Adventure EV

Battery Boxes

Weight Gain…

by on Nov.17, 2009, under Batteries, Battery Boxes, Design, EV Land Rover

Well, you can’t get something for nothing.  For now, the Rover remains on a strict diet.  But the plan for the not-so-distant-future involves bulking up a bit.  Actually, quite a bit.

I’ve got the motor.  I’ve got the motor controller.  You’ve seen those.  The last major bit of the equation is the batteries, and you havent’ seen those yet.  This is largely because they’re on their way to the USA by way of a large container ship from China.  At least, I hope they are.

While the engine in an ICE car is the heaviest, most expensive part of the drivetrain system, with an EV this task falls to the batteries.  In my case, about 790lbs worth of these:

Thundersky 160Ah LiFePO4 Cell

That is a 3.2v 160Ah LiFePO4 cell from Thundersky.  It’s about one quarter the size of a conventional, large lead acid battery, the kind you’d find in a pickup truck.  My conversion is using 64 of these battery cells, for a total nominal voltage of about 205v (240v peak charge).

I’ll try and give some perspective on the battery system.

Battery capacity is usually given in units of kilowatts (kW).  That’s 1000 watts.  If you were to burn ten 100 watt lightbulbs for one hour,  or one 100 watt lightbulb for 10 hours, you’d consume 1kW of energy.  If you had a battery that contained 1kW of energy… well you get the picture.  An average home in the US consumes about 30 kW of electricity a day.

One gallon of petrol (I use the word petrol because it’s petroleum, a liquid, while gas is… gaseous) contains about 36 kW of energy.  My battery system will contain about 33 kW of energy (3.2v x 160Ah x 64 cells = 32,768 watts).  Not even the equivalent of one gallon of petrol.  But, I’ve projected my EV’s range at between 50 and 75 miles utilizing a conservative 80% of the battery pack’s storage capacity, for cell longevity.  The low end of the range is calculated with the vehicle travelling at a constant 65 mph, while 75 miles is with the vehicle travelling 40 mph.

The Land Rover is horrendously, aerodynamically inefficient (there’s a tire on the bonnet… which is what I call the hood because, it’s British… the Rover, I mean), which explains the differences in range at the different speeds.  At speeds above 40mph most of the power used to move a vehicle goes into fighting the air, since drag goes up with the square of speed.  Every doubling of speed requires four times the power to counteract the atmospheric drag.  As a comparison, a Toyota Camry has a total drag index of 7.57, while my Land Rover comes in more like 18.  A Prius is a low 6.24, while GM’s electric car from the past, the EV1 was a very slick 3.95.

Rover Sunset

Since it takes more energy to push a boxy truck through the air, the Land Rover is at a disadvantage, but for speeds around 40mph drag becomes less of an issue.  And for its intended purpose, as a vehicle designed for local trips into town, 40mph is perfect… as is the range capability.  In fact, here in the Rockies the atmosphere is up to 20% thinner than at sea level, so that’s 20% less air to push.  With an ICE car, that means a corresponding loss of 20% power (20% less oxygen in which to mix and burn with petrol), but the electric motor doesn’t care.  Chalk up a bonus for EV power at altitude!

Imagine the numbers, though.  I’ll have less energy than a gallon of petrol onboard, and while counting on using only 80% of that, I’ll be able to travel 75 miles.  Astute math students will notice that’s more than 75 mpg (it’s actually more like 90 mpg)… in a 38 year old Land Rover!  That’s about five times more efficient than the 15-18 mpg the truck delivered with its internal combustion engine.

Throw the electric drivetrain in something lighter, smaller, more aerodynamic (pick any modern sedan or wagon), and you might see 125 miles of range, or 130 mpg.

In an ICE most of the energy extracted from the petrol is wasted as heat.  Only 20% makes its way into motion.  With an electric motor very little energy is wasted as heat meaning almost all of the power goes into turning the motor shaft.  Some are capable of being 98% efficient at turning electrical potential energy into kinetic energy.

But back to the batteries for second.  I mentioned in a previous post that I had managed to extract about 700 lbs of ICE related equipment that I wouldn’t need.  Well, with the batteries alone, it’s all coming back… and then some.  But the Rover will be faster, stronger than before.  So it’s more like 700+ pounds of muscle.  Beefcake!  Beefcaaaake!!!

