Here is a picture of the void where the 2nd battery can go.
Could any of these batteries be worth?
I would probably go with using the Super Soco OEM batteries. Just to be on the safe side.
Maleta 60 voltios Ebike batería de litio 60 V 30AH Li-Ion batería con 50Amp BMS para 2000 W 3000 W scooter Motor 5A cargador
http://s.aliexpress.com/zaeiaE3E?fromSns=Copy to Clipboard
Would like to buy the original, but the price is significantly higher .. Have bought several lithium batteries on Aliexpress, including for my Oset electrical trial, it works well without notice after two years. If you look closely at Ali sellers, you can get Japanese cells, (Samsung, Panasonic, LG) instead of Chinese for just a little extra. All for the nearly half of the price.
All batteries came with built-in BMS (battery Monitoring System) for balancing across the cells packs.
What stops me is that I rather wanted to hook up a battery at a time, would like to see how the two batteries connected in parallel in practice.
Would like to find a battery that can operate independently of the original.
I would like to make one myself and connect it parallel to the stock one. I got a quote for 160cells 18650 Samsung 30B 3ah and 4.35V fully charged (like the ones that SOCO has). With a suitable BMS and plenty of work I can give it another 1.8Kwh of energy. Price for the cells is less than 550USD with transport and customs included.
But I won't stop there.
I have bought a DC to DC converter to make use of a 29Volt pack that I had made from previous projects. This is a 16ah pack which will play the role of a Reserve Tank in case I run out of battery. The Converter is capable of giving any DC volt I like and power can be up to 1300W. I tested it with my 60volt e-bike and with power set at 100W I managed to charge the battery up 0.5volts in 10mins. So I reckon in an emergency I can adjust it at 300w or more and charge fast to get some kms and return home. So the battery pack and the converter in a back-pack can save me in a long ride.
