Lightweight lithium ion battery

I use lipo batteries on my bicycle, I have 4 packs of 6s 5a hooked up 2 in paralell the, the 2 packs hooked in series, giving me approx 49v with 10amps, to get my bicycle to go almost 30mpg and a little over 10 miles in distance.
the thing with lipo is, you don't want to run down to 0 volts, since each of these 6s batteries are 6 cells, you charge maximun of about 4.1v per cell and discharge to no less than 3.6v per cell. If you run a lipo battery down below 3.0v per cell, you risk a fire that you can't put out. Also if you want longevity in you battery pack keep it between 3.6v and 4.1v, and you can charge them around 1000 times.
I have given you experience in the bicycle world, in an auto, four 6s batteries can give you about 24v with 20amps, if you hook all 4 batteries in paralell, that might be too much for your needs, as I buy these 6s cells from a hobby shop at about $45 per single 6s battery. If you want around 14v, just get 3s batteries, should be around $20 each.