The discharge rate of the cells should play into the calculations as well. If the cells have a 3C max rate, 45 90 ah cells could produce about 52 HP (270 amps max@144v), resulting in a somewhat underpowered vehicle. How much does exceeding the discharge rate reduce the life of the pack? Is there a discharge rate that, once passed, makes the pack subject to random cell failures (i.e. more than 10C will damage the internal structure of a certain brand cell )?
Tom Keenan On Jul 21, 2014, at 11:21 PM, EVDL Administrator via EV <ev@lists.evdl.org> wrote: > 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 _______________________________________________ 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)
