Newsletter

Planning


If you have made the decesion that you want to convert a car into electric the planning phase can be started. It is very important to be realistic during the preparations. You have to be able to draw up your demands and you have to have no doubt about your possibilities. There is no more sorrowful sight than a left-off-dream.

Car choosing

It is worth to have a look at other people's transitions. How they succeeded with the conversion. You can find some very good albums on the internet with the photocollection of self-made electric vehicles. For example on www.austinev.org/evalbum you can see more than 1200 photos of electric conversions. People having thickker purse have more audacious dreams. Their "products" can be seen on www.metricmind.com/photo.htm .

If you are amazed by those creations it is time to meditate what kind of car you want to have in electric version. Eventually any kind of car can be converted into electric. But some point of view is worth to be taken into consideration.

Let's have a look at what kind of cars can come up from the huge choice. Three categories are the most suitable for conversion:


L6e category:

 
Axiam   Axiam
     
 
Ligier   Ligier
     

In the reality these vehicles are not cars but little motors. To be concerned a car such motor it has to be subjected to the next conditions:

  1. Its own mass (without batteries) cannot be more than 350 kg.
  2. The power of the driving motor cannot be more than 4 kW.
  3. The designed speed limit cannot be more than 45 km/h.

Driving and owning such car is ruled differently in each country. But it can be said that to keep such car is much more cheaper than that of a "normal" car.


Cultic cars:

 
Citroen duck   Renault4
     
 
VW beetle   Mini
     

Sport cars from the 60-s, 70-s and 80-s:

 
850 Spider   Alfa Romeo Spider
     
 
FIAT 850   FIAT x1/9
     
 
MG Midget   Triumph TR7
     
 
Porsche 914   Porsche 924
     

Motor choosing

Which motor to choose is determined mainly by the speed limit you want to reach. In this matter you have to be retained. Although the price of motors does not increase so quickly as the power output does but the price and weight of the neccessary batteries can be a serious limitation.

To count the motor power output from the desired speed limit you can use the next formula:



motor = (P rolling + P airresistance ) / η = ( m f * g * f * v + γ * c w * A * v³ / 2 ) / η


where:

motor = the nominal power output of the electric motor (kW)
rolling = the needed power to surmount rolling resistance (kW)
airresistance = the needed power to surmount air-resistance (kW)
η = the efficiency of the drive chain (typically 0.9 - 0.95)
f = permitted biggest mass (kg)
g = gravitative acceleration (9,81 m/s²)
f = rolling resistance factor (typically 0.01 - 0.02)
γ = the density of the air (1,2 kg/m³)
w = airresistance factor
A = dimension of forehead surface (m²)


Regarding the experience you can reach about the next speed limit with the different motors:


4kW - 50-60 km/h
7.5kW - 70-80 km/h
11kW - 90-110 km/h
21kW - 120-140 km/h

The relatively low end speed can be deceiving. As the used electric motors can give down 2-3 times of their power rate for a short period by acceleration these cars seem to be fairly agile.

The 4 kW motors are used first of all in moped cars, sometimes in cultic cars. The most ideal power output for cultic vehicles is the 7.5kW motor. Into the little sport cars at least the 11kW motors suit the best.


Battery pack

The weakest link of chain in an electric vehicle is the battery pack. We have to make the most compromise in this case. This is the most expensive and the soonest used up part of the vehicle.

To determine the capacity of the battery pack you have to decide how long distance you want to make with full charged batteries. The consumption of an electric car depends in large scale on the style it is driven plus on the conditions of the road. It can vary from 100Wh/km upto 200Wh/km. Generally the manufacturers and distributors of the electric cars are counting with the 100Wh/km to paint a better picture about the car's features. While under bad road conditions and too dynamic driving style this value can even reach the 200Wh/km.

Counting with the avarage 150Wh/km seems to be a good approach. Let's take a look at how much energy we can take with ourselves. There is stored about 12*150 = 1800Wh energy in a 12V/150Ah battery. But you should not use this energy until the last "drop" because it shortens the probable span of life of the battery in large scale. It is a rather good estimation if we count with 1500Wh as takable energy. From above numbers you can see that a 12V/150Ah battery is enough for about 10km. It means about 10Wh per Ah.

