You can get custom batteries but there are some issues:
1) do they have the proper voltage and amperage specs?
2) do they have the correct plug? Some machines have standard plugs, some, like Sager, have oddities that are difficult to find.
3) will they fit in the space provided for the batteries?
4) what savings, if any, will there be? Generally, buying batteries in bulk results in substantial discounts and since batteries do have a shelf life, they aren't something that can be stockpiled. So your notebook manufacturer buys thousands if not hundreds of thousands which gets them a really great price vs your buying one or two, which will get you a really lousy price.
5) most batteries are made up of cells, the easiest way to get to the voltage you want is to wire the cells up in either series (to boost voltage) or parallel (to boost capacity). Typically LIon cells run 3.7 volts (+/-). To get to your input voltage (generally the adaptor plug unless you make up a plug for the battery port) you would wire up a number of cells (ie - for the Sager, its 20 volts DC so you'd have 6 cells in series to get 22.8 volts - better a bit more than too little). However, you need to find out what range of voltages your computer can handle, this isn't commonly published information and the only effective way to find out is to try successfully higher voltages until either you reach the voltage you have or you burn up your power control circuits. Another option is to introduce a DC/DC converter to regulate the voltage and perhaps the amperage as well. Remember your battery voltage will vary with use. However, DC/DC converters are hard to find, very hard to find. Almost easier to invert to AC, use a transformer and then re-invert to DC at the voltage you want. Unfortunately, that's usually not very efficient.
This all goes back to why there aren't many people building notebooks at home. There's really no standard parts available and making them yourself is often very expensive.
Which is why I am migrating to SFF computers. Still portable, though not as much on battery (but my use of battery power is less than 5% and usually just to get from room to room, plus I am set up with portable power either from my cars or with a charging station on wheels). Almost all the parts are standard off the shelf items. I have a huge choice in CPUs, GPUs, sound cards, HDDs, media drives, whatever. And its substantially less cost for the same performance.
