Adventure EV

Batteries

Final Battery Installation

by on Nov.06, 2010, under Batteries, Battery Boxes, EV Land Rover

And we’re back…  Are you?  Drop me a comment or a pm if you’re still following the build!

Is everyone psyched?  I am…  After a more than half year diversion (isn’t it just like paying gigs to get in the way?) I’m back in the southwest, back in the garage, and back to work on my Land Rover EV conversion.

SWRainbow_v01

Strapping

When last we met I had just received four crates of lithium battery cells.  The final part of the Rover build is to get the cells from the crates onto the truck.  Here’s what they all look like splayed out on the floor in roughly the configuration of the four battery boxes.

BasicLayout_v01

There’s something missing from these groups of cells…  the aluminum end-plates that sandwich cells together.  Unfortunately, I designed my boxes around the dimensions of the cells themselves.  I hadn’t accounted for extra width of the end-plates.  Some allowance for extra space was made, but not enough.  The stock end-plates add at least another inch to the totals which in some locations couldn’t be spared.

That’s OK.  While the cells should be strapped together to prevent side-wall swelling the tight fit into the metal battery boxes should be adequate containment.  Besides, technically the cells don’t start to swell until they’re over-charged, and I don’t anticipate putting them through that kind of abuse.

In the meantime, I’ve procured a set of polyester-strapping tools, from an EBay seller, to bundle easy-to-handle groups of cells together.  It’s the same stuff used for package shipping.  Unlike steel or plastic, polyester strapping retains its elasticity/tension while remaining very strong.  This means that the cells will always be under positive compression.  The polyester straps are quite easy to work with, as well.  Will the strapping alone prevent swelling?  Not sure, but in this case the battery boxes themselves will provide that duty.

The strapping kit consists of a roll of polyester strapping, a tensioner/cutter tool, a crimper, and crimps.

StrappingTools_v01

StrappedCells_v01

Final Box Construction

Once all the cells are strapped together they are placed in their appropriate battery boxes.  As mentioned in a previous post regarding heating during winter, each box contains a battery heater designed to keep the cells at room temperature.  Batteries tend to lose their ability to pump out power at cold temperatures.  Keeping them at room temperature ensures that the EV’s performance during the winter doesn’t suffer.  The heaters are designed to be plugged into a standard 120v circuit while the EV is charging.  They don’t leach power from the battery pack.  As the EV is driven, the cells will naturally heat up due to internal resistance negating a need for active heating during discharge.

Each box is lined with layer of Ionomer foam (purchased from the ever helpful McMaster-Carr). The battery pads are secured to pieces of aluminum sheet which act as a second floor.  The heat from the pads diffuses through the aluminum plate allowing an even heating of the cells sitting above.   Each heater pad is designed to work at 120v and output 35 watts.  Every heater is wired in parallel to create one large 120v,  420 watt load.

InsulatedBox_v01HeaterPads_v01

Balancing

Before everything can be buttoned up the cells need initial balancing.  This is a crucial step.  It’s impossible to know the charge state of each cell as they come from the factory.  There’s a good chance that they’re all near the same level of charge, but initial balancing is the only way to be sure.  Forgoing this procedure could result in the destruction of individual cells.

Imagine each cell is like a bottle of water.   When they’re all linked together in series their energy levels increase at the same time when charged.  If one were half full, at the start of the procedure while another 3/4 full their levels wouldn’t arrive at the top at the same time.  Balancing makes sure that the levels are all equalized before-hand.  Overfilling would be disastrous.

This procedure should only really need to be done once, for all intents and purposes.  There’s a caveat to that which I’ll get into later.

The best way to make sure all the cells are filled the same amount is to connect them all in parallel.  All the positive terminals get connected together and all the negative terminals get connected together.   This allows the cells with more energy to drain into the cells with less, and vice versa.  It creates one giant battery pack that delivers 3.2v at a capacity of 10,240 amp-hours.  That’s equivalent to a REALLY BIG pair of AA batteries or a battery pack that could power a 4G iPhone for  nine years.

14 gauge wire from a 50′ extension cord was used to tie all the cells together temporarily.  There are better ways to do this.  Line all the cells up in a line with all the positive terminals on one side and all the negative terminals on the other bridged with two pieces of angle iron, for one.  But all my cells were already configured for an alternating-connecting, series configuration, hence the crisscrossing maze of wires.

