Electric Porsche 944

Archive for April, 2015

Recycled 18650 Bicycle Battery

by on Apr.08, 2015, under Electric Car

In previous posts I’ve mentioned accumulating and testing used laptop batteries and other sources of 18650 size cells.  A project came up that makes good use of 91 of these cells (the range extender for the car may or may not still happen).  Many people have already done this, but the following is my method of assembling a 24v 26Ah bicycle battery specifically designed to fit in the existing case and charge using the existing charger.  The bike originally used a pair of 12v 12Ah lead acid batteries.

The first step was to layout the connection scheme for the 91 cells and create a drawing and then tool paths to drill and engrave the copper clad board which will be used for the bulk of the cell connections.

Bike battery - SketchUp Make_2015-04-08_14-59-26

Then drill and engrave the PCB material.

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The layout of the PCB combined with the layout and orientation of the cells gives the desired series/parallel arrangement.

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Tin the tops of the cells, take care not to heat the cells too much, others have said this can damage the cells.

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I used resistor leads for the connections from cell to PCB, bend the lead as shown to make a good electrical and mechanical connection to the top of the cells.

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Solder the lead in the very center of the cell.

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Trim the lead, leaving roughly 1/2″ sticking up.  I found that each resistor would make 4 connections.  After all the cells are done, the PCB is placed overtop of the protruding leads, this takes careful alignment to make sure they all stick through, careful soldering in the previous step helps out a lot here.

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Bend and solder the leads to the PCB making sure not to get too close to the engraved dividing lines.

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Repeat on the other side with the appropriate PCB and as many modules are being assembled.  Due to the shape/size required I had to make 4 stacks of 5 cells to the side of the main blocks, these were simply soldered together with wire.

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Complete the electrical connections (don’t forget to include a fuse!)

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Connect the cell log 8 for monitoring and plug in the charger.

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Once the pack passes basic tests use electrical tape or other insulating material to cover the exposed electrical connections.

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For this application the pack will be balanced on the first charge and simply monitored and rebalanced as required.  If the pack won’t stay balanced using this method either a top balancing BMS or RC charger with balancing leads may be adapted for long term use.  The existing lead acid charger charges at 3A CC-CV to 28.82v which is 4.117v per cell on a 7S13P pack, this is low enough that minor cell imbalance over time shouldn’t be an issue.

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The whole system is a compact box with built in charger.  The original battery was a series pair of 12v 12Ah SLA UPS style batteries, at 50% discharge that’s about 6Ah if Peukert doesn’t decrease it even more.  The new battery is 25.9v nominal, giving a slightly higher average voltage during discharge and a capacity of 26Ah, though it shouldn’t be discharged 100% it can be done with slightly accelerated aging.  This gives 4.3x the Ah’s but with the higher voltage during discharge it’s about 5x the usable energy in a package that weights about 2/3 as much as the original lead.

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The reconversion of a Porsche 914

by on Apr.07, 2015, under Electric Car

The Porsche 914 is an iconic car from the 70’s that has stood up against the test of time.  It has great looks, great performance (not exactly powerful, but great handling).  Parts are available, but not always easy to find.  There have been countless engine transplants, and an abundance of EV conversions.  The car simply makes a great platform for customization.

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This particular car was initially converted to electric drive using a Series DC motor, and lead acid batteries by the previous owner.  The car worked, but it never had very good range and was significantly overweight.  Later in life the car was then converted to LiFePO4 with the same motor/controller combo.  This made a significant difference, but range was still limited, and the DC motor less than ideal.

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The next chapter in the 914’s life is a switch to a 3 phase AC induction motor and inverter combo made by Siemens along with the Lithium battery out of a Chevy Volt, two of them in fact totalling 30kwh.  With regenerative braking and a 30kwh battery, the cars range will be up around 200km on a charge.

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Along with the EV reconversion, the car is getting a facelift.  Most of the rubber on the car has deteriorated, the tail lights and marker lights are faded/cracked, these will all be replaced with new parts.  The headlights will be upgraded to LED, this not only reduces power consumption but drastically increases the quality and quantity of light produced.  All side marker/brake/turn signal bulbs will also be upgraded to LED’s.  The brakes have already been upgraded, and the wheels are the lighter and more modern Porsche 911 style.

