parallel 18650 's

[SNORKY]

Hi,
Please will someone have a go at answering my question?
If I put two 18650 (2.6 amp) cells in parallel to run an LEDfor 3 hrs (one cell will not last long enough before the voltage drops off) ….willthis be dangerous …..Will one cell deplete before the other kicks in …..willone cell try to charge the other…or will the batteries work in tandem anddischarge at the same rate ……in essence I wish to make a 2 cell in parallel batterypack …please what are the pit falls is doing this …..im using a 3.7v led driver @1 Amp Hr in thecircuit (the driver has a low voltage warning system at 2.5v)
Snorky :sick2:

 

[moderator007]

In parallel they act as one 5200mah cell. Just make sure their voltage is the same before connecting them in parallel and you will be fine.

 

[SNORKY]

Many thanks for the reply…..i really appreciate it
Snorky
:twothumbs

 

[Cataract]

The two cells would discharge at the same rate. However, you want 2 cells with the same capacity and once you pair them up, keep them together. There are no risks of reverse charging one cell here -that can only happen if the cells are in series or if you invert the polarity of one of the cells The worst that can happen is that one cell will charge the other untill they are equal, but you definitely don't want one weak and one new battery together just to be on the safe side. Check your cells every now and then and when one of them doesn't charge up to the same voltage, it might be time to change them. I can't think of any other pitfall than that.

By putting 2 cells in parallel, both will discharge at half the rate they normally would (more or less, internal battery resistance is the only real variable here), thus giving you twice the runtime and twice the amp capacity.

Edit: I see Moderator 007 found a simpler way to express it :)

 

[Aquanaut]

http://batteryuniversity.com/learn/article/serial_and_parallel_battery_configurations

discusses laptop battery packs: "Such a configuration is called 4S2P, meaning 4 cells are in series and 2 strings of these in parallel."

They discuss requirements and limitations.

 

[Prototype3a]

Best practice is to use a pair of new cells together or at least a pair that you purchased together and each have the same number of cycles on them. If the cells have different ages/cycles they can discharge unevenly and one cell may pick up more of the load than the other.

 

[spencer]

In parallel they will always have the same voltage. It is my understanding that two cells of different capacities (IF THEY ARE THE SAME VOLTAGE) will discharge at the same rate. (IE they will both reach x voltage at the same time).

Don't try this until someone else confirms it. I'm just putting it up right now for discussion.

 

[Microa]

What do you think when one of them dying earlier than his partner after a few years of service?

 

[Prototype3a]

The key here is the internal resistances of the cells. As batteries age their internal resistances increase. The cell with the lower internal resistance will discharge faster. This is true for all series/parallel combinations. This is why we have 'balancing' chargers for r/c purposes. These carefully fully charge all the cells in a pack to correct for the small differences in state of charge and internal resistance.

 

[Aquanaut]

What do you think when one of them dying earlier than his partner after a few years of service?

I have taken apart two laptop battery packs, and in each case there was one cell that had died and could not be recharged. Most likely, this is the failure mechanism of packs that have parallel connected cells. Series connected cells can usually be balanced charged.

BTW, the other good cells from the laptop packs are still doing just fine in my 18650 lights. This is an excellent source of "free" cells.

 

[SNORKY]

Hi All,
Just finished a 3hr Night time Mountain bike ride (and whata cold one it was ….minus 5 Celsius)
So… I pared up 2 cells both @ 4.1v ……I run the light flatout on max power for the full 3 hrs …just measured the voltage of both cells …….andboth measured 3.83v ….the light did not dim at all
The only thing I cannot answer is that if the cells werecycled the same amount of times in the past , as these are reclaimed laptopcells and most probably from different packs , as I opened several old packs atonce
I would like to thank everyone who helped me with my concern
Kindest regards …and a merry xmas Snorky
:wave:

 

[langham]

Sorry I am late to the party, but the only requirement for placing two DC power sources in parallel is that the voltages match. All of what is said earlier is true, but the voltage of 2 batteries in parallel will always be the same, unless they can not get to the same voltage for some reason, which means that one cell will continuously discharge to the other to try and bring it back up to the higher voltage. Think of 2 tanks with a pipe that runs between the 2 and you have a hose connected in the middle. No matter what capacity the 2 tanks are as long as they both contain the same liquid they will have the same height of liquid left in them. If one tank has a smaller capacity the larger tank will have to discharge at a higher capacity in order for its level to be the same as the smaller tank. Both tanks will empty at the same time though. If you had more liquid in one tank than the other when you connected the 2 all that would happen is the one with more liquid level (or voltage) will transfer liquid into the other tank to make them equal. The problem with this occurring in a battery is that it might happen very fast and the current could create a significant amount of heat in the process, known as power loss. Another thing that has to be considered is that the battery with the higher capacity will discharge at a higher rate which will cause it to wear faster, but at only 1A of current total this should be no problem for a lithium cell.

 

[Microa]

The problem is you can not predict if the weak battery suddenly short circuit. You only aware of the capacity of the battery is becoming less. I think we should put a fuse between each cell.

 

[Norm]

The problem is you can not predict if the weak battery suddenly short circuit. You only aware of the capacity of the battery is becoming less. I think we should put a fuse between each cell.

Used protected cells, your protection will kick in, problem solved.

Norm

 

[langham]

Used protected cells, your protection will kick in, problem solved.

Norm

He is right, you don't have to worry about anything if they are protected except for thermal stuff that would still cause inefficiency. You can purchase protection circuits on the internet very cheap.

 

[Microa]

The two cells are connect in parallel. The protection circuit can not switch them off. In #5 the link to batteryuniversity's information also point out the risk of fire in this case.

 

[Norm]

The two cells are connect in parallel. The protection circuit can not switch them off.

Please explain?

If one cell goes short it will trip the protection on the other cell, just the same way any short circuit will trip the protection.

Norm

 

[Microa]

This is a typical Li-ion battery protection circuit. You see the batteries itselves have their own loop which can not be switched off by the protection circuit.

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[Microa]

OK. I got what you mean. Each battery should have an individual protection circuit for itself.

 

[Norm]

OK. I got what you mean. Each battery should have an individual protection circuit for itself.

Yes :)

Norm