ParallelBalancing_v02ParallelBalancing_v01

In addition to wiring all the cells in parallel I used a Mastech, adjustable, regulated power supply to charge the batteries to 3.8 volts which is essentially the top of the LiFePO4 chemistry charge curve for these Thundersky cells.  This insures that not only are all the cells balanced, but they’re balanced when full making it very difficult to over-fill a individual cell when charging in series.

Mastech_v01

Mastech power supply providing an initial balancing charge at 3.6 volts

The way to the power supply works is simple.  You wire it in parallel with the cells, and set it to output a set voltage (in the case of the picture 3.6v).  The batteries all fill identically until they reach the set voltage.  My Mastech can output a maximum of 20 amps.  The battery pack I’m trying to balance is 10,240 amp-hours in capacity.   If the cells started off empty  it would take at least 21 days to fully charge the pack.

Luckily, they weren’t empty… and I cheated a bit and first wired them in series so I could use my big EV charger, an Elcon PFC-5000.  Rather than the pidly 76 watts I was limited to with the Mastech, the Elcon could charge at 5000 watts, but I had to make sure to monitor the individual cells with a voltmeter to ensure a cell didn’t go above 3.8v.  Once the first cell hit 3.8v, I wired the pack in parallel and finished off the balancing with the Mastech for about three days.

Hookup

Once the balancing was complete, I rewired for the final series configuration using the supplied copper bus-bars and stainless steel hardware provided with the cells.  Not liking the standard lockwashers that came with the cells, I went to my local Fastenal and picked up some toothed washers (also known as star washers) which do a better job of holding hardware together especially when heat-cycled.  If you can get your hands on Belleville spring washers, that’s another good choice.  I also picked up larger washers to better spread the clamping forces.  After the battery terminal was cleaned up with a red Scotchbright pad, a very slight amount of NoAlOx anti-oxident paste was used between junctions to ensure clean connections.

My boxes are designed to be sealed against the elements, which I can do quite easily with lithium since it doesn’t vent gas like standard lead-acid cells.   I use marine power pass-throughs to transfer the high voltage between boxes and to the motor contoller.

Wood blocks wedged under the frame tops prevent the cells from moving vertically.  Once everything is in place, the cells can’t budge… yet removal for maintenance remains relatively easy.  Of course, one of the reasons to go with lithium cells is to negate the need for maintenance like having to add water to the cells of a lead-acid battery.

WiredUpBox_v01

Battery box wired in series with copper bus-bars

BoxTop_v01

Marine power pass-throughs transfer electricity in and out of the sealed boxes

BalancingBox_v01

A completed box sits next to another box balancing in parallel. Tape covers the power outputs to prevent accidental short circuits. The white wire is for the battery heaters

What!?! No BMS????? (Opinion follows… )

EV affecionados will notice that there doesn’t appear to be a Battery Management System (BMS) in place.  And they’d be right.

To use a BMS or not use a BMS?  It’s a hotly contested debate within the DIY EV community with opinions and tempers raging on both sides.

Basically, a BMS is used to insure that each cell within a battery pack doesn’t become over-charged or over-discharged since these two events are more or less catastrophic with lithium chemistries.

Active battery managment is much more important for chemistries like Lithium-Cobalt.  These are lighter-weight cells similar to the type used in laptop batteries.  Allow them to over-charge and they have a tendency to catch fire or explode.

Thankfully LiFePO4 is much more inert.  However, a very expensive cell can be rendered dead-as-a-door-nail if either end of the charge spectrum is breached.  While it’s relatively simple to tell the voltage of an entire battery pack, it’s much more difficult to drill down to the individual cell level.  It requires a lot more circuitry.  Enter the BMS.

Many designs exist, from those that monitor cell voltages and report them to a user-interface, to solutions that actively try and balance the cells by shifting charge levels around, to simple management that just employs bleeding excess charge away from a particular cell.  All the systems, however are designed to deal with over-discharge by throwing up an alarm, or over-charge by limiting charge current to a cell.

Some question whether management is even necessary.  Through a combination of anecodotal and emperical evidence it’s been found that once a set of battery cells is balanced it tends to stay balanced as long as it isn’t pushed to extremes.   As long as the user isn’t attempting to drain all of the energy from the battery pack as well as charge it to its absolute limits, all should be well.

I’ve designed my EV to be somewhat over-engineered.  While it should have a range of over 80 miles, I don’t intend on every using more than 50 miles.   And chances are, 35 miles may be a good average.  That could be considered a waste of battery storage capability and efficiency, but regardless it puts a lot less stress on the pack negating a need for a BMS.

