The order has been made! Four 5Ah, 14.8v batteries (which will be made up into a single 10Ah, 29.6v battery), four voltage alarms (one for each, so that it buzzes when the voltage gets down to a level that means "stop using the battery or it will fail permanently, 3.0v is recommended, but I think I'll set it at a more conservative 3.2v), an iMax 150 watt charger (and another, much cheaper, 80 watt charger which claims to be iMax but which I suspect is actually an iMax clone), a battery status displayer and balancer so I can measure and diagnose problems, a parallel charging board (which I suspect I won't be using), a 4s 6-way parallel balance lead (which I think I'll be using instead).
Also 20 connector plugs and sockets so I can wire up my charger and driver harnesses, 100 meters of 4mm cables, 100 meters of 2.5 mm cable (for the wiring harnesses; 100 meters is far far more than I need, but it comes in 100 m reels, and I'm sure to find uses for it in future), a temperature sensor that will connect to the iMax (and, I hope, tell it if the battery gets too hot so it can shut off charging), a temperature sensor for the bike, so that I'll know about any hot-battery problem while I'm riding, a car fuse (25 amps) for the charger and driver harnesses. Cable at a cross-section of 2.5 mm should be enough to carry 20 amps (the most I'm expecting), but I'm going to use 4 mm wherever possible
Lots of safety here. Because those 2 kg of batteries are storing a million joules of energy. In practical terms, that would heat a kilogram of water to 240 degrees C. So I'm doing everything I can think of to make sure that all that energy doesn't get out of control. I'm not worried about the volts - 33 volts isn;t enough to give you a shock. It's the energy that's the thing.
The only "consumable" here is the battery - they only last a couple of years. Everything else should last pretty much indefinitely. The batteries themselves cost £68, which I think is really cheap.
The weight on the bike will be about 2.1 kg, compared with the 3.12 kg of my existing batteries, but should have a little bit more energy (I'm thinking maybe 15% more). I could build bigger batteries, but in my experience, it's really difficult to know how much power is left in a battery, but you certainly know when it's empty. So I'd rather have three 10Ah batteries (which I can carry in one pannier) than one 30Ah biggie.
Now what I have to do, is wait for the deliveries!