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### Topic: 18650 in series or parallel in battery bank, which approach is better  (Read 807 times)previous topic - next topic

If i need 12Vs from 18650 power bank then i will connect four 3.7V cells in series to get 14.8V and to increase capacity i will have 3 or series arrange in for example 4s3p bank

But what if i connect all 18650 in parallel which will increase the capacity of the power bank but voltage will be 3.7   and to get 12 volts i will use step up circuit to make it 12v

the question is what are the disadvantages of this approach.    in parallel the voltage is increased but capacity will be equal to whatever a single 18650 assuming all 18650 are of same brand with same capacity.

Why people use parallel batteries to increase voltage but keeping the capacity the same  while if they connect in parallel it will increase the capacity of the charge and to get higher voltage use step up circuit

#### slipstick

Confusing question. No-one connects batteries in parallel and keeps the capacity the same and I have no idea what "3 or series arrangement" might mean.

How you connect cells depends of what you want to drive. If you need 15V it would be daft to connect 4 cells in parallel and then add a step up converter when 4 in series would do it without any additional circuitry (which is bound to waste some power because nothing is 100% efficient).

But the main point is that whether you use series or parallel connection the amount of energy in a given number of cells is always the same. So the efficiency of any additional circuitry needed makes the difference.

Steve

#### raschemmel

#2
##### Nov 17, 2019, 07:31 pmLast Edit: Nov 17, 2019, 07:33 pm by raschemmel
So far I haven't seen any Application Design Critera
current specs.
What's the load and the the minimum operating
time between battery changes ?
In other wirds , what is the application mAh spec ?

Have a look on following images from  https://batteryuniversity.com/learn/article/serial_and_parallel_battery_configurations

In following 18650 are connected in series      each cell has 3400mAh     the whole battery bank will give 14.4V  but the total capacity is 3400mAh

Now look at following 18650 connected in parallel        in this case the voltage is 3.7V   but the capacity has been increased

Now i can connect a step up Buck and boost converter like following

Now ICs like these will lower voltage and produce higher voltage

But in the parallel circuit we can have more total capacity

So which approach is better if i need 12V battery bank  ( i know higher voltage may consume more current )

#### raschemmel

#4
##### Nov 17, 2019, 10:15 pmLast Edit: Nov 18, 2019, 01:26 am by raschemmel
Quote
If i need 12Vs from 18650 power bank then i will connect four 3.7V cells in series to get 14.8V and to increase capacity i will have 3 or series arrange in for example 4s3p bank

But what if i connect all 18650 in parallel which will increase the capacity of the power bank but voltage will be 3.7   and to get 12 volts i will use step up circuit to make it 12v

the question is what are the disadvantages of this approach.    in parallel the voltage is increased but capacity will be equal to whatever a single 18650 assuming all 18650 are of same brand with same capacity.

Why people use parallel batteries to increase voltage but keeping the capacity the same  while if they connect in parallel it will increase the capacity of the charge and to get higher voltage use step up circuit
You still haven't answered the questions :
Quote
What's the load and the the minimum operating
time between battery changes ?
In other words , what is the application mAh spec ?
Let me repeat it for clarity:

A- WHAT IS THE APPLICATION CURRENT REQUIREMENT ?

B- HOW LONG DOES YOUR APPLICATION NEED TO RUN AT ONE SESSION WITHOUT STOPPING TO
CHANGE THE BATTERY ?

Do you know how to calculate that ?
ie:

Example
application contains the following devices:
1. Device A (current draw 50mA)
2. Device B (current draw 170mA)
3. Device C (current draw 200mA)
total current draw= 870mA

Application needs to run 12 hours without powering down for a battery change.

870mA * 12 = 10440mAh

Run requirement = 10440mAh
10440/3400= 3.07 batteries =>either round up to 4 or reduce time to 11 hrs, 43min.

Minimum current capability at any given time = 870mA (0.870A)
18650 current capacity = 3400mAh

Does it matter which you calculate first ?
Will there be any difference ?
Typically you calculate the number of batteries to get the voltage you need.
Then you calculate how many of those SERIES battery packs you need to get the operation time
required by the application.

Conclusion:
Application requires FOUR batteries IN SERIES to get 14.8V (3 batteries is only 11.1V)
Application requires FOUR  SERIES 18650 battery packs (of FOUR batteries) in PARALLEL) .
This is a 4S4P battery pack.

How many batteries do you need in SERIES ?
Your post starts with "If I need 12V ...."

12V/3.7 = 3.24 batteries, (3*3.7=11.1V)
If 11.1v is unacceptable then you have to round up to FOUR 3.7V batteries to get 14.8V

Even if you use 4 batteries IN SERIES, you STILL need to tell us HOW LONG THIS APPLICATION
NEEDS TO RUN CONTINUOUSLY WITHOUT POWERING DOWN.
Once you tell us that , we can tell you (or you can calculate) how many 4-series battery packs
you need to put IN PARALLEL.

If you say FOUR, then the resulting battery pack is a 4S4P.  (because 3 in series will not give you 12V)
If you say THREE, then the resulting battery pack is a 4S3P. (reducing run time to 11 hrs 43 min))

Quote
But what if i connect all 18650 in parallel which will increase the capacity of the power bank but voltage will be 3.7   and to get 12 volts i will use step up circuit to make it 12v

You lose current capability by using the step up because it is unlikely to be able to source the
current the batteries can source without it, in addition you waste energy through the silicon devices
in the converter.

