Too Busy For Words - The PaulWay Blog

14 10 2010

Thu, 14 Oct 2010

Putting coulombs on the charge card

(I have to nod my head to my good friend Dunx for the title, though an explanation would be otiose.)

Finally I've put the order in for the batteries for the electric motorbike.

This is not a small decision when the cost of the batteries is over $3500. It's more so when there's that feeling that you're going to regret buying something that you're still not quite convinced will fit in the space available. Add to this the feeling that you wanted something larger and more capable but that would be simply impossible to fit, or almost an order of magnitude more expensive. And there's the looming question of how, exactly, is the whole thing going to perform. That's a lot of buyer choice anxiety right there.

We can reduce the choices down somewhat by starting from the fixed things. The engine, muffler, exhaust, air intakes and various peripherals that I removed from the bike and am not going to put back weigh around 84 kilos; that's how much weight, roughly, I can put back in the bike in battery pack. I'm adding the extra weight of the motor over the rear wheel (16kg), the motor controller, DC-DC converter and some other stuff, so it's probably going to be slightly heavier at 'dry weight'. This is something that the engineer can sign off on; we haven't made the whole thing unregisterable yet.

Likewise, to run the motor at 100km/hr requires 96 volts; at 3.2 volts per cell that's 30 cells, but I want to be able to comfortably exceed that for emergencies. A rather unfriendly email from a battery supply company taught me that the next size up of charger is 38 cells - and the cells really don't like being charged much more than their peak voltage. So 38 cells it is then. (If it turns out that I can't fit 38 in the bike, either due to space or weight, I can more definitely fit 30 anyway, and I'll just have to accept that its top speed isn't quite what I'd hoped.)

38 cells fitting into 84 kg makes for a weight per cell of around 2.21kg. By a happy chance, that is almost exactly the weight of the ThunderSky 60Ah cells. Unhappily for me I was hoping to use 90Ah cells as they have a better energy density. It was, I think, this unhappiness that then caused some of the delays.

It started when showing off my bike at the Canberra Electric Vehicle Festival 2010. A friend and I were talking about batteries and he mentioned 'pouch' cells as being his preferred battery component. These are like flattish bags with two metal tabs sticking out of them; Their chief advantage is much better amp-hour per kilogram and amp-hour per cubic centimeter. You can't squash them and you have to build a rigid pack around them, but that's OK since you have to do that for the vehicle anyway.

And some of them can deliver phenomenal quantities of current - the 70Ah cells I started looking at could deliver 350A continuous and 560A peak - enough to give 50mm² cabling a serious headache. Serious electric sports cars use these packs for their mind-cudgelling power delivery. Then we got a quote for them and stopped salivating: at over $13,000 for an equivalent pack to the ThunderSkys mentioned above this was seriously out of my league - at least for this bike.

As is often the way with me I then spent a quantity of time researching different batteries, finding various options and constructing elaborate spreadsheets to compare them. It compares rectangular (including pouch type) and cylindrical, calculating $/Ah, g/Ah and cc/Ah for comparison. The ThunderSkys come in at a very competitive $1.6/Ah but a modest ~37g/Ah and ~23cc/Ah; the best cells get down to ~22g/Ah and 10cc/Ah but an excitingly expensive ~$4.6/Ah. Then again you have to wade through who sells which cells - and half of the manufacturers won't deal with you unless you're ordering at least 1000 cells in one go.

In the end I was back where I started - 60Ah ThunderSkys, which I ordered today from EVWorks. $90 odd for delivery is cheap compared to the cost of the things being delivered, and especially to the $1000 or so charged by a USAdian company to send the same cells here. And with EVWorks I could buy pretty much all the remaining components - contactor (to turn the thing on from the key switch), cables, battery management system, charger, cell interconnects, cable lugs (yes, I get to use lugnuts), and conduit. I'm sure there's something I've forgotten but I can't think of what.

Anyway, the main task now is to read up the National Code Of Practice NCOP14 Guidelines for Electric Drive - its basics are covered on EVWorks's website - and work out what I have to do to make sure it will pass inspection. And also not fry myself or the batteries.

Oh, and the fairings I ordered (from the UK) have arrived too - I may try and get it looking nicer and take some photos. I have to have something better looking than this...

posted at: 23:00 | path: /personal/ebike | permanent link to this entry


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