Adventure EV

Battery Boxes

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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Battery Box Update

by on Dec.15, 2009, under Battery Boxes, Controller, EV Land Rover, Fabrication, Heater

Did I already do a battery box update?

The one continuous thread throughout the conversion has been the fabrication of the battery boxes.  It seems these things take forever to build.  But it’s been cold… and I’m being whiny.  I must admit that they are pretty heavy duty, though.  Far more robust than they probably need to be, but probably beefy enough to handle off-road rigors, if necessary.  Like everything in this Land Rover, they’re built tough… very fitting.

Most people build a tray that the battery cells sit on, usually located somewhere in the engine bay, or more commonly, in the trunk.  I wanted to build sealed enclosures that sit completely under the vehicle, out of the passenger compartment.  Since I’m using lithium cells this shouldn’t be a problem.  Flooded lead-acid batteries, in comparison, vent hydrogen gas when they charge.  You wouldn’t want one of those in a sealed container…

There are four boxes in total, carrying 64 LiFePO4 battery cells.  Two sit on either side of the vehicle where the stock fuel tank locations were, just under the seats.  A larger rear box sits tucked up between the rear axle and rear frame crossmember.  The final box sits in the front of the engine bay.  All of it is made from 1/8″ mild steel in various forms; angle iron, square tube, strap, sheet.  Aluminum sheet is used to fill in the gaps and cut down on weight, but even with that saving measure I’m guessing all the boxes will add at least 150 pounds to the rig.  Not great, but they’ll last forever and be able to take some abuse.

Here’s one of the side box frames being held in position under the chassis by a Harbor Freight transmission jack.  Once full with batteries the jack will be the only way to get the boxes in place (the rear box will weigh about 260 pounds), a very worthwhile investment!

Side FrameThe rear box sits between the rear road springs and hangs from the rear crossmember.  Another piece of angle iron stretching between the frame rails anchors the front mount.  There’s a nice, empty space under the short-wheelbase Rover.  I had previous modified a Jeep fuel tank to fit back there.  Now the space is home to a different fuel.

Rear Frame

Crossmember DetailThe Rover has an unusually short rear overhang.  Great for off-roading.  I probably lose a couple of degrees with the rear box hanging down, but it shouldn’t pose a problem.  All in all, it’s quite an elegant fit.

Rear Clearance Since the top of the boxes are angle iron to provide strength and a lip to seal the top lid against, they pose an obstruction for the cells, so notches were strategically cut to allow groups of strapped cells access.  Not having the cells made this hard.  I just have to trust that the dimensions will allow for the clearance.

The final frames were painted with POR-15, and aluminum sides were cut to size.

Frames OutsideOK, granted the thin aluminum sheet isn’t exactly the toughest stuff in the world… But finally, something that cuts like butter!  And here’s the tool that does it.  Harbor Freight electric metal shears that make quick work of the box walls.  Say what you will about the quality of Harbor Freight stuff, but it’s cheap, and gets the job done for the few times people like me need something done.  And having the right tool for the job makes all the difference!

Shears

Once the sides are cut, they’re riveted to the frames.  A combination of the paint and sealing caulk ensures no galvanic reaction between the steel and aluminum, and helps seal the box from the elements.  Here’s Dad helping out with the riveting.

Dad Helping

A very nearly finished rear box.  I suppose I could leave it this way, but the plan is to spray self-etching primer on everything and coat with a semi-gloss black.  However, it’s been too cold to do any of spray painting.  All in good time.

Rear BoxI’ve sized the boxes to be slightly larger than the cells so that I can place some insulating foam around the perimeter.  This will help against shock and increase the insulation factor for the cold season.

The LiFePO4 cells should be fine just sitting in the cold, but they don’t like being charged in sub-zero weather.  To help performance in colder months, heaters are employed.  The bottom of each box gets two layers of aluminum.  One layer acts as the exterior wall.  A layer of foam (temperature tolerant Ionomer Foam from McMaster-Carr)  sits on that, and then thin battery heater plates from KTA Services, Inc sit on the foam.  These heaters are rated at 35w each and run off of 120VAC.  The idea is that these heaters will connect directly to wall power when the EV is charging in the winter.  As the cells discharge during normal driving, they should create enough internal heat to suffice without the heater pads active.  The second layer of aluminum sits on top of the battery pads, not only to protect them, but also to help spread the heat under the cells.

Heater

The hardest box to build was the front box.  I had originally designed the rear box to contain three rows of eight cells, for a total of 24 cells, but the rear differential pumpkin got in the way.  One of the rows of eight had to go, and in its place I got a compromised sideways row of three.  I had to find a place for five more cells.