Let's see a practical example: We have an electric car installed with 72V/7.5kW motor and with 6 pcs of 12V/150Ah lead acid gel batteries. The quantity of stored energy is about 9kWh. The estimated efficient distance is 60km (9000/150). If the batteries are rather new, they are in good condition, we drive carefully plus the weather is warm enough (in cold weather the capacity of the lead acid batteries decreases significantly), we can easily reach even the 100km with one full charge, it is not impossible at all.

In the case of LiFePO4 batteries the situation is much better: its weight is much less, you can take out the energy more quickly, the capacity of the battery does not depend in so much rate on the temperature and the 4 times as long span of life can be attractive. Counting similarly to the lead acid gel batteries we can get the result that the energy stored in a 3.2V/90Ah lithium battery is enough for about 2km.


Variations

Let's have a look at some installations in table form:


Motor type Battery pack End speed Efficient distance mass to be installed
4 kW/48V 4 db 12V/150Ah Pb 50-60 km/h 40 km 224 kg
4 kW/48V 8 db 12V/120Ah Pb 50-60 km/h 64 km 310 kg
7.5 kW/72V 6 db 12V/150Ah Pb 70-80 km/h 60 km 310 kg
7.5 kW/108V 9 db 12V/120Ah Pb 70-80 km/h 72 km 360 kg
11 kW/144V 12 db 12V/120Ah Pb 90-110 km/h 96 km 455 kg
11 kW/144V 48 db 3.2V/90Ah LiFePO4 100-120 km/h 96 km 224 kg
11 kW/144V 48 db 3.2V/160Ah LiFePO4 100-120 km/h 170 km 349 kg

You can see that the end speed has been increased in the case of lithium batteries. There are two reasons of it: the smaller mass and the higher nominal voltage (48*3.2V = 153.6V). Increasing the voltage is the most simply way of tuning an electric vehicle.

Lead acid gel or lithium batteries? This is the question. If we look at the costs of long term there is no difference. As many times the lithium batteries are more expensive than the lead acid gel batteries, their span of life is as many times longer. The lithium batteries have other better features, too: the mass is third (regarding the same quantity of stored energy in lead acid and lithium batteries), they can give down the energy quickly, they can be charged quickly, they can withstand cold weather very well.

What are the advantages of the lead acid batteries? Only the price. If you do not have enough money to buy lithium batteries for longer term, you have to choose lead acid gel batteries. Then you can postpone the decision with some years, when the first lead acid battery pack reaches its end of life. This happens in the case of every-day usage after about 2 years, in the case of only week-end usage after 5-6 years. Then you can put this question again to yourself and if you can afford it you can buy the more expensive battery pack only at this time. The technology develops rapidly. We hope that at your second decision the price of lithium batteries will not be so high.


Financing

(what you will probably exceed)

A serious planning cannot miss the estimates on the costs. Although there are only a few people who really does it. Who would start a new hobby if he would know at the beginning how much money he would spend on it? Now we are trying to do the impossible but only for ourselves.

Let's have a look at what kind of costs we shall meet:

The two ends:


  1. The smallest motor, minimal lead acid gel batteries, L6e or cultic vehicle, 50-60 km/h end speed, 30-40km efficient distance, battery exchange every 20.000 km. (Minimum ev)
  2. Little sport car, the biggest motor, lithium batteries, 100-120 km/h end speed, about 100 km efficient distance, battery exchange every 200.000 km. (Maximum ev)

Minimum ev Maximum ev
type price in EURO type price in EURO
Motor + controller DC/T4-48+MC48-350 1.080,- DC/T11-144+MC144-300 1.870,-
Battery 4*6FM120EVX 920,- 48*LFP90AHA 9.216,-
Charger 2*BC24/10 216,- 3*LIBC67.2/20 1.080,-
Cables, etc.   160,-   240,-
Price for the make *   600,-   1.000,-
Plans+authority*   220,-   220,-
Options   - Servo brake 400,-
    -------------------   --------------------
Total:   3.196,-   14.026,-

* - price in Hungary


It is worth to mention the savings, too: