Update 1 - batteries

I've now got a few ideas of where I want to head and thought I'd write them up.

My basic requirement is to be able to do 100KM/hr so I can drive anywhere in Canberra without being a hazard to traffic, and to be able to drive for at least an hour at 80KM/hr.

For a motor I'm looking at an EnerTrac MHM-602 Hub Motor for the motorbike.  This saves having a gearbox, chain, clutch and a lot of other paraphernalia, which cuts down on weight, number of things to go wrong, and gives me more space in the engine bay.  This motor will do 100KM/hr at 96V and draw 50A while doing so with a normal bike and rider.

The motor controller I'm thinking of is a Kelly KBL12401I.  It will go up to 120V, can do regenerative braking in any one of three modes, can cope with up to 400A over one minute and 200A over five, and has an RS-232 connection to allow it to be programmed and get statistics.

Now the big question is what size of cells and how many do I get to make up the battery.  There are several factors to consider here:

• The larger the amp-hour size of the cells, the longer they last but the larger and heavier they are and the more they cost.
• Each cell goes from 3.2 to 3.5 volts in regular operation and can go as high as 4.8V and as low as 2.5.  Beyond that you have cooked your cell.
• The larger the cumulative battery voltage, the less current it needs to supply to achieve a particular amount of power.  Since resistance is proportional to the square of the current, at higher voltage smaller cables can supply a larger amount of power without heating up too much.
• The batteries have to fit in the space available in the engine bay and be carried by the bike frame and motor.  The fairings may provide more or less space in some areas.  The more weight higher up on the frame, the more likely the bike is to overbalance.  The batteries should also be mounted more or less upright.
• Cells are also rated in 'C' - which is related to the amount of amps they can instantaneously deliver as a fraction of the cell's capacity.  A 100Ah cell that's rated at 1C is capable of delivering 100A - a 50Ah cell rated at 2C can deliver the same current but for half as long.
• A cell might be rated at 100Ah but that doesn't mean that you can draw 100 amps from it for an hour.  If you do that you will probably kill it.  Planning the battery capacity therefore needs to be conservative.

That's a lot of variables to juggle.
To supply 96V from cells at 3.2V each I need 30 cells.  For 100Ah Thunder Sky cells weighing 3.3Kg each this works out at just under 100Kg.  This is probably quite a bit more than the weight of the engine, fuel tank, exhaust, carburettor and so forth.  So finding a bike frame that will take the weight, can take it low down, and will still be comfortable and fun to ride is going to be the next challenge.