A typical replacement battery for my beloved Tidalforce bikes is a 36V 10Ah battery of some sort. I've tried sealed lead acid (SLA) batteries,power tool batteries (Bosch FatPacks), LiFEPO4 packs, and Ping packs (http://www.pingbattery.com/). All of which have their pluses and minuses.
Of course, my favorite as of right now are LiPO packs made from Turnigy Multistar cells. They are extremely compact, relatively light weight, and fairly inexpensive. They come in easy to configure package configurations and put out more than enough current to drive my favorite electric bike.
This configuration is a 12S (45.6V) 10Ah pack that is quite slim. The voltage is right about the maximum that the Tidalforce system will tolerate. Anything higher and the system will go into overvoltage protection (OVP) and shut down until the voltage drops below OVP. One solution is to not chage it up to its maximum voltage, which has the added bonus of extending battery life.
Anyway, here are some photos. I plan on mounting it to my bike's water bottle mounts this weekend and test ride it to see how it does.
I had some coroplast around so thought I'd make a sort of protective padding for the battery to see how well it works/lasts.