Too Busy For Words - the PaulWay Blog

Sat 13th Nov, 2010

If only I'd set this up earlier

One of the problems with building an electric vehicle is making the battery pack. There are a number of interdependent variables - weight, amp-hour capacity, volume, maximum voltage, and cost - and many trade-offs to be made. Even if you limit yourself to a particular voltage (because you have a particular motor) and total pack weight (because you have a maximum 'kerb weight' for your vehicle) there's still a lot of options. And you want to know a fair bit about each choice - cost, number of cells (which determines the number of joints you'll have to make), maximum amperage deliverable, and so on.

At about the time that I heard about 'pouch' cells - lithium cells that have a tough but flexible plastic wrapper instead of a hard outer casing, often used in mobile phones and portable electronic devices - I started making a spreadsheet of these options. I threw together a few formulas to determine some of the information about my specific pack but then I shared the (Google) spreadsheet with a friend and he put in his own calculations. Then I realised some of the cells I'd been looking at were Lithium Polymer (normal operation voltage 3.7V) rather than Lithium Iron Phosphate (3.2V) and so some of my packs could have fewer cells. And of course, it hit me - I needed a database.

So I hacked one together in Django, and transferred all my information into the database (by hand). Then I wrote the code to put together a pack of batteries based on your requirements. I released it to the denizens of the Canberra EV group with little fanfare. I added a few extra bits and pieces, like simple displays of the cells by manufacturer and online store. And now I present to you this finely crafted link to release it, still on my testing server at home, to the rest of the world.

Feedback, as always, is welcomed.

As an aside, it is a difficult thing to get some of this information. Many manufacturers are Chinese, with badly-translated, hastily thrown together and hard-to-use websites. Many I'm not sure there's any point in listing since they seem only to cater for manufacturers buying 1000 or more cells or wanting custom engineering. Some, like Saft, seem to have very little information and what little is there looks awesome, but they sell to the defence industries and I've never got a reply from my email to them. Some, like one Australian manufacturer I won't name here, are very hard to deal with by email and seem to think that websites are brochures, not sales desks. Many of the Chinese manufacturers, especially of cylindrical cells, work through Alibaba and various other direct-ship quote-based pseudo-services. A lot of the sellers of cylindrical cells seem to be selling the same thing ('Headway' cells), which triggers my scam detectors. For me, it's just not worth trying to trawl through all those sites trying to find useful information.

I am willing to hear from anyone that can supply more information for me, especially online stores, but I prefer to get full information (especially for cells) with verifiable information. There's a lot of "we're totally coming out with a radical new cell that is ultra-cool and is far more powerful than anyone else's, but it's all hush-hush because we don't want our competitors to steal our technology" claims out there, and I don't want to list speculative numbers. Give me a URL to a full data sheet.

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