how much economy of scale is there in one big LI battery vs its equivalent 2 or more small ones?

Factual Questions
1

in other words, suppose we want to make a battery whose weight (volume too, ideally) can be adjusted downward if needed at the expense of battery life. Let’s say if I only want an hour of charge, why lug 4 hours worth of battery.

How expensive would it be to make a battery that is easy to disassemble into several components? If some components are essential for battery operation (maybe a DC-AC converter or something of that nature) they can stay, but I want to understand how much of the gadget could hypothetically be made optional/pluggable. And how much of a cost penalty would it be to do something like that?

2

These already exist - they are called “cells” and you can buy them at any battery store. For example - CR123A

3

You can achieve the effect you are describing by connecting individual batteries in parallel. You’ll get the same voltage as from any individual battery, but together they will be able to deliver the same level of power for a longer period of time. This is actually what the term “battery” (in the electrical sense) originally referred to.

From wikipedia: “Battery, 18th and 19th century term for a number of capacitors or Leyden jars connected in parallel, to increase the charge storage.”

I’m no electrical engineer, but I believe the scale is purely linear. Barring any issues arising from heat, increased resistance in the cabling, etc., two batteries in parallel will last twice as long as a single battery, three batteries will last three times as long, etc.

4

One big factor is scale of manufacture. The one size fits all leads to economies of scale.

I have seen 9 volts batteries built as 9 volt batteries. I think it is more common now to have 6 small cells in them.

5

I suspect that any economy of scale in this would be overwhelmed by the cost of needing extra connectors for the multiple small batteries.

To say nothing of the additional repair work involved over the life of the computer. These additional parts are added points of failure. And likely ones, since the OP seems to contemplate disconnecting and reconnecting batteries frequently, and most laptop battery connectors are not built to withstand this. Normally the battery connector is used only a few times over the lifetime of the laptop.

6

Due to the chemistry involved, LI cells have a nominal voltage of 3.7VDC. Therefore, your laptop has to have several of these cells to make up the needed voltage. All the cells could certainly have less capacity than needed for four hours, but you would save little in weight or expense.