cells in parallel can only be at the same voltage (once they initially equalize). When initially connecting you need to consider their voltage delta and IRs to prohibit possible overcurrent.

Also when one cell is bad or less healthy then the other cells it will place extra loaf on the good cells.

Of course the load will be distributed based on their IR as in any such parallel circuit.
 
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InvisibleFrodo

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This is false. Cells in parallel can only be at the same voltage (once they initially equalize). When initially connecting you need to consider their voltage delta and IRs to prohibit possible overcurrent.



Of course the load will be distributed based on their IR as in any such parallel circuit.

What about what I said is false? Are you saying a healthy cell and an unhealthy cell can be wired into the same pack and their voltage will remain the same after the voltage has been "equalized"?
I won't go into specifics of what's happening when you have 2 cells of uneven capability/charge state/cell health both running together, but for as long as it can hold up, most of the voltage and wattage comes from the healthy cell. It will be heavily taxed.
 

peter yetman

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Surely, if you have two cells of different capacity (due to age and condition) when charging, one cell can be full while the other better condition cell will still take charge. Won't the full cell still see the charging voltage until the charger cuts off for the second cell and lead to overcharging of the full cell.
That would lead me to think that the cells should be of similar condition for a safe process.
Of course I could be wrong, I often am.
P
 
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Gauss163

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What about what I said is false? Are you saying a healthy cell and an unhealthy cell can be wired into the same pack and their voltage will remain the same after the voltage has been "equalized"?

I quoted the false statements in my reply in post #7. Namely, it is impossible for only some cells in a parallel pack to be overcharged (or overdischarged). When connected in parallel they will always be at the same voltage (with current or not).

Note that in parallel their effective charge termination current will be altered based on current distribution, but this is often a good thing (e.g. it permits high IR cells to get a fuller charge). Such paralleling is often used to trick the charger into using a lower termination current when it lacks such an option.

I won't go into specifics of what's happening when you have 2 cells of uneven capability/charge state/cell health both running together, but for as long as it can hold up, most of the voltage and wattage comes from the healthy cell. It will be heavily taxed.

Again, of course the load will be distributed based on the resistances. But you can calculate the numbers and design accordingly to ensure that no cell is overloaded.

But note that the OP asks about parallel charging. This is perfectly safe and is done all the time by RC vehicle hobbyists to shorten charge times - even with quite dissimilar packs (having equal charge termination voltage of course). Search any RC hobby forum on this topic and you will find many posts describing how to parallel charge packs, and even graphs illustrating what happens in parallel.

I suspect some of the confusion above stems from warnings about cells in serial (vs. parallel), where some of the above concerns do apply.

Note: in fact the charge termination voltages don't need to be equal, but you need to stop the charge at the lowest one. For example you could charge a 4.20V and 4.35V cell in parallel, stopping at 4.20V. But then you'd need to do a final (short) charge on the 4.35V cell to finish it. This is generally much quicker than charging both separately. I often do things like that to save time.
 
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Lynx_Arc

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I agree with having both cells healthy in a pack as having a bad cell can cause issues in both charging and discharging and if the load on the pair is too much for one cell alone then the healthy cell can be pushed too hard and if your charging rate is based on 2 healthy cells the bad cell will be not need to be charged much and can cause the healthy cell to be charged at higher than expected rates and can either reduce the life of the cell or if charged near limit can cause issues. If the two cells are both healthy but not close to the same capacity and charge/discharge rates don't count on both cells being healthy then shouldn't be an issue.
 

Gauss163

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I agree with having both cells healthy in a pack as having a bad cell can cause issues in both charging and discharging and if the load on the pair is too much for one cell alone then the healthy cell can be pushed too hard

Both cells don't need to be "healthy". Rather, you need to know how their IR compare so you can determine how the current will distribute to the cells in the parallel pack. Then you can determine the max safe (dis)charge current for the pack.

and if your charging rate is based on 2 healthy cells the bad cell will be not need to be charged much and can cause the healthy cell to be charged at higher than expected rates and can either reduce the life of the cell or if charged near limit can cause issues.

Well of course you shouldn't choose a rate that doesn't correspond to the actual cells in use.

If the two cells are both healthy but not close to the same capacity and charge/discharge rates don't count on both cells being healthy then shouldn't be an issue.

The above sentence doesn't parse. What do you mean?
 
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Lynx_Arc

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Both cells don't need to be "healthy". Rather, you need to know how their IR compare so you can determine how the current will distribute to the cells in the parallel pack. Then you can determine the max safe (dis)charge current for the pack.
Yes an unhealthy cell will have a considerably higher IR and cause you to reevaluate safe current levels in use.
Well of course you shouldn't choose a rate that doesn't correspond to the actual cells in use.
You shouldn't but often people "think" a cell is healthy and try to get the max in/out of it because they are clueless to charge/discharge rates on less than healthy cells.
The above sentence doesn't parse. What do you mean?

I mean if your charge and discharge levels are safe for the one healthy cell having a second mediocre to unhealthy cell in parallel should be a lot less of an issue in use.
 

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