I am a tech-nerd, however, and in the future I may install a BMS just to learn more about what actually goes on with the battery cells… in which case I might install an Elithion BMS system.

Next is wiring it all up.

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Fuel tanks.

by on Feb.12, 2010, under Batteries, EV Land Rover

Just before I had to leave the southwestern Rockies for California’s warmer wetter climes a set of large crates arrived containing my fuel tanks, Lithium Iron Phosphate (LiFePO4) cells manufactured by Thunder Sky Energy Group in China.

Crates of LiFePO4 cells

In total, the battery system is capable of containing about 33 kWh of energy which should be able to give my Land Rover an approximate range of between 55 and 75 miles, when conservatively discharged to 80% capacity.  Or with the amount of electricity I’m currently using, enough electricity storage to power my house for two days.

Each cell is 3.2 volts-nominal and stores 160Ah of energy.  I’ve got 64 of them.  The calculation to determine how much energy a group of cells can store is:

Total Capacity = cell voltage x cell capacity x number of cells = 3.2V x 160Ah x 64 = 32,768 watt-hours.

And the calculation of how much range achievable is:

Range = pack capacity / vehicle watt-hours per mile = 32,768 /  500 = 65.536 miles

This last range calculation is tricky and highly variable.  It’s only the roughest estimate based on the average efficiency of my vehicle based off of parameters like weight, aerodynamic drag, average speed, rolling resistance, and drivetrain drag.  Driving slower will decrease the watt-hr/mile usage, while driving faster will increase it.

Based off of the real-world experiences of others, however, an EV conversion will range between 200 and 500 watt-hrs/mile, the former figure being with a conversion of something light, small, and aerodynamically efficient like a Geo Metro, the latter being a heavier, bulkier conversion of… say, a 40 year old Land Rover.

Each crate contains 16 cells.

Crate of 16 cells

Each cell is about the size of a large, hardbacked, Tolstoy novel and weighs about 5.6kg (12.32 lbs) for a total pack weight of just over 350 kgs (or about 760 lbs).  That’s about the same amount I took out of the Land Rover in ICE components.

Cell detail

The cells come grouped in sets of four which just about matches the size of a conventional 12v Lead-Acid car battery, but you can specify alternate groupings if desired.  You can see the aluminum end-plates and strapping hardware below.  It’s used to prevent the cells from swelling when taking a charge.  Swelling increases internal resistance, which reduces power output.  This behavior is really only prevalent in prismatic lithium cells.  The trade-off is a reduced cost and a cell that is capable of storing a large amount of energy.  Having to strap the cells together becomes a minor inconvenience.

Grouped cells

In designing my battery boxes, I added a bit of space for cell end-plates, but I had no idea that the stock hardware from Thunder Sky would be as robust and thick as it is.  Unfortunately, these cell blocks don’t fit my boxes,  but all is not lost.  I will probably discard the provided plates and use the structure of the battery box itself to accomplish the same task.

Also included in the shipment was a box full of harware for hooking up the electrical side of things.  Here we have aluminum bolts and laminated, copper interconnect bars wrapped with heat shrink tubing.  The copper bars are used to connect the individual cells together, while larger and longer runs of 2/0-sized welding cable will connected the battery boxes together and to the motor controller and charger.

Included hardware

The next task is installing all these cells in the battery boxes, mounting them to the Land Rover’s frame, and wiring it all up.  Oh, and sort out some kind of battery management/monitoring system.  More on that later… when I’ve sorted it.

None of this will happen until the spring, however, as other work demands attention.

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The Good, The Bad, and the Ugly

by on Dec.24, 2009, under Batteries, EV Land Rover, The Knowledge

Sorry for the gap in posts.  It’s been a very hectic couple of weeks!

Funding an electric vehicle conversion…  You’re not entirely alone here, depending on where you are.  It’s odd that there’s no real support for alternative fuel vehicle conversions at the Federal level, considering the country’s “green awareness” climate, but individual states, and sometimes even individual counties offer tax incentives for alternative fuel vehicles.  That’s good news.

Sometimes the programs are quite generous, but due to ongoing difficulties with state budgets most are being cut back quite a bit for 2010 and beyond.  That’s bad.