Cartman from TV's South Park

Where to put 790 pounds of batteries?  The Land Rover, having a truck-based, ladder chassis, means that there’s a lot of space under the vehicle around the frame in which to hide stuff.  After taking lots of measurements and using Google’s free Sketchup program, I was able to find a very close-to-scale model of my Land Rover online.  This allowed me to mock-up component placement to give me an idea what I could fit where.  It’s been tremendously helpful, and Sketchup is pretty easy to use.

RoverEVConversion_HV

You can click on the same image in the gallery below to see a larger version.  The yellow boxes are the battery cells, and you can also see a rudimentary layout of the electrical runs.

I completed this mock-up several weeks before heading to Taos to work on the Rover… and it has proven to be very accurate.  There will be one modification to the rear cells, however.  The Sketchup model’s rear differential pumpkin isn’t fully realized.  On the real thing, it interferes with the rear battery box as laid out, so I’ll have to turn some cells sideways and figure out a location for two orphaned cells, to make room back there.  They’ll probably go up front somewhere.

The side fuel tank locations are being used as they exist on the truck.  I modified my truck for a rear fuel tank a long time ago, so there’s a position there.  The front is now bereft of engine and cooling radiator, so there’s plenty of space up there.

There are a couple of nice side-benefits to this setup.  For one, the cells sit low in the chassis which lowers the vehicle’s center of gravity.  This is a benefit to safety and capability.  Secondly, much more weight transitions from the front to the rear compared to the ICE layout.  This should improve weight distribution from the ICE’s 60/40 front rear split.  In fact, there will be about 300 lbs of batteries in the rear, 160lbs on either side, and 240lbs in the front.

Wait!  Said astute math students will note that, that is more than 790 pounds.  Indeed, all those battery cells must be contained within four separate battery boxes, which I will be constructing out of 1.5″ x 1.5″ x 1/8″ steel angle iron and some thin sheets of aluminum.

But before I can bust the metal out, I need to know for sure that the battery boxes will fit the truck.  Pre-visualizing the layout is great for a start, but as with the differential pumpkin, what you see in the virtual world is not always what you get.

So I spent a cold, snowy, windy Sunday building battery boxes out of cardboard… and then crawling around under the truck  discovering what would really fit.  This is when I was alerted to the differential clearance issue.

Battery Box Mock Up

Now that I have the actual dimensions I need, I can get started on building the real boxes.  It’s been over a decade since I last picked up a MIG gun, but it’s like riding a bike!  It comes back quickly.  This was my first weld on some practice pieces.  They can only get better.

First Weld

For those curious, I’m using a 110v Lincoln Electric Weldpak 100 MIG welder that I picked up at Home Depot in the mid-90s for about $300.  It’s got a gas conversion kit, and I’m running a 75%/25% Argon/CO2 mix.  I’m amazed that 12 years later there’s still a gas in the cylinder!  I’m probably running near the limits of what this particular model can handle, though it shouldn’t be a problem.

MIG welders make it really easy for the lay-person to weld.  I originally purchased the unit to make sheet-metal repairs to the Land Rover’s rusted steel bulkhead.  I self-taught myself, so just about anyone with any creative aptitude and desire should be able to pick it up.  And obviously, it’s been a handy skill to have.

Speaking of which, how did I do the performance projections on my Land Rover?  I used a performance analysis spreadsheet that I found at Brian Hughes’ MR2 EV Conversion site.  We’ll see how the real thing turns out.

The internet, in general, has been where I’ve learned most of what I think I know now.  Only finishing this project will answer whether I actually know anything useful, but for those wanting to learn more about EVs, conversion, batteries, motors, all this stuff… there are a couple of great web forums with people far more experienced than me giving their two cents.  One site in particular, the DIY Electric Car forums, has been where I spend most of my time learning.  Imagine that, an entire site dedicated to homebuilt EVs!

Another interesting resource to peruse is the EVAlbum, a web site dedicated to people’s DIY conversions.  Everything and anything has been converted, and you can find out what goodies were used and how well the resulting EV performs.  Fun for finding really weird stuff.

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