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Some of the more mundane but important upgrades include a washer fluid pump, the original Porsche 914 used air from the spare tire to pressurize the washer fluid bottle and using a fluid valve in the steering column allowed the driver to spray washer fluid on the windscreen.  There are compatible parts from newer Porsches that allow the control of an electric pump which will simplify and modernize this oddly implemented feature.

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The mis matched, broken and worn parts throughout the car will be replaced with original or better parts that will make this car equally at home as a daily driver and at a car show.  The original (and correct) door hardware shown on the left vs the compatible but incorrect parts on the right (passenger and drivers door respectively).

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The functional but incorrect Amps gauge in the instrument cluster will also be replaced with the correct “fuel” gauge found in the car originally.  The stock fuel gauge will be interfaced with the Ah counter to give the driver an indication of the remaining “fuel”.

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To incorporate the necessary EV instrumentation into the 914, a 2″ round gauge made by EV Display will be located in the center console replacing the clock or volt meter commonly found there.  This can display Voltage/Current/Battery Temp/Amp Hours/State of Charge/Fuel Gage/Wattage/Watt Hours, well any two of these items at a given time.

EV Display V3 Kit medium (1)

To give modern functionality and a more informative display an Android tablet will be incorporated into the center console which in conjunction of the EV Display and Torque app will allow a customizable gauge cluster to be created in software.  Real time availability of motor power will allow the driver to choose the most efficient gear for cruising, unlike gas vehicles 5th gear may not be the most energy efficient highway gear.

minibms.mybigcommerce.comtemplatefilesUsing Torque Pro with EV Display

Having an Android tablet integrated into the car will also allow the use of GPS Navigation, Audio apps, and while parked even let you watch movies or play games.  The audio output will be routed through the indash stereo.

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The Porsche 914 and it’s VW cousins of the era were never known for good heat.  The air cooled engines simply didn’t have an efficient method of collecting the waste heat and getting it into the cabin.  Many people augmented the stock heater with a VW gas heater which was a factory option in some countries.  Now it may seem counterintuitive to put a gas heater into an electric car, however there is one thing that gas is good at; making heat.  The propulsion of an automobile has a total efficiency in the teens for an average vehicle up into the 20’s for a very fuel efficient vehicle.  On the other hand home furnaces can reach into the upper 90% efficiency range burning natural gas or similar fuels, we typically don’t heat our homes with electricity for a reason.  The range impact on an electric vehicle from the heater can easily be 25% or even more in very cold weather.  The other problem with cold weather is most batteries don’t perform as well in the cold to begin with.  If the car is limited to 75% of it’s range just because it’s cold, then we take another 25% in order to heat the cabin we’ve significantly reduced the usable range of the vehicle.  The VW gas heaters are VERY fuel efficient, one of the most common is rated at 0.32 – 0.38 litres per hour with constant usage and in most cases the car is going to get too hot with constant use, so running the heater intermittently for 1/2 – 2/3 of the drive will be more comfortable and efficient.  Assuming a speed of 100km/h (typical of Deerfoot or Stony Trail here in Calgary) the car would burn 0.16 – 0.25 Litres of regular gasoline for a 100km drive with intermittent use (16 to 25 cents at today’s prices) the electricity cost to drive that 100km would be roughly $1.20.

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Above you can see the vintage heater almost ready for testing before it gets torn down for cleanup and repainting.

 

The 914 is a very popular car for conversions partially because of  the large amount of space that can be used for batteries.  The front trunk, rear trunk, engine bay and fuel tank location are all suitable spaces for mounting EV components.  In this case, part of the front trunk will be used for batteries, holding just over 1/3 of the pack.  The Chevy Volt battery is liquid heated and cooled, this is a very efficient way to control temperature and something that will be maintained in the 914.  The liquid heater from the Volt will be reused and capable of bringing the battery pack up to safe charging temperature or simply warming the battery on a cold morning using wall power instead of battery power for better range and power.  The cars cabin may be included in this loop to preheat both the car and the battery to further reduce the need for gas or electric heat while driving.

