Slightly technical (and possibly stupid) question about batteries

[WelshBloke]

Why arent smartphone batteries a stack of smaller parallel batteries rather than one big one?

If you had, say, 5 400mah batteries in parallel it would be the same as one 2000mah battery yes?

Then with a bit of funky connecting you could get the charge time for your 2000mah stack to be the same as for a 400mah one couldnt you?

 

[Morbus]

Where did you read that smartphone batteries aren't smaller batteries stacked together? They have cells, like laptop batteries, don't they?

 

[richaron]

If it's not a bunch of cells then it's not a "battery".

/me puts on grandpa pants

 

[WelshBloke]

So why does the charging time go up with capacity then?

If the "stack" were wired up properly couldnt you charge each individual battery simultaneously and independently?

 

[TheStu]

Where did you read that smartphone batteries aren't smaller batteries stacked together? They have cells, like laptop batteries, don't they?

My understanding of how Li-Ion cells are built is that they are a single sheet of material that is wrapped/rolled up. So for something like a cell phone 'battery', it's probably a single 'cell' at the end of the day. With a laptop battery it is a bunch of those cells hooked up together, sometimes in a hard case, sometimes not.

Apparently round Li-Ion 'batteries' are made the same way, which I didn't know.

 

[richaron]

Conventionally cells were stacked together to combine fundamental properties of said cells. E.g. a battery of 3 cells each making 1 Volt can make a 3 Volt battery.

If the "stack" were wired up properly couldnt you charge each individual battery simultaneously and independently?

"battery"? Charge one cell in my above example & it won't drive a 3V device.

Edit: dude above might be correct I'm talking basics and origin of the term "battery"

 

[TheStu]

So why does the charging time go up with capacity then?

If the "stack" were wired up properly couldnt you charge each individual battery simultaneously and independently?

If the power source in a cell phone were composed of multiple cells, I can't say for sure (I just don't have the background knowledge for it) whether it would charge faster or not; my gut says no, but I think that depends on how it is wired, and the controller. But I can say that 5x400 mAh cells would take up more space than 1x2000 mAh cell, if only for the additional wiring necessary to wire them up together. And in a cell phone, space is at an extreme premium.

 

[JeffMD]

From wiki;

"Li-ion cells (as distinct from entire batteries) are available in various shapes, which can generally be divided into four groups:

Small cylindrical (solid body without terminals, such as those used in laptop batteries)
Large cylindrical (solid body with large threaded terminals)
Pouch (soft, flat body, such as those used in cell phones)
Prismatic (semi-hard plastic case with large threaded terminals, such as vehicles' traction packs)
Cells with a cylindrical shape are made in a characteristic "swiss roll" manner (known as a "jelly roll" in the US), which means it is a single long sandwich of positive electrode, separator, negative electrode and separator rolled into a single spool. The main disadvantage of this method of construction is that the cell will have a higher series inductance.

The absence of a case gives pouch cells the highest gravimetric energy density; however, for many practical applications they still require an external means of containment to prevent expansion when their state-of-charge (SOC) level is high, and for general structural stability of the battery pack of which they are part."

 

[ChronoReverse]

So why does the charging time go up with capacity then?

If the "stack" were wired up properly couldnt you charge each individual battery simultaneously and independently?

The reason is because the limitation of charging speed isn't the battery but rather the charger or charging circuitry of the phone.



A good analogy for electricity is the flow of water. When you increase tank size but the flow of water into the tank is still the same then it takes longer to fill. It doesn't matter whether it's literally a single tank or multiple tanks combined. Likewise for the cells in a battery.



Common mobile chargers are usually 500mA, 1A, 1.5A or 2(.1)A. This is their maximum current capacity and is equivalent to the flow rate of water. Therefore, 2A charger will fill a 1000mAh battery in 30 minutes. A 2000mAh battery will take 1 hour and this is (almost) regardless of the physical construction of the battery. Furthermore, the charging circuits of your phone have their own limitation. My old Nexus S had a limit of about 1A while my GS4's limit appears to be 2A.



In any case, your idea of charging multiple cells in parallel would actually work; it's the reason why you can charge the enormous battery(ies) of a Tesla S in the same time it takes to charge your little cell phone battery. The thing is that your source needs to be able to provide all that juice!

 

[WelshBloke]

Common mobile chargers are usually 500mA, 1A, 1.5A or 2(.1)A. This is their maximum current capacity and is equivalent to the flow rate of water. Therefore, 2A charger will fill a 1000mAh battery in 30 minutes. A 2000mAh battery will take 1 hour and this is (almost) regardless of the physical construction of the battery.

I was under the impression that the changing ampage was limited because "bad things" happen when you pump too much into the batteries too quickly.
So 1.5A into a 2000mah battery would take X number of minutes, 1.5A into a 400mah battery would take X/5ish ?

In any case, your idea of charging multiple cells in parallel would actually work; it's the reason why you can charge the enormous battery of a Tesla S in the same time it takes to charge your little cell phone battery. The thing is that your source needs to be able to provide all that juice!

