There are people that have spent $1000 or more for LiPO4 batteries and management systems. The guys that are spending the big bucks are usually in it for the thrills... it's harder to make the economic argument for ebikes over other options. (cheaper than a car however)
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Actually the Lifepo4s have come done a bit in price now Safe, for around 700 bucks you can get 48v 20ah Headways including charger. You will get in excess of 50 kilometers with these (my good mate has a set thats what he gets and no pedalling as he has now working pedals)
If you can get your hands on Li-ion cells they are also very good. Makita and Bosh battery packs use these, they require now BMS A123 cells as per used in Dewalt 36v packs are also very popular amongst the ES crew they do require a BMS. Some people though will by say 4 Dewalt packs and leave them in tact buy a dewalt charger and charge them as if they were to be used in a power tool. If you have a service center for any of the mentioned manufactures hit them up for any faulty packs they have, you will get good cells from them one or two maybe iffy turf them keep the rest ...Just a thought.
As safe suggests NiCads are an option but like the Li-Ions your need a **** load of them for any sort of range and they are heavy, not as heavy as Lead Acid but still heavier than the other alternatives.
There are guys that have spent way less than a grand and get big kilometers, if you are strapped for cash though SLAs are the option to get you started, i have them on my first ride and they are going great, my absolute max commute though is under 10 kilometers, if i wanted range SLAs wouldn't be the best option. And yes they are hella heavy.
Lipos are the other alternative but this tech is best left to the more experienced individuals as they can be very volitle if treated badly ...They are the most energy dense battery available though 4 22v 5000mah packs weigh the same as one 12v 18ah SLA..Surprised you dont use them safe road racing is all about HP to weight ratio, lipos would drastically decrease the weight of your bike.
Best of luck...
KiM