Hi eso6686,
Welcome to AC and the RWA forum!
Any chance you can share the minimum spec of the battery pack requirements so I can see if I can get something or get someone to built something similar?
You could go with external battery packs - but you need to be careful. You need TWO 4-cell packs ... one LiFePO4 pack at 12.8V, and one pack at -12.8V. I would go with at least 1,000mAh capacity. You also want to make sure you have circuitry to balance each of the 4 cells in each battery pack, along with the other very important stuff like over-voltage protection, under-voltage protection, over-current protection, etc.
The charging voltages (from the modded Z1ES) for the packs are +15V, and -15V. Linear regulated with resistor current limiting. All built in.
I actually have made an external battery supply for a customer before (2 of the Red Wine Audio 10,000mAh packs, the same we used in the Black Lightning supply, so the play time on battery was well over 24 hours!). A 3-pin jack on the rear panel of the Sony connected the Sony to the external battery supply via a small cable. No need to switch the battery pack ON or OFF, or to connect an external charger. The modded Sony has that all built in and it was all automatic.
Feel free to email me if you wanted a Level 3 mod with external battery power solution. We can work it out, but please note that I wouldn't be able to get to doing this for at least a month. But I am very confident that you will find the results well worth the wait.

Vinnie