I built an experimental battery pack using two 19V 4900mAh LiPo packs in series with a simple balancer board, all wrapped up in a shrink-fit sleeve. The weight is 1,22 kg and gives about half the range compared to my 10Ah 36V GM pack that weights around 5,2 kg. The LiPo (lithium-polymer) pack fits in a slightly expanded bottle holder but it is not a safe battery pack, missing temperature sensors, waterproofing and mechanical protection. All this is required to have a safe pack.
Many houses and garages have, and will, catch fire due to ebike battery packs not being built safe enough. Check out
http://www.extraenergy.org where some examples can be found. Also call your local fire department and ask if they know about any fires started by lithium batteries...