I have 13A at 240V at the wall, 5 400mah batteries simultaneously shouldn't be a problem.

 

[WelshBloke]

But I can say that 5x400 mAh cells would take up more space than 1x2000 mAh cell, if only for the additional wiring necessary to wire them up together. And in a cell phone, space is at an extreme premium.

I doubt it would take up a great deal of space. At the most crude you could stagger individual contacts for the "cells" along the edge of the battery. Copper strips take up very little space.
You'd have to beef up the initial charging circuit but that couldn't be that hard surely?

 

[ChronoReverse]

So 1.5A into a 2000mah battery would take X number of minutes, 1.5A into a 400mah battery would take X/5ish ?

Nope. The capacity of a 2000mAh battery and 5x400mAh batteries in parallel are exactly equal. So charging at 1.5A would take exactly the same amount of time (1 hour 20 minutes).

Again, the water analogy. If you have a single 1 gallon tank and four quarter gallon tanks, would they not fill at the same rate if you're using the same hose?





As an aside, each individual cell can only take so much charging current (and can only discharge so much for that matter) so you need parallel cells to increase the capability in both directions. If you do that, then the limitation is the "smarts" to manage the charging evenly (and correctly) as well as heat management. A complex and difficult task to do.

I have 13A at 240V at the wall, 5 400mah batteries simultaneously shouldn't be a problem.

Again, the issue here isn't so much the battery but that our charging hardware is limited. The best charger you can get is 2.6A and most phones and tablets won't charge that fast anyway because the management circuitry won't do it. Fast charging is also detrimental to battery durability. It also generates a lot of heat in the phone and battery.

Furthermore, it's misleading to say 240V 13A since that's your mains power running on nice thick cable. You're carrying power over a thin Lightning or USB cable instead. If you wanted to double from 2.6A to 5.2A that's really pushing it; think heat and power loss even over short cables.

 

[WelshBloke]

Nope. The capacity of a 2000mAh battery and 5x400mAh batteries in parallel are exactly equal. So charging at 1.5A would take exactly the same amount of time (1 hour 20 minutes).

That's if you're putting 1.5A in spread out over all the cells. I'm talking about feeding each 400mah cell 1.5A.

Its a way of pumping 7.5A into the battery without overloading it.

Of course if it's impossible to make the charging circuitry work it's a moot point.

 

[ChronoReverse]

That's if you're putting 1.5A in spread out over all the cells. I'm talking about feeding each 400mah cell 1.5A.

Its a way of pumping 7.5A into the battery without overloading it.

The point is that if you're feeding 7.5A then it'll charge faster for both the 2000mAh and the 5x400mAh. The really is no difference electrically.

BUT, as I mentioned each cell does have a limited charging capacity. So you're quite right that using parallel cells increases this. As I said, this is how the large capacity battery packs in EVs or airplanes do it but it requires complex and difficult management circuitry to do so safely and properly.

Unfortunately, you can't just wire them literally in parallel since each cell isn't absolutely identical. One might charge faster than another or have less capacity such that current flows from one cell to another causing Bad Things (TM) to happen. Hence the management circuitry.

Of course if it's impossible to make the charging circuitry work it's a moot point.

In any case, for the case of phones, the charging circuitry and chargers themselves are limited so it's as you said, a moot point.

 

[WelshBloke]

BUT, as I mentioned each cell does have a limited charging capacity. So you're quite right that using parallel cells increases this. As I said, this is how the large capacity battery packs in EVs or airplanes do it but it requires complex and difficult management circuitry to do so safely and properly.

Those are pumping huge amounts of current aren't they?

In any case, for the case of phones, the charging circuitry and chargers themselves are limited so it's as you said, a moot point.

But I have chargers for other things that pump out significantly more than 2A.

 

[ChronoReverse]

Those are pumping huge amounts of current aren't they?

They're literally charging up a bomb =)

But I have chargers for other things that pump out significantly more than 2A.

They probably have thicker wires than USB or Lightning too I'd wager and the device with the battery probably has better heat dissipation. Think about how warm cellphones already get when charging and imagine twice the heat.

It's not impossible but it's an engineering challenge and a pretty expensive challenge at that. They're already using techniques like Qualcomm's Quickcharge to increase charging speeds but there's unlikely to be huge leaps available.

 

[Dman8777]

My guess is that heat is a limiting factor in consumer electronics. My tablet and phone both get pretty damn warm when they charge. If that's actually the case, the only way multiple batteries would help is if they were physically seperate units and spread heat around the chassis. That obviously adds to the cost and complicates the design.

 

[Brian Stirling]

A typical smartphone battery is actually a single cell with a rated voltage of about 3.7 volts. The actual voltage will depend on the state of charge and the current flow and can range from a high of 4.2V to about 3.3V.


Brian

 

[IronWing]

With multi-cell batteries the charger has to allow for cell equalization so that all cells get charged fully instead of having different cells at different charge conditions and different voltages. Uneven charging can lead to currents between cells after the charger is disconnected. Cell equalization can take time.