The Rover does have a bunch of hiding places for more battery capacity, but rather than try and mount a fifth battery box, I decided to modify the front box.  It turns out space is becoming a premium if I want to keep everything moderately contained and relatively simple.

Instead of only needing to house 16 cells, the front box was widened to contain 18, and a small side box was welded to the back providing the space for the final three cells.  It’s weird, but it works as well as it can without the actual cells on hand.  I really hope I’ve left enough wiggle room.

The front battery box will have a clear acrylic lid for extra “bling-factor.”

Here’s the basis for the front box.

Simple Front FrameThe frame bolts directly into the frame rails.  Another set of brackets was fabricated to carry the bigger electronic items, the charger and motor controller.  The charger is another piece I don’t actually have yet, so I’m hoping the dimensions I found online are correct.

TrayThe charger sits a few inches above the motor, behind the battery box, and the controller sits a few inches above the charger.  The front of the “components” frame is bolted to the back of the battery frame so that it can be removed separately if needed.  The rear of the frame bolts directly to the vehicle’s bulkhead/firewall.

Once everything is tight the whole assembly ain’t goin’ nowhere.  It’s extremely solid!

Full Frames

Again, gotta love that access!  The Rover makes this conversion so easy in some ways.

Hopefully, all the miscellaneous electronics (fuses, contactor, shunt, relays, etc.) will sit behind the controller, up against the bulkhead.

Wait, I have the controller!  What’s that look like in place?

Electrics FinishedWhat are four, deep-cycle, flooded lead-acid batteries doing in the battery box?

Controller UpdateWhy, helping test out the motor controller, of course…  That’s an ethernet cable attached to the Soliton-1, with the controller’s configuration page on my netbook…

I can’t end the story there… can I?

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Battery Box Progress

by on Nov.18, 2009, under Batteries, Battery Boxes, EV Land Rover, Fabrication

I certainly expected to learn a lot embarking on this endeavor, and I have.  But something I didn’t really need to learn was that cutting 1.5″ x 15.” x 1/8″ angle iron with a 12″ mitre saw and an abrasive cut-off wheel… takes FOREVER!!!

So far I’ve cut pieces for three of the four battery boxes… 12 pieces for each box, two cuts for each piece, for 72 cuts altogether.  45 degree angles are the worst.  It’s like cutting thicker material.  The wheel just spins round and round… and round… and round before it cuts through.  I thought it would be like butter,  but it’s more like cutting through a can with a nail file, metal dust everywhere.

Cutting Angle Iron

And the abrasive cut-off wheel doesn’t cut cleanly at all.  Every time I complete a cut I have to de-burr, bevel, and smooth the edge with a 4″ angle grinder.  It just gets tedious.  Took a couple of days.  Word of advice, don’t do this without ear defenders and other normal safety gear.  You’ll go mental without the ear protection… and deaf.

I’ve heard of chop saws that use carbide-tipped metal blades that do cut metal like butter… and cleanly.  If I was doing it for a living I’d purchase one in a second, but they’re ex-pens-ive.  Another word of advice, get one of those!

But then it was on to welding, and for some reason welding is far more enjoyable.  I dunno, there’s something about melting two pieces together by brute force… and a lot of electricity… and sparks.

Welding

So after the past couple of days, the two frames below are the result of the fruits of my labor.  And what do you know, they actually kinda fit.  Tomorrow I complete the rear box frame… the big one.  That shouldn’t take long.  The welding is relatively quick and painless.  But I’ve run out of shielding gas… wish me luck trying to find some.

I purchased some thin gauge aluminum sheet which will act as the box walls.  That will be sealed in and riveted, not welded, and I can cut that stuff with electric metal shears.  Hopefully this time it will cut like butter.

Side Box Frames

I also pick up my motor spacer tomorrow…  hopefully.  It’s Taos.  Every time I call to see if it’s done it hasn’t been started.  Life moves at a different pace out here.  Luckily I’ve been preoccupied with the fabrication work, and I haven’t been pushing the machinist.

I dropped my flywheel off earlier in the week to have the starter teeth and a bit of the back side shaved off.  Maybe I can cut its weight by a third.  It’s a heavy bugger, so any weight savings would be nice.  It was a spur of the moment decision, so I failed to weigh it prior.  Oh well… heavy.  That’s how much it weighed.

Maybe I’ll never see my parts again…

For those looking for sources for parts and pieces:

1.5″ x 1.5″ x 1/8″ Hot Rolled Steel Angle – 80 ft @ $87.75 ($1.10 linear foot)

60 sq/ft .040″ 5052 Aluminum Sheet – $140.19 ($2.34 sq/ft)

Purchased at Metal Supermarkets in Albuquerque, NM.  You might find an outlet near you, or you can order online.

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