Imagine a list of paper which you need to fold into compact form. It would be really hard to fold it into "L".

Batteries made of long sheets folded then into rectangle

http://images.indianexpress.com/2017/01/galaxynote7_info_1.j...

Another guess is with an L shaped battery, thermal expansion makes it tricky because the L shaped compartment has a sharp inner corner which runs this risk of stressing the battery when it expands. A square compartment does not have that issue.

In any event, Apple knows how to produce weird shaped batteries. Just look at the Macbook. But it's in a nicely shape space.

Irregular in 3 dimensions (terraced).

Explanation as to why this isn't the case:

TL;DR - putting the cells in parallel without changing their C-rating does nothing for charging them faster. You're limited by the C-rating of the cells, which is usually determined by the quality of components and the anode/cathode design.

Putting batteries in parallel maintains the voltage of one cell but multiplies the current by the number of cells in the pack.

E.g. if you have two Lithium Polymer cells in parallel, the nominal voltage will be 3.8V. At 5W, the current would be ~1.3A

Putting batteries in series adds the cell voltages while keeping current the same for the pack.

e.g. if you have two Lithium Polymer cells in series, the nominal voltage of the pack would be 7.6V and at 5W the current would be ~0.66A. Notice compared to the parallel example: voltage is doubled, current is halved.

What you need to compare here is the so-called "C-rating" (capacity rating) of the cell. All the C-rating means is that if you provide this much current, the cell is full in an hour. A C.10 rating would be the current over 10 hours. C.20 current over 20 hours, etc.

e.g. you have a 3.8V LiPo cell that's 2000mAh. A C-rating of 1 means you charge or discharge at 2A and it's full/dead in an hour.

So, putting batteries in parallel keeps their voltage the same but "doubles" the current.

However this entire time we've been talking about current, when we should be talking about power.

If your 2000mAh battery has a C-rating of 1, meaning you can charge it at maximum at the capacity of the battery, then in parallel you have 4000mAh (2x2000mAh) at 3.8V, meaning you can safely charge it at 4000mAx3.8V = ~15W

Put the same two cells in series. Now you have 2000mAh at 7.6V. How fast can you charge it? 2000mAx7.6V = ~15W.

Consumer electronics don't need a high C-rating because the cells don't need to charge/discharge at extremely high rates (as they would in a quadcopter, for example).

The main benefit of putting cells in series is that you raise the voltage, which lowers the current and thus means you can use smaller (and cheaper) wires to carry the same power.

Putting the cells in parallel would increase the current for the same power, but it does not change the C-rating and thus the amount of power you can safely put into the cells.

Because with your method you are doubling the power.

Imagine you have one charger with a fixed output of 10W (2A @ 5V). Now you have a USB splitter cable which allows you to plug two devices into the same charger.

What happens? Most likely both devices will charge at 1A (5W) instead of a single device charging at 2A (10W). The power output of the charger doesn't magically increase to 4A (20W) because you plugged in a USB splitter cable.

Increasing the number of battery cells is equivalent to adding more splitter cables to the charger. The input power doesn't change, but the power to each cell (device) is decreased proportionally to the number of cells (devices) drawing power.

- Cheaper to produce / higher yields than an L shape?

- LiIon batteries have a power density <-> energy density tradeoff. Perhaps the batteries have different chemistries so as to get the best of both worlds.