Tuesday, June 8, 2010

New Battery Racks

Loyal readers will remember that I've decided to take this opportunity to redo the battery trays in the front of the car. Since I had to disassemble the entire front of the car anyway, I figured now was the time to do it. Of course I should mention why. Two reasons really.

First, I've come to find out that these batteries really are supposed to be mounted vertically. Originally, I'd read it didn't matter, and you could install them in any orientation you wanted except upside down. But since then, I've seen in multiple places that if you don't mount them vertically, the electrolyte will pool to one side of the battery, shortening the batteries life. Well, I thought the electrolyte was a paste, but I turned one on it's side the other day and could hear a faint but distinct gurgling coming from the battery.

Second, when going through the pack with a wrench trying to tighten the lugs on the batteries, I found it extremely difficult to get a wrench on some of them, and in fact, I couldn't reach 7 or 8 of them at all! That meant over time the constant heating and cooling of the terminal would cause the lug to work it's way loose (I already saw this on the lugs I could reach) eventually creating enough of a gap that there would be arcing and eventually fire. So, vertical it is!

Here is a shot of the work I did today. there will be more horizontal pieces running between the sides. Three to be exact. They will support two rows of 9 batteries just over the motor. That means I need to find room for 12 more. As it happens, the space just behind the radiator, and in front of motor is wide enough (by 4 mm) for 12 batteries. So they should all fit pretty well, plus I got rid of some steel that weighed quite a bit more than the aluminum.

I'm also taking this opportunity to replace the vacuum pump that I've come to despise so much. It is by far the loudest, most annoying thing on the car. I'd bought the pump off of eBay about a year ago for the bargain price of $110. It was one of the few parts I thought I could save money on because a vacuum pump is a vacuum pump. Right? Well it turns out there are pumps that are a lot quieter.

Here you see the original one on the left. It measured in at a wopping 74 dBA. The new pump on the right puts out a relatively quiet 58 dBA. The dB scale is logarithmic, and every 10 dB increase means that the sound you perceive is twice as loud. So this new pump will be roughly 1.6 times quieter than the old one. Was that worth the $300 I laid out for it? Well, let me throw this little bone into the mix. The old pump drew 8 amps, the new one, 2.5 amps. Now are you impressed? I thought so.

The old one still works great, it's just loud. I'll probably put it back on eBay.


Zameer Anwar said...

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Tim Catellier said...

Thanks for the information Zameer. I think the pump I got is actually better suited for the job because it was designed specifically for EV conversions. It's optimized for that environment. The older pump is more of a generic design. Not bad, just not designed for EVs.

Gam0ra said...

Man you are WAY too worried about fire. Flaming cars are COOL.

Tim Catellier said...

Heh, I'm crazy that way ;-)

John said...

Hi Tim, I have just discovered your EV blog a few days ago. I think you are doing a fine conversion job on that BMW Z3! But I did notice that you are not strapping the prismatic cells together as the manufacturer recommends. It turns out that the individual cells can swell rather dramatically into a foot ball like shape if they are not constrained with some kind of strapping system that holds them tightly together. I understand this only happens when the cells are being charged up. I noted in your blog that you intentionally put gaps in between your cells to help with cooling, you might want to revisit this.

Tim Catellier said...

Right you are John! I didn't know much about this at the time. The only cells that are mounted in such a way that they could swell are some of the cells in the very back of the trunk battery box. That's a total of 11 cells. The rest are squeezed together tight (that's what I plan for the cells in front as well, as I redo the mounts). My understanding is that the problem is exacerbated by over charging. Well I won't be doing that, but I do think I'm going to have to keep an eye on them. Thanks for the comment John!