Thursday, 27 December 2007

EV Design Notes

Electric Vehicle Design Notes:

This post documents the decisions made to choose the various components in the design of an electric vehicle for daily family use.

I have drawn heavily from the ideas from http://www.austinev.org/evalbum/1149 for the motor and controller.

Requirements:
Range: at least 40km
Size: Able to seat 4-5 people (I have kids)
Acceleration: I would like to be better than stock, but time will tell.
Top speed: Minimum 80kmh
Cost: as little as possible, in order to ensure a reliable and long lasting design.


Vehicle Choice:
Must seat 4 people. Most EVs are small cars that end up having to have the rear seats removed in order to stay below the manufacturers GVM after installing batteries and passengers. This rules out compact 4 door cars. For this reason I have chosen a small van or ute / pickup. This way I should be able to stay well below the GVM as vans tend to have a high load carrying capacity without modification. ~1000kg

The following are candidates

Suzuki carry - eliminated
Suzuki APV - eliminated
Townace SBV - eliminated
Townace - eliminated
Toyota Hilux / Tacoma Ute - Purchased


Unfortunately wind loading and vehicle mass will detract from the performance however a more modern Short Bonet van will be the best compromise.

A Toyota Townace SBV model or a Suzuki APV are my current preference, however cost a bit more than I would like to spend.
I am looking for a van with high kilometres, rear seats, good body with no panel damage.

The following might be a contender. 1998 – 2002 TOWNACE / LITEACE NOAH.

After further research a Hilx Exta Cab (RN90R) Ute was chosen as it has 4 seats and has the load carying capacity. It is also the same tare weight as a Townace Van.

Other notes:



  • No air conditioning necessary, however may be able to be turned into a heat pump. – To be investigated.

  • No power steering however there are some ways to add electric assist that use a lot of energy. Will need to check for older models that may have a compatible manual steering box. There is a 1999 Townace SBV model without power steering. The Hilux hand a manual and power steer option.

  • Manual or Auto. Gearbox will be removed, but auto will leave the interior cleaner looking as there is no clutch.

  • Rear wheel drive essential.
  • Low Differential ratio will be essential.

  • An aftermarket VDO / Autogauge / Acewell does a nice all in one Didital unit / Speco for locally made.

  • Check GVM vs battery mass (330kg).


Motor:
An AC industrial drive has been chosen. This allows ‘off the shelf’ industrial components to be used. A 15 kilowatt motor will be used as opposed to the 11kw in the Suzuki. A 15kw, 4 pole unit will weigh in at 95kg. An aluminium framed motor will be vital in order to minimise weight.

The price this should be about $400 for a used unit.

An AC motor has been chosen as this is the general trend in the future, no brushes and reversible for direct drive and regenerative braking.

4 pole should give 4000RPM, direct drive to the drive shaft. Top speed will be 80-90 depending on diff ratio and wheel size.


Drive:
A standard industrial variable frequency drive will be used as per link above. A second hand drive will be used if possible. The drive should be able to output about 4 or more times the motors rated capacity for short durations (under acceleration). This may be limited by the batteries, especially SLA.

Regenerative braking is a requirement. A braking resistor will also be necessary

Customisation of drive configuration will be interesting. Could have an economy and power switch.
The drive should have removable display to mount on dash board.

Estimated used price ~$1300

Monitoring:
I hope a lot of data will be available from the drives remote display, but if not perhaps an SBC, pocket PC or Palm can be used to extract data from the drive.

Drive Train:
I will have direct drive to drive shaft for simplicity. I will likely keep the gearbox in storage ‘just in case’ I change my mind.
I will need to ensure that the motor can not be put in reverse unless the vehicle is stationary according to the RTA rules. On further discussion with the Suzuki Guy performance is good without a gearbox. And throwing it into reverse will just decelerate the vehicle.

It seems that the max RPM of the EV1 is 7000 RPM with a differential reduction of 10.9 and the ranger is 13000 RPM with a reduction of about 12.5. A ratio of 4000RPM with a dif ratio of 4-5 seems comparable.

Batteries:
As much as I would like to use Lithium Ion batteries, Deep Cycle AGM SLA batteries will likely be chosen for cost reasons. These will also not need an expensive charger and not initially need a battery management system (BMS).

On further reading, a BMS will be important but not essential on day one. This could be a whole topic on its own. Current thinking is to build rather than buy due to cost considerations. The design would incorporate a microcontroller, a-d converter and a FET run in PWM mode to bypass up to 500ma of charge and discharge current. The controllers would talk through a common electrically isolated bus to each other and optionally a central control unit for parameter tuning and monitoring (and flash program upgrades?). The sum of the bypass voltage (say 13.8 volts or even 15 volts for cyclic charging) (13.8*50 = 690 volts must be higher than the charger cut-off voltage or else the charger will think that the pack never reaches full charge.

50 x 20AH 12 volt batteries will be used (13.800kWh). These will weigh about 322kg. Larger batteries would be preferred but this will be the max weight permissible.

Lithium Ion batteries from http://www.ev-power.com.au/ would cost USD $12784 – this includes a 15% discount (40ah, 188 packs) and would also require a BMS.

Used UPS batteries could be sourced for initial trials.

The batteries would be installed under the rear seats of the van in angle iron frames. SLA batteries should not need venting. Flooded cells will need venting to the exterior.

The packs will be isolated by relays when the ignition is off, and also a crash detector (inertial switch) is available to disconnect everything in case the worst happens.



Charger:
The VSD drive can cater for charging by shunt regulating the charger at a voltage threshold, but this would be a waste of energy

12 x 48v SLA chargers will be bought from e-crazyman on Ebay. (+ one 24v) These will charge from flat in 8 hours. They are 2 stage units.

I would like to also install solar panels on the roof for daytime charging. This is Sydney after all. The trick here will be to find a say 48v to 700v inverter. 15 watt panels $125 each total 60 watt costing $500. Inverter price is estimated at 1k.

Solar will not be considered initially as electricity is relatively cheep and the payback period would be very long.

I will also add a charging outlet in my shed for off peak cheaper charging over night.

12 volt system:
Install 20A inverter to run lights, radio etc. and optionally to charge an accessory battery. There will be 2 10A SMPS, one from +300 the other from -300 with common outputs.

Heating:
Install ceramic heater core if necessary. Could consider the braking resistor for a heat source.

A reverse cycle heat pump may be able to be used. Could use original aircon fittings. Can an AC pump be reversed? Could use “in window” type pump on a small VFD.

Car restorers use 12 volt ceramic ‘camping’ heaters.

Vehicle Modifications:

Battery installation under seats (5 rows of 10 under middle Some may need to be in front of vehicle to maintain original load balance.
Frame to carry Electric motor
Frame to carry electronics (under ‘hood’ or behind front seats.)
Modified drive shaft
Tyres – low rolling resistance tyres, optional initially.
Minimise weight. Remove unnecessary components and sell (motor, bull bar, tow bar, fuel tank, exhaust,)
Vacuum pump and cylinder for brakes.
Remove all unnecessary wiring
Install LED lighting except for headlamps. (electronic flasher may be required if indicators do not provide sufficient load)

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