#### gilshultz

Basic battery 101: When batteries are hooked up In series, the voltage is increased. For example, two - 6 Volt batteries connected in series produce 12 Volts. When batteries are hooked up in parallel, the voltage remains the same, but the power (or available current) is increased. This means that the batteries would last longer. There are advantages to both however batteries are not a constant voltage device, each chemistry has its own discharge curve which you should study.  There are Buck, Boost and SEPIC converters many which operate in the 90%+ range.  Using a linear regulator is tempting but they are only about 50% efficient and all that heat it produces comes form your batteries and is not available for your load.  If this does not answer your question more information will be needed.
Good Luck & Have Fun!
Gil

#### MrMark

#6
##### Nov 18, 2019, 02:44 pmLast Edit: Nov 18, 2019, 02:45 pm by MrMark
Broadly speaking one can build a more efficient power system with a series connected battery array and a buck converter but the charging circuit will be more complicated and the subsystem will require charge balancing.

If one is driving a high current (relative to battery capacity) load it's better to be regulating the battery output down to a lower voltage than to be trying to step it up.  In a low current cost sensitive application you're more likely to see a boost converter configuration.  So, to raschemmel's point, which is better depends upon the particular application.

#### raschemmel

#7
##### Nov 18, 2019, 06:26 pmLast Edit: Nov 18, 2019, 06:29 pm by raschemmel
To my knowledge the OP made no mention of chaging circuit and as must know , 18650 usb
chargers are available in single and mutipack
configurations which is why I went to such
legths to obtain the OPs continous runtime spec,
to no avail. It is STILL unknown what the
current draw is for the application or how
long it needs to run before shutting down to swap
batteries. This information may only be
obtained with a small room, with a single char, a bright overhead light
and a "good cop/bad cop" team interogation team.

#### gilshultz

Try this website, "https://www.powerstream.com/BPD.htm"  it has a good explanation of series, parallel and how to chose by: PowerStream, September 30, 2019, Engineering Guidelines for Designing Battery Packs".  When designing a battery pack I first determine what my load is, and how long I need it to operate.  I then consider how I can charge it and what will work best for the application.  Now I am ready to look at battery chemistry and the battery pack configuration.  Not all batteries work great in parallel and or in series, this has a big impact on the charging circuitry. Looking at all of the factors I make my choice.  Assuming the batteries are the same, the capacity increases by the amount the additional battery adds regardless if it is in series or parallel.  Assume (keeping it simple) we have 2 3Volt 1Amp batteries.  Put them in series you get 6Volts at 1 amp or 6 watts.  Put them in parallel we get 3Volts at 2 amps or 6 watts. Another thing to be careful when putting them in series the weakest battery becomes the dominate factor in the battery pack.  In parallel for the most part it is swamped out. Personally I prefer converters Buck, Boost or SEPIC on my battery packs as it keeps the voltage stable.
Good Luck & Have Fun!
Gil

#### raschemmel

#9
##### Nov 18, 2019, 07:51 pmLast Edit: Nov 18, 2019, 07:57 pm by raschemmel
Quote
Personally I prefer converters Buck, Boost or SEPIC on my battery packs as it keeps the voltage stable.
I don't see how a battery voltage can be unstable. Surely the battery can source more current than the
converter so I am not seeing what it is you think would make the battery voltage unstable.

Based on what we know so far, the OP doesn't have a choice.
He is forced to use a buck converter because the battery voltage will be 14.8V
and the application requires 12V so it appears there will have to be a buck converter.

#### wvmarle

#10
##### Nov 20, 2019, 08:45 am
I don't see how a battery voltage can be unstable
Voltage drops as the batteries discharge. That's probably what the OP is referring to.

Whether that's an issue depends on the application... which still has not been answered. So far we're still in XY territory.
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

#### raschemmel

#11
##### Nov 20, 2019, 09:47 am
"
Voltage drops as the batteries discharge. That's probably what the OP is referring to."

It wasn't the OP that said that.
It doesn't matter, you can't get 12V without
a buck converter anyway.

#### krupski

#12
##### Nov 20, 2019, 10:39 amLast Edit: Nov 20, 2019, 10:39 am by krupski Reason: The dang editor stripped all my paragraph newlines again. ARGHHHH!!!
If i need 12Vs from 18650 power bank then i will connect four 3.7V cells in series to get 14.8V and to increase capacity i will have 3 or series arrange in for example 4s3p bank

But what if i connect all 18650 in parallel which will increase the capacity of the power bank but voltage will be 3.7   and to get 12 volts i will use step up circuit to make it 12v

the question is what are the disadvantages of this approach.    in parallel the voltage is increased but capacity will be equal to whatever a single 18650 assuming all 18650 are of same brand with same capacity.

Why people use parallel batteries to increase voltage but keeping the capacity the same  while if they connect in parallel it will increase the capacity of the charge and to get higher voltage use step up circuit

You are fooling yourself. You NEVER get something for nothing.

Lets say you need 12 volts. You can get this by putting six 2 volt cells in series. If you instead put six cells in parallel, you get 6 times the capacity, but a 2 volt to 12 volt up converter will draw 6 times the current (i.e. you gain nothing).

And since any conversion has less than 100% efficiency, you actually LOSE capacity by up converting 2 volts to 12.

Gentlemen may prefer Blondes, but Real Men prefer Redheads!

#### krupski

#13
##### Nov 20, 2019, 10:44 amLast Edit: Nov 20, 2019, 10:44 am by krupski Reason: The dang editor stripped all my paragraph newlines again. ARGHHHH!!!
A- WHAT IS THE APPLICATION CURRENT REQUIREMENT ?
Doesn't matter. The OP thinks he can get something for nothing.

The three laws of thermodynamics:

* You can't win.
* You can't break even.
* You can't stop playing.
Gentlemen may prefer Blondes, but Real Men prefer Redheads!

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