Step 11: A Sneaky Premade A123 Pack option
That's the high-end cordless power tools market. Specifically, the DC9360 36 volt lithium ion battery for DeWalt's 36 volt power tools line has been the subject of great praise by the crazy electric bike guy community. A DC9360 battery and charger (Dewalt part number DC9000) set can usually be obtained on Ebay or Amazon for $200 or so, and the battery itself around $120. For 10 A123 cells with an integrated battery manager, that's quite a steal at market prices. It is much more expensive than the Hobbyking option, but A123 cells are serious quality.
The DC9360 does need a bit of hackery to turn it into a full fledged EV battery, however.
The battery has three terminals:
1. Positive, which is connected directly to the cells inside.
2. Negative terminal connected through a hard 15 amp fuse inside the BMS module , distinct from the fuse described below. Pulling more than 15 amps from this would just blow the fuse.
3. Another negative terminal connected through an internal power MOSFET.
Several hacks have been documented on the BMS module's internal workings which indicate this FET is used as a speed controller for the tool motor. A document which details how to connect to the BMS such that the FET is turned on (allowing the higher current output to be used) is here . It also describes a multi-battery "ORing" system which any number of batteries can be paralleled and swapped in and out. I can't vouch for that circuit, however, since I've never built one.
Another option which has been validated is to simple solder wires directly from the positive terminal and negative terminal of the cells, completely bypassing the BMS for discharge purposes, but continuing to use it for charging. This operation usually leaves a discharge cable exiting the rear of the pack and leaves the tabs and connections on top unmodified.
Once cell interconnect inside the pack is deliberately made narrow so it acts as a fuse if the current draw becomes too high or the battery is shorted past the BMS. The location of the battery-level fuse is detailed in this RCGroups post (the post itself describes the complete removal of the BMS module, which is great if you want to make your own packs, but otherwise not helpful). This narrow area can be bridged using copper desoldering braid, grounding braid, or just some pieces of wire. A large soldering iron (60 to 80 watts) with a high thermal mass tip should be used to solder the reinforcement without heating up the cell too much.
Many resources on the Intertubes have been compiled by Crazy Electric Bike Guys about chopping and screwing these batteries - here's another one.