On 21 Jul 2014 at 14:12, Ben Goren via EV wrote: > I'm really only looking for a battery pack big enough for local trips; > a couple dozen miles would be plenty. I suspect that the smallest pack > that puts out the necessary voltage is going to be just about the > right size ...
Hmm. Maybe. Let's do some math. A typical subcompact conversion uses around 250 Wh/mi. (Some have done as well as 150-160 Wh/mi, carefully driven.) You're not going to match that with a heavier, less aerodynamic car that wasn't designed with fuel efficiency in mind, however. What can you do? Well, in the EV Photo Album I found a '66 Mustang conversion that does 375 Wh/mi. http://www.evalbum.com/733 Here's a '65 ragtop that does 330 Wh/mi. That sounds a little better. http://www.evalbum.com/2056 One other similar Mustang entry estimated his energy use as "at most 500 Wh/mi," but we'll take that with a grain of salt that since he doesn't seem to have measured it accurately yet. Now, these are BEV conversions. You're leaving in all the ICE bits for a car that will probably weigh in the 2800-3000lb range before conversion, maybe 3500 or more after. You'll might also pass up the skinny LRR tires for sticky high performance ones. So let's be conservative (and assume you probably won't drive like I do ;-) and allow 400 Wh/mi. You want about 25 miles of range. So you need 10 kWh, but you don't want to size your battery to that. You probably want to not exceed 80% DOD for good life, but let's say you're OK with giving up a little battery life, and go to 90%. So you actually need a battery capacity of about 11.1kWh. Doing that with lead in the space you have is going to be a non-starter, so I guess we'll go right to lithium. A LiFePO4 cell has a nominal voltage of 3.2v. Let's say you're going to use a 144v drive system, so you'll need 45 cells. (In a more reasonable world, you'd determine how much energy and power you need, and size the battery from that. THEN you'd decide what motor and controlller would work with that battery. However, we'll assume for now that you've already chosen a motor and controller, because it makes the calculations easier.) We'll start with, as you suggest, "the smallest pack that puts out the necessary voltage." The laptop-size cells are obvously too small unless you go to massive paralleling a la Tesla, so let's jump to something like Headway 38140 cells. I'm no lithium expert, but I'd guess that these are about the smallest lithium cells practical for an EV (and a pretty light one at that). The 38140s are 12ah cells, 38mm in diameter and 152mm long. Because they're cylindrical, packaging isn't optimum, but (allowing a little wiggle room) a 5x9 matrix would be 152mm (6") high, 200mm (7.9") wide, and 360mm (14.2") long. Your battery box will need to be at least 25-50mm (1-2") larger in all dimensions than this, to allow for wiring, ventilation, and (in the winter) insulation. So you're looking at maybe 8"h x 10"w x 16"l. I don't know how that compares to a banker's box, not being familiar with such a critter. Each cell is good for about 38.4 Wh, so 45 of them would be ... 1.73kWh. Hmm. That would get you about 4.3 miles of range. We're going to need something larger. How about CALB? The smallest one EVSource sells (that's not an endorsement, it's just one place I found offering them online) is 40ah. Each cell is 181mm x 115mm x 46mm. Again allowing a couple mm per cell for breathing room between (the lithium experts here can give you more info on whether this is necessary), your battery is now 181mm (7.1") high x 585mm (23") long x 432mm (17") wide. Again add at least 25-50mm (1-2") to each dimension to allow for battery box sizing, to get to around 9"h x 25"l by 19"w, maybe even a little higher to allow for installing a BMS. Now with 45 cells we're talking 5.8kWh and a practical range of 13 miles with new cells, declining to perhaps 9.8 miles after a few years of use. Still only about half what you want. (But can you live with it?) Let's see what we can do to get closer to your target range. The Winston WB- LYP90AHA is a 90ah cell. This will give you nearly 13kWh for a practical range of 29 miles, perhaps 22 miles as they age. Now we're talking. Each cell is 218mm high x 143mm long x 61mm wide. Sticking with our 5 x 9 configuration, we now have 218mm (8.6") x 1100mm (43") x 567mm (22.3") for a battery box about 11"h x 45"w x 24"d. This is a significant distance from "the smallest pack that puts out the necessary voltage," unfortunately, but still smaller than a lead pack would be. The specs in Wikipedia say the Mustang Mk I is about 68" wide. Would there be enough room for this battery in the forward part of the trunk, between the wheel wells and above the rear axle? I'm thinking that otherwise you'd lose a fair bit of trunk space. Or maybe you could fit a half-size gas tank, and use some of the freed-up space for part of the battery. David Roden - Akron, Ohio, USA EVDL Administrator = = = = = = = = = = = = = = = = = = = = = = = = = = = = = EVDL Information: http://www.evdl.org/help/ = = = = = = = = = = = = = = = = = = = = = = = = = = = = = Note: mail sent to "evpost" and "etpost" addresses will not reach me. To send a private message, please obtain my email address from the webpage http://www.evdl.org/help/ . = = = = = = = = = = = = = = = = = = = = = = = = = = = = = _______________________________________________ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org For EV drag racing discussion, please use NEDRA (http://groups.yahoo.com/group/NEDRA)
