Adventure EV

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.


5 Comments for this entry

  • Tom

    I’m still following, glad to see you back 🙂

  • Alan Buckley

    Great, and good luck, I’m looking forward to the finish.

  • Harri

    Good to see update! I followed your build daily when converting my Range Rover.When i got driving i thought that you might be too busy buzzing around wilderness and forgot to tell us about it.
    Well, you will be amazed when you do!

  • Charles

    Yes, still following this, and very happy to see you back at it!

  • Kees Verwaal

    Hi there,

    today 29 july 2011 i am very curious if there are any more improvals and about the total cost of this project.
    Over here in the netherlands the fuel is very expensive (petrol €1,70 and dies €1,30 a litre) so i like to hear from you.

    Kind regards
    Kees

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