In order to take full advantage of one of these programs I need to have my Land Rover certified as an electric vehicle and registered before the end of 2009.  I’m also heading back to the east coast for the upcoming holidays.  It doesn’t leave me a lot of time to get stuff done.  That’s ugly.

Ahh, if that were all…

In order to ensure my shot at taking advantage of one of these programs, my Dad graciously booked a vehicle inspection in Broomfield, CO, just outside Boulder, CO, for December 17.  Broomfield is 350 miles away.  Ohhh, that’s bad.

And my batteries have been delayed from China.  That’s really ugly.

In fact, they will not get to me by the end of 2009, and that’s just sad.  But it is what it is.

Here’s the good.  Christmas came a bit early this year:

Of course those four, deep-cycle, lead-acid batteries weren’t just for testing the controller!  They were, in fact, part of my backup plan.  That’s my Dad driving, and me on the wobble-cam.  Sorry about that.  My budget on this one is all going into the build…

First impressions:  Torque from zero rpm is a very good thing.  The Soliton-1 motor controller is amazingly quiet, there is no high pitched whining as the motor spins up, only the slight whirring of the two small cooling fans.  The motor is amazingly quiet.  In fact, pretty much the only sound you can hear in the video is the tires on snow.  A Prius in electric mode makes more noise.

Colorado doesn’t care whether the electric vehicle is finished.  They just care if it’s an electric vehicle, and that means no ICE.

Motor Bay

Temporary 48 volt lead-acid setup while waiting for the LiFePO4 cells to arrive. No ICE to be found. High tech broom handle acts as bonnet stay.

So with a top speed of 20 mph and range of… who-knows-but-I-don’t-want-to-test-it, we loaded the Land Rover onto the back of a U-Haul auto-transport attached to my other Land Rover (a Range Rover) and towed the rig up to the Broomfield Technical Center.

Loaded Up

I was actually surprised how well the Range Rover handled the extra weight.  All told, it was moving about 10,000 pounds over mountain passes at 5000-8500ft ASL, and even then we managed to maintain 65-70mph most of the way and average 11.5 mpg with nary a wiggle from the rear.  How the H2 Hummer, which doesn’t weigh 10,000 pounds, achieves less than 10 mpg is beyond me.

New and Old

Don't worry about the Range Rover's sagging rear. Once on the move, the air suspension raises to keep everything level. The Series Land Rover has no idea what that means.

This is definitely one arena where ICE will win out over EV.  It’s not that electric motors can’t provide enough power (diesel locomotives, after all, run electric traction motors,) it’s that the amount of energy required to move 10,000 pounds at highway speeds for 350 miles is just immense…more than 15 times the amount of energy I can store in a single charge of my battery-pack-to-be.

Testing

So we made it to our appointment with the state tech inspection station (apparently my camera did not, as witnessed by the terrible phone pic), and the truck passed with flying colors!  I didn’t even have to drive it off the trailer, which slightly disappointed me, but who was I to argue…  They just checked the VIN number, popped the bonnet to ensure there was no ICE in there, and issued a document indicating that a new title with change of fuel status to electric be issued.  The guy at the inspection center was gracious, very helpful and mentioned that there were quite a few electric conversions in the Boulder, CO area, but no Land Rover’s that he’d seen.

Pass Screen

Screen at the testing center reads: "This vehicle has been converted to dedicated electric power."

The next day we applied for a new title and registered the car.  So it’s all legal now.  And it’s technically an electric car.  So I’m claiming a bit of success regarding the challenge of converting the Land Rover to electric power by the end of 2009.

It’s not done, of course.  I still have quite a bit to do to fulfill my original design goals.  Pretty much all the fabrication is complete, save for some small bits here and there.  But I still have to load in the lithium cells, and obviously that can’t and won’t happen until 2010.  I’m actually writing this stuck in an airport on my way back to the east coast while a blizzard rages.  I won’t be back to the project until next year.

Temp Setup

Chances are that I won’t really be able to complete the project until the spring.  The last few weeks have been difficult, a true learning experience, and ultimately satisfyingly fun, but I need to get back to working on some projects that pay the bills.

This is a good thing.  The weather will be warmer, and I’ll have a battery management system (BMS) design in place.  I can finish painting.  And, I can work out why my clutch only disengages 90% of the way, I suspect my motor adapter spacer is too thick by about a 1/16th to a ¼ of an inch.

But don’t worry, I’ll try and get some pictures of those lithium cells.  They should come in just as I get back in the new year.

So have a great Holidays, everyone.  Stay safe!

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