26650 instead of 18650 battery in power banks

Thread Starter

Niladri Pal

Joined Jun 21, 2017
18
Can i use 26650 instead of 18650 battery to make a power bank?
If yes then why normaly power banks only made of 18650 cells instead of 26650?
 

Thread Starter

Niladri Pal

Joined Jun 21, 2017
18
No difference in capacity just the diameter of the cells 18mm or 26mm.
Ya thanks for the rply.
But why reputed companies dosnt use 26650 cells.it has greater power consumption capacity and power bank size can be reduced with maximum power storage.So why the big companies are using this 18650 cells instead off 26650?
 

Thread Starter

Niladri Pal

Joined Jun 21, 2017
18
Capacity can vary.
The 26650 is bigger.
The other one is cheaper option for a power bank.

I would choose 26650 if I were to make one
Ya thanks for the rply.
But why reputed companies dosnt use 26650 cells.it has greater power consumption capacity and power bank size can be reduced with maximum power storage.So why the big companies are using this 18650 cells instead off 26650?
 

Thread Starter

Niladri Pal

Joined Jun 21, 2017
18
You should ask them why.
My guess is cost reduction. Reputable companies also wants profit.
How will the profit come also.
Suppose manufacturing a 5000 mah power bank it will take 2 or 3 18650 cells to met the requirements.
But for 22650 it will only take a single cell, so comparing that it will take less cost to make a power bank with 22650 cells.
 

MrAl

Joined Jun 17, 2014
6,649
Ya thanks for the rply.
But why reputed companies dosnt use 26650 cells.it has greater power consumption capacity and power bank size can be reduced with maximum power storage.So why the big companies are using this 18650 cells instead off 26650?

Hi,

On the technical end it's mostly about energy density, and cross sectional area vs surface area.
On the financial side it's about materials cost and how many cells are sold over a certain time period.

The energy density controls the volume of the cell, the cross sectional area and surface area determine the maximum current draw for a well designed cell.

The volume of a 26650 cell is roughly 2 times the 18650.
The cross sectional area is about 2 times the 18650.

This means that theoretically the bigger cell can hold 2x more charge and discharge at a 2x higher current.

Since the bigger cell diameter is 26/18=13/9 times, the nearest equal square is a sqaure of the bigger cells with one side having 9 cells, and the smaller cells with one side having 13 cells. The capacity difference is:
162x for the bigger cells, and 169x for the smaller cells. Thus, the smaller cells take up less space by 13/18 (about 72 percent) when arranged in a perfect square leaving space for air flow that is equal to the space lost in packing circles side by side in a grid.
You'd have to look at more close packing to see if you can get an advantage that way, but then there would be less air space for cooling too which would have to be accounted for by determining the difference in cooling for both packs.

The cost factor probably plays a bigger part though. We'd have to compare equally scaled ratings which means we'd have to look up the larger cells with 2x current and 2x charge storage and compare pricing.

We'd also have to see if there is any significant self discharge issues for the larger cells vs the smaller cells.
 
Last edited:

Thread Starter

Niladri Pal

Joined Jun 21, 2017
18
Hi,

On the technical end it's mostly about energy density, and cross sectional area vs surface area.
On the financial side it's about materials cost and how many cells are sold over a certain time period.

The energy density controls the volume of the cell, the cross sectional area and surface area determine the maximum current draw for a well designed cell.

The volume of a 26650 cell is roughly 2 times the 18650.
The cross sectional area is about 2 times the 18650.

This means that theoretically the bigger cell can hold 2x more charge and discharge at a 2x higher current.

Since the bigger cell diameter is 26/18=13/9 times, the nearest equal square is a sqaure of the bigger cells with one side having 9 cells, and the smaller cells with one side having 13 cells. The capacity difference is:
162x for the bigger cells, and 169x for the smaller cells. Thus, the smaller cells take up less space by 13/18 (about 72 percent) when arranged in a perfect square leaving space for air flow that is equal to the space lost in packing circles side by side in a grid.
You'd have to look at more close packing to see if you can get an advantage that way, but then there would be less air space for cooling too which would have to be accounted for by determining the difference in cooling for both packs.

The cost factor probably plays a bigger part though. We'd have to compare equally scaled ratings which means we'd have to look up the larger cells with 2x current and 2x charge storage and compare pricing.

We'd also have to see if there is any significant self discharge issues for the larger cells vs the smaller cells.

Lithium-ion, including lithium-polymer batteries used in Power Banks and smartphones eventually lose their capacity over time (typically between 200 up to 1000 cycles depending on the battery cell quality and chemical composition) . The bigger the battery is, the less cycles will need to charge it and the longer it will last. Smartphones discharge their battery daily, thus has shorter battery lifetime than a large capacity power banks, so is it will be better to make a power bank with big battery?
 

MrAl

Joined Jun 17, 2014
6,649
Lithium-ion, including lithium-polymer batteries used in Power Banks and smartphones eventually lose their capacity over time (typically between 200 up to 1000 cycles depending on the battery cell quality and chemical composition) . The bigger the battery is, the less cycles will need to charge it and the longer it will last. Smartphones discharge their battery daily, thus has shorter battery lifetime than a large capacity power banks, so is it will be better to make a power bank with big battery?

Hello there,

That is a good point, but many things normalize when we examine the applications.

The big cell has 2 times the capacity and 2 times the current rating as the smaller cell, if they are built the same way. So for example to have two equal packs that can deliver the same current, we could find 2 big cells in one pack vs 4 small cells in a second pack for comparison.

Now the current draw is say 1x for both packs, because in any given application any pack would have to supply the same amount of current. The current in each cell in the small cell pack is 0.25x per cel, the current in each cell in the large pack is 0.5x per cell. Because the big cell pack as 2x the charge storage, it discharges in 1/2 the time, but because each cells see 2x the current as the smaller pack, both packs discharge in the same amount of time. This means that if we see 100 cycles on the small cell pack we also see 100 cycles on the larger cell pack.

This means there's no advantage for this particular criterion of cycle life in a given application. You may still be able to find an advantage for some other criterion however that we did not explore yet.

To add to my previous post where the criterion was the working volume, we can fit 4 small cells (18650) in the same SQUARE (cuboidal) space as a single big cell (22650). Note the emphasis on "SQUARE" because this does not work for a round space. Many things built today are rectangular so this would mean we'd see a 2x energy density for smaller cells over larger cells. If the enclosing space was ROUND (cylindrical) though, we'd fit 3 cells in a space that has just approximately 7.7 percent wider diameter than the larger cell, so we'd get about 1.5x the energy density but at the cost of about 8 percent larger diameter. So it is starting to look like the smaller cells pack better.
Also, just 2 smaller cells pack into the same space as one larger cell with just 1/2 the spatial density yet have the same capacity and current capability, so again we see a savings in space using the smaller cells.

Can the smaller cells be a little safer too? If only one cell has a bad problem, we only have to deal with 1/2 the energy so it could be better. I guess this depends how the pack is built too though.
 
Last edited:
Top