Hi Sam.....I've also got an Arri 2c.....years ago I bought a CP xtal replacement motor for that draws only about 2 amps at 16 vdc.
in the past I've found some gell cells....normally used in emergency lighting battery packs.
Gell cells have no memory.
they come in 6 volt blocks....3 of them for 18 vdc....years ago they were about $20.00 each.
Building a box to carry them in was a make shift thing..
I used anywhere from 5 to 8 amp hour blocks...wired them together....and also made my own chargers for them.
I actually did 2 12vdc packs for my Arri 16BL.....and sent them out on a shoot.
They ran 16,000 ft of film...when I asked them how the batteries worked out....they told me that
they didn't change to the second battery or charge the first one that they were using.