Compromises often have to be made. So here is compromise number 1 of the project – the batteries. I had three choices for batteries:
- Lithium polymer cells. Big ones, and a bunch of them. This turned out to be way too expensive for now, and is a future upgrade path for this thing if it works and my finances dictate it.
- A series of 4/3AF cells. They could fill the space between the wheels most efficiently, had the capacity, but were nameless and unproven. It would have provided 36 volts, 4.2AH, and upwards of 50 amps.
- A bunch of Sanyo N3000CR full-C NiCads, old school style. I have about 40 of these cells, which are proven in the world of big combat robots. They are potentially good to 100+ amps burst discharge, but are only 3 amp hours in capacity.
Due to time and wiser economics, I decided to forego the nameless cells for now and use the N3000CRs. I won’t have much run time with this, especially at full power, but the amp rating means I’ll run less of a risk of exploding batteries. Life would be perfect if they had made the ElectricX2 one inch longer, but in the end, I can only fit 28 cells in the middle channel between the wheels. Maybe I can hide another two somewhere! 33.6 volts is reasonable enough for now.
Batteries are often the limiting performance factor in any electric vehicle. I didn’t feel like lugging giant 12 volt lead-acid batteries around, even though they would have been the simplest solution. The batteries as they stand are completely hidden under the frame, which makes for a very clean outside appearance.
Here’s some pics of the battery pack work. It’s not complete yet, as I haven’t determined the location of the rest of the electronics. The project page has been updated to reflect this change.
Laying out the cells in the middle channel for a quick test fit. The full-C cells span almost the entire width of the channel, leaving just enough space for wire and cable. The 28th cell sticks out in front a bit, and will be armored to prevent malicious foreign objects from damaging the cell.
After gluing the cells together, I soldered them with 1/16″ x 1/4″ interconnects, roughly the equivalent of a 14 gauge wire between each cell. Because of the odd 28th cell, the pack had to be bridged with a wire in the middle. I used 12 gauge for this. End leads weren’t attached at this point since I have yet to lay out the rest of the electrical system.
Fitting the pack, now about an eighth inch wider, back into the middle channel. I made absolutely sure to insulate the metal terminals from the metal chassis lest I short out all 28 cells simultaneously. That would be an amazing fire. To make it more interesting, this was the 666th picture taken by my camera. Battery leads will exit out the back. A skin of aluminum or UHMW plastic will cover the bottom – I am leaning towards UHMW since it’s slippery and will make curbhopping easier. As if I’m going to curbhop and all.
Each extra run of wire adds more system resistance, so I might skip mounting the Whattmeter up on the handle bars – that’s nearly 5 feet of wire each way. Instead, it might just be strapped to the rails and be read intermittently.
After the 100A motor controller and battery charger arrives, this thing is pretty much done. Anything after that is vanity and luxury… but of course as long as I have time left, there will be vanity and luxury.