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Above the test fit and temperature monitoring of a small portion of the Chevy Volt battery pack.  The Volt battery comes as a complete unit configured for a very specific purpose as far as electrical and cooling connections are concerned.  Due to the size and shape of the 914, not all of the stock modules will fit the car as is.  In order to reconfigure the modules some extra parts are required, it’s not going to be as easy as dropping by the local Chevy dealership and requesting a very specific battery cooling part, the battery is likely not meant to be serviced outside the factory, only replaced.  While very specific the parts aren’t very complicated, simple plastic pieces needed to plug unnecessary cooling ports to start.  These parts were 3d printed using ABS material which has a suitable temperature range and chemical stability to work in the heating/cooling system.

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The stock black gasket is on the left, with a 3d printed flexible version in white at the top, this white version won’t be used unless a replacement is required.  The center and right show the two types of blanking plates required to block off the cooling ports on one end of a battery module, one holds a gasket, the other seals up against the gasket in the last battery plate.  The 3d printed parts have studs incorporated into them similar to the stock pieces and are meant to be melted once installed to hold them in place.  The existing cooling ports and end plates are all interchangeable and can be configured as desired, I simply found I didn’t have enough blanking plates to seal off the ends of a few of the module groups in my planned configuration.  Unlike the Volt where the first two battery assemblies are in a row then feeding the final module through the “side” ports I found the need to have some of the assemblies side by side with cooling ports on one end only.

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The Chevy Volt battery can be easily broken down into its 1kwh and 2kwh building blocks, 6 and 12 cell respectively then rearranged and bolted back together in the shape/size you desire just like Lego.  Changing the total length of a module does require different bolts to put it back together, but M6 x 1 threaded rod works perfectly for this.  Making a module shorter can be done two ways, either replacing the existing bolts with M6 threaded rod or cutting the existing bolts and threading the end with an M6 x 1 die (The stock bolts are only threaded about 1 inch on the end).  Remember you need four threaded rods per module.  M6 x 1 is commonly available in 1m lengths which is slightly longer than what is required to make a 4x 2kwh block using the “side” cooling ports.

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Reassembly (compression) using a ratchet strap makes it easy to get the bolts started and using the built in wire guides and a 1″ strap keeps everything in place with minimal chance of the strap sliding off or out of place.  For longer modules like the one shown, using two ratchet straps, with a ratchet on each side make it easier to evenly compress the modules and get the nuts on the threaded rods started.

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Steel or Nylon straping used for banding skids for shipping should work well to replace the stock steel(?) band around the top of the battery assembly.  Nylon is probably cheaper/easier/safer for use around a battery and doesn’t require fancy tools to install.

 

In the name of recycling even parts of the battery assembly base can be reused to make new battery mounts and reuse the existing hardware and mounting method.  First find all of the somewhat camouflaged spot welds and drill them out with a 15/64th’s drill bit (or similar) being careful to get as close to center as possible.  Then enlarge the holes with an 11/32nd’s (or similar) drill bit to completely remove the spot weld.  This is slightly larger than the spot weld itself allowing for being slightly off center.

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After all of the spot welds in a given section are drilled out the clamping portion of the base can be pried off.  There was some sort of adhesive in the bonding of these two pieces but tapping flat screwdrivers or other long wedges between the two pieces will separate them nicely.

 

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The liberated plate now has the required mounting lip and studs to reuse the original clamping bars.  This plate can be welded to the bottom of a battery box or directly to the vehicle, I would recommend welding all of the previously drilled out spot weld holes to make a nice solid mount, an adhesive like liquid nails probably wouldn’t hurt, but might make removing it even more difficult than from the Chevy plate.

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The finished product will be a smooth and quiet easy to drive 100kw, 200km convertible ready for the daily commute or the open road.  The combination of small lightweight car and a fairly large battery pack (30kwh) will give this Porsche 914 more range than almost all of the OEM EV’s available today (with the exception of Tesla).  Charging has not been decided yet, however a 10-12kw solution would extend the total daily usable distance by a significant margin allowing the car to charge from 20% to 80% in about an hour and a half.

 

 

 

 

 

 

 

 

 

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