I was wondering if when you connect some batteries in parallel, will it double everything (but volts).
For example, I have: One 3.7v 18650 battery 2000mah, with a maximum continuous discharge current of 1AMP.
And it powers a 4v bulb rated at 1AMP.
If now I put 5 of those batteries in parallel, does it mean now my batteries represent; a: 3.7v with 10000mah and my maximum continuous discharge current would be now 5amp?
Also, could I put now a bulb drawing 3.7v and 5amps? (without risk).
Will batteries in parallel with the previous example carries the total drawing power between each other?
So if my bulb were 3.7v and 20amps, every cell would be giving: 4AMPS? is it like that?
And final question, if you dont know what’s the maximum continuous discharge rate a battery can give, is measuring temperature safe to know it can resist the demand or not? (what would be the maximum temperature? 50°c? 60°c?)
In parallel, you do get higher current and higher capacity, but not higher voltage.
For your first example, yes, your 3.7V nominal battery will have 10 000mAh of capacity, and will be able to discharge at 5A
For your second example, if your load pulls 20A, and you have 5 cells, that means current is shared among 5 cells, being 20A/5 cells = 4A/cell. You need cells that can support that 4A draw.
I would say the most you should go is 50°C, and the top of the range should be 60°C. However, since you are only pulling 4A per cell at 20A, with adequate cells, like a Samsung 25R, the cell won’t even get warm.
1. When in parallel yes. Do not put a depleted cell with a charged cell though. A maximum difference of 0.1V is crucial.
2. In parallel, it does not matter much. In series, they need to be well matched. Not really that dangerous in single use items such as flashlights where we charge cells externally, so this isn’t too much of a problem either. Stick to quality cells in all cases, and at the same voltage.
3. In parallel, as long as cells do not differ greatly in voltage, it isn’t a problem.
In series, it is optimal to use identical cells.
Thanks, I have been wondering about this, that is how close the voltage on two batteries is when you put them in parallel. As long the cells are withing 0.1V, is there any other sort of balancing necessary before making the connection?
0.4V/0.1Ohm= 4A of current. That is a lot of current for an NCR18650B. In fact, over double the rated charge current. Not dangerous since the current goes down as the cells equalize, but damaging to cycle life.
Imagine now we have 2-3 cells that are full, and one empty.
That means we have 8-12A of current flowing into the lower charged cell! That is downright dangerous for the cell, and could result in a failed cell in a few cycle, and dangerous fast charging resulting in a lot of heat.
Finally, imagine if you had Samsung 30Qs. 0.04Ohms internal resistance.
With 2 cells in parallel, that would mean over 10A of current to one cell! Even a 30Q can’t handle that safely.
With 4 cells in parallel, that would mean 30A! That would probably make the cell explode in age.
TLDR: Don’t put empty cells in parallel with charged ones.
Thanks for putting that in plain terms BlueSword. I can understand that math, and it gives me the ability to determine what is the max delta for a given cell, based on the delta voltage and cell internal resistance. But I will add that for the NCR18650B case, (as the other case), the resistance is really 2x the resistance of any one cell, so the current is about half of what is calculated above, or 2A for a 0.4V delta and 0.1 ohm per cell.
This is true, there are plenty of YT videos of people trying to make them blow up by shorting, overcharging, heating, hammering, etc.
Usually they just get very hot, sometimes vent.
They would remain at the same voltage in parallel regardless of their capacity and discharge.
One cell may be supplying more current than another or heating up a lot more, but it would still be balanced because the voltage in parallel is always equal.
It would only be unbalanced if you mix different chemistry which has different nominal voltage.
Basic electronics says that the voltage over V1 is the same as V2 because they are attached to the same wire.
Sorry, you can’t break physics laws in the real world.
It is not possible to have two (significantly) different voltages at the same piece of wire.
The discharge rate of a cell does not limit how much power can go in or out, it is simply a suggestion for maximum continuous or peak discharge.
No matter what “discharge rate” it has, it will always become balanced in parallel, even if that means heating up and possibly venting.