Electric Bikes - Battery Build My Own?

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Is it hard to solder? Is it hard or bad to build my own battery?
I found these:
http://batteryjunction.com/1pc12v50mahn.html
Wasn't sure if it would be worth it to build these batteries into a 36V 5Ah pack. What do you think?
It would be a hell of a lot cheaper than buying a premade one.
You can make any voltage pack out of them that you want, I have no idea why they put the 24 volts there and I bet they don't either.
Soldering is pretty easy but doing a nimh battery pack as a first effort is a big ask due to the fact you want good current flow so good joints and if you over heat the terminals you will damage the battery.
Also they don't state max. discharge and max. continuous discharge rates, so depending upon the motor they may not cope.