Too Busy For Words - the PaulWay Blog

Sun 3rd Apr, 2011

Great minds think alike

The next major task for the electric motorbike is to build a frame that can store the battery. They need to be held in place with a lot of strength - the battery frame and its contents need to be able to withstand a 20G impact from the front, for example, and it needs to be able to fit the 38 cells I have as low as possible to make the bike handle well. I'd made up some cardboard mock-up cells and had worked out a way to get them into the frame - just.

Then I'd realised that I'd forgotten the space on top of the cells - there needs to be about a three centimetre gap above the top of the cell for the interconnects and cell management module. And then I needed to include the thickness of the actual frame. And I realised that one of the frames would be sticking right where my toes were supposed to be. This needed a rethink.

I've found that I get a lot better results and ideas when I brainstorm with friends, so I invited my two brothers-in-law to come over and play with some cardboard boxes. I showed them what we had to work with and, between drinks, we started rearranging things. We also talked about the actual requirements, and we started discarding the ideas which had been limiting me - like whether we used the side fairings I'd bought for it.

It's often said that progress doesn't happen with the word 'Eureka' as often as it does with the phrase 'that's interesting...'. And sure enough, Trev came up with the break-through idea by just rearranging things and observing, testing and improving. Then I made a few suggestions, then Rob made a few more, then we talked about how to build it, and before an hour was up we had a pretty much complete idea of how to construct the whole thing. And it all fits lower than the original fuel tank!

The key insight, as it later appeared, was that making room for the cell interconnections inside the frame chews up lots of space. Turning the cells on their side used horizontal space, which is less constrained. By putting the interconnects on the outside, I can see every cell's monitoring state and check the connections easily. The only variation is the bottom row, which faces forward because that way I can fit six batteries across the bottom row and still have room for my toes.

This photo illustrates the whole thing. The bottom layer is six cells across (facing forwards), and each other layer is made of eight batteries facing side-out. (The smaller squares are one cell, then I made three- and four-wide groups for ease of construction and reduction in cardboard). You can see on the right the rear upper engine mount which sticks into the second layer of cells - when this is removed that row can move an inch back; this in turn allows the front group to move down, which then in turn allows the top group to move forward. There is plenty of room there even with plates between each row.

Then the whole outside is covered in a solid plate of something - we're currently debating whether steel, aluminium or acrylic is better - that provides water and abrasion resistance. Steel plate with laser cut holes with an acrylic plate behind it is my current favourite - the holes provide viewports through to see the battery management system, and also mean that the bike has pinpoints of green light glowing from it at night. Inside the plate there are rubber offsets which both keep the plate away from the interconnects and press the cells into their niches. The outside plates attach at hinge points at the bottom and bolts go through from the top to a central plate which holds the whole thing together.

If the outer plate isn't seen to be enough to hold the cells in place, I have idea to fix that as well: put a steel strap around each group of cells and, on the ends, attach a flat plate to which a bolt has been attached. This then bolts down to a rod attached to the plate in the middle. The steel strap will sit inside the channel on the side of the batteries, holding them just as well as the frame I made up before.

The method of construction is still a little up in the air. Rob likes the idea of just bending acrylic into place around wooden moulds. Trev likes aluminium for lightness and for the horizontal plates to support the cells I think that's good. The central vertical plate I think I would make from steel and I would definitely prefer the outer plate to be steel for abrasion and impact resistance. All these can be cut from plate using water jet cutting - in fact, this design lends itself much more to water jet cutting than other designs I've considered. That makes it cheaper, as well as much more awesome.

Hopefully I'll have some plans together this week and I can start taking them around to fabrication places to see what they'd charge to make. The other good thing about this plan is that it's probably even possible for me to make it so that I can simply bolt the bits together myself, rather than having to pay someone to put it together. It feels like I've got the whole project back on track!

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