Apologies for the multiple rough posts, I will try to organise better as the project goes on. This helps me maintain a record of the processes used on specific areas of the project. 

Ok, so the E590 donor laptop uses a 3S LiPo battery, and the X60 uses a 4S LiPo battery, so this give a fews options to enable the swappable battery functionality. 

1. Remove a cell from the X60 pack to make it 3S and rewire it to include the BMS board from the E590 instead of the original. (performed on either the 8 cell or 4 cell battery)

Pros

      Minimal custom plastics fabrication

Cons

      Wasted space

      Reduced maximum capacity (effects can be reduced by using 3500mah 18650 cells)

2. Modify housing of external battery to somehow house the E590 battery

Pros

       Battery should just work

Cons

       Incompatible physical dimensions, and space use inefficiency

       Lots of custom chassis work.

3. Find another compatible battery from a sister system which would better adapt into the X60 housing. Either internally or with external battery. Could alow for a power bank style external battery instead of interfacing directly (nb wasted power in voltage conversion). Could implement a more open version of the dual battery system.

Questions / Thoughts:

What is the pin out of the new E590 battery? 1 less pin than the internal battery?

Will the BMS not turn on again after disconnecting existing cells? (concerns for research into batteyr packs). Will the "jump start" technique work on modern battery boards?

How will the BMS handle the change in cell capacity? Or will it just be fine?

If the E590 battery cannot have it's board removed is the battery lock limited to model destined only for the E590 or is it compatible with other lenovo batteries form different systems with same connectors?

If lenovo boards just don't work is there a way to get around the battery manufacturer lock on the modern E590, for example the X230 allowed the use of non brand batteries by flashing the embedded controller (no idea how to write software for this).?

In the E470 and similar these laptops have both an internal battery and external pack which use both the newer internal battery connector and old external battery simultneously. There is mention online  of using both batteries simulataneously. This is done as far as I'm aware by dual diodes of each battery to balance voltage enabling dual or single batteries. in combination with mosfets to turn off the primary battery creating a battery use hierachy; using the external battery first then the internal. Dependent on space left in the chassis this could be an interested topic to explore.

Currently, hoping that the extra pin is the one that when bridged simply activates the battery.

Update 30/1/19

Opened up both the E590 and X390 to see if a battery swap is possible. Discovered that although they use the same connector the pinout seems to be different colourwise and the voltage slightly different on the X390 (but likely within the range of the dc dc converters anyway). I am unsure if this is a difference between the X and E series or is specific to the model, year or something else,

The benefit of the dissassembly was that the structure of the batteries are very similar with either 3 (E590) or 6 smaller (X390) cells to acheive a "3S" voltage. Next to the neatly stacked or lined up cells, is the bms board which is "indicated by a difference in thickness along a single edge of the cuboid battery shapes. 

Potentially strange:

E590 - Red - Red - White - Yellow - Greenish - Blueish - Black - Black

X390 - Black - Black - Yellow - White - Greenish - Blueish - Red - Red

This could also be potentially solved by swapping wires. (I would like to avoid this incase there is a good reason for the slightly different operating voltages (if not to just reduce battery wear) etc)