# Parallel batteries double everything?

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.

Yes, you’re correct.
Check the manufacturer specifications or a real world test by someone to know what current you can pull form it.

Hmmm …

For the last example , Yes and No …

A) The batteries should self balance when discharging ( Should )
B) Discharge at the same rate , only if they are well matched .
C) If you use poorly matched batteries imbalance can occur

Batteries need to be matched for discharge and capacity as both can create imbalance .
This is problematic for most batteries as they near depletion .

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.

Gracias guys.

Now I see why cars and bikes do use several 18650 in serie and parallel, not necesarry they are high drain batteries.

There’s also a video in youtube where someone starts a car using several 18650 batteries in paralell and series.

Interesting.

But previously you stated that each battery has “maximum continuous discharge current of 1 AMP”, so how could it suddenly give you 4 Amps?

But yes, if each cell could discharge at 4 amps, and you had 5 of such cells in parallel, then you could draw 20 amps max total.

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?

Batteries only need to be balanced during charging (unless each cell is charged individually).
If they are in parallel they balance themselves, so you only need to care about series.

I’ve put batteries in parallel with up to a .4 difference in voltage and didn’t see any noticeable problems. I guess there could have been something that I just didn’t catch.

Wow. That is dangerous.

Say you have a 4.2V cell, and a 3.8V cell.

There is a 0.4V delta.

Imagine we just have 2 NCR18650Bs in parallel.

They have an internal resistance of 0.100Ohms.

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.

First of all, it can do 5A just fine.
A few seconds of 5A current will make 0 difference to it’s lifespan.

Second, when you put the two unbalanced cells in parallel the total resistance will be 200mOhm, so 2 amps.

Third, a 30Q can easily handle 15A continuous, and busts of about 30-50A no problem.

Fourth, they do not explode.

Imbalance levels ?
Batteries can tolerate quite a bit of abuse or we would have far more Boom / flame / injury - reports .

Imbalance is interesting and I have been doing some watching in this regard since RC batteries tend to do more flaming than flashlight batteries .
Where the Danger seems to be - is near depletion .

There are two major causes for imbalance

1) Discharge capability - Simply batteries are not all equal . Even the same brand / manufacturing sample lot can have variance . This is especially undesirable in series .

2) Capacity - Again there is variance . - Where this is usually problematic is as batteries near depletion .

3) The Higher the current - the more these variances will effect the balance state of the batteries .

Even batteries run parallel can become imbalanced , due to capacity and discharge capability . And once again the variable here as to how bad things can be , is the discharge rate .

Bottom line - Own a Multi Meter - and know what you batteries are doing , don’t trust to blind luck - especially in high performance applications .

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.

The variable is ” DISCHARGE RATE ” …
At low discharge the self balancing is true …
But once you factor in a higher discharge rate , then things can change .
The other variable is capacity …

Also the other variable is the batteries themselves …
A lot of things are simple on paper , but often the real world teaches us different .

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.

My fault - Once again I have left to many holes in my explanation - Thats on me !
So I guess I simply cant say things like variables - Thats on me !

• I have failed to explain correctly - It’s like 1.30 am . Im tired . Thats my bad !

@Enderman, I was not speaking about discharge currents.

I was speaking of charge currents.

A 30Q receiving 20A of charge current when the cell is rated for a max of 4A charging current is dangerous for the cell.