# Electric conversion

1. Aug 30, 2010

### karthik.s

hi, my name is karthik. i am a student and have taken a project.
i am planning to convert a 22 seater mini bus to electric. it produces max power of 70 hp at 3000rpm and max torque of 222 N-m at 1750-2000rpm. max gradeability is 30%. final drive ratio is 6.574:1. max speed i am looking at is 70kmph. avg speed is 40kmph.
what power motor do you suggest to use? i am planninig to use brushless dc motor. also what voltage motor to use?
what batteries to use? how many of them? what Ah value?
i am thinking of 12V lead acid batteries but not able to calculate the Ah value. the vehicle travels around 6hrs a day and covering 240 km.

2. Aug 30, 2010

### Staff: Mentor

I'm sorry, but what you are suggesting there is an extremely tall order. You'd need several dozen car batteries at the very least.

Do you know what fuel economy this mini bus gets?

3. Aug 31, 2010

### karthik.s

hey russ, thanks for your response. my project is for public transport corporation, so there is lots of space to place the batteries and also since fuel not being used the costs can be recovered in a year. the vehicle presently gives around 4km to a litre, hence the conversion to electric. please help me out with the calculations.

4. Aug 31, 2010

### Staff: Mentor

Ok. So the actual efficiency difference between diesel and electric depends on the type of driving: diesel vehicles have highly variable efficiency based on the driving (city vs highway), but electrics do not. At the very least, you probably waste half the output of the gas engine, then the engine itself is only about 33% efficient. So figure an electric could do the equivalent of about 24 km/L.

Now, the heat capacity of diesel is about 37MJ/L, so you need 370 MJ to go 240 km. 370 MJ is 103 kWh. A typical car battery has a capacity of 40 amp-hours at about 13 V, so that's about 200 car batteries.

Also, it isn't exactly accurate to say there is no fuel: you are using electricity and you'll still have to pay for that. Where I live, commercial electricity runs about $.1 per kWh, so the cost would be$10 per day. Diesel is around $.9 per liter, so that's about$55 per day. The retrofit won't be cheap, so I don't know if you can really recover it in a year, even if your bus is capable of carrying 3,000 kg of batteries.

5. Sep 1, 2010

### karthik.s

check out the file i have uploaded. more specifically the 45kw brushless dc motor matches the max power and max torque of our vehicle.
now considering 45 kw motor.12 V lead acid batteries have power density 280w/kg and energy density 34 w-hr/kg, we would require 160kg of batteries. that is 8 batteries. each lead acid battery weighs 22kg.
put in series it would produce 96V. but the battery voltage in the file for 45kw motor is 320V.
160kg of batteries produce energy 5440w-hr i.e 57Ah and if it has to run for 6hrs it means it is taking only 9amps whereas rated current in the file is 165amps.
can that motor be run at 96V and 9amps.what is wrong in the above calculation?
also the product of battery voltage and rated current in the file does not give rated power. what exactly is going on in this calculation?

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6. Sep 1, 2010

### uart

Yes 5kW-hr is NOT enough to power the Bus for the time/distance you require. Russ's rough calculations are in the right ballpark. Around 100 to 150 kW-hr's total battery energy is a about the minimum energy requirements. (BTW you'd need a fairly efficient design and reasonably easy terrain to even achieve those figures, the requirements could easily be higher.) So multiply all your figures by 20 and you'll have a more reasonable estimate of how many batteries are needed.

Now work out the cost of those deep cycle batteries and it's probably over \$30,000. Now figure that they'll last a maximum of one to two years under that duty and you'll soon realize that long range battery powered vehicles aren't necessarily that economical at this time (though that situation is getting better with improving battery technology).

7. Sep 1, 2010

### karthik.s

well i have uploaded another file of a series hybrid bus. though its hybrid after all motor powers it and batteries power the motor. so it should be no different to my calc.
if you see that report a 160kw motor is being driven by 26 batteries with two 13 series in parallel ,so generating 156V. but what is the Ah value of those batteries and how much duration can they be driven before recharging. i guess this will answer my doubts.
looking forward to the reply...

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8. Sep 1, 2010

### uart

Yes there is a big difference. It's a difference between power requirements versus energy requirement. The series hybrid doesn't reduce the electric motor requirements at all (as compared to a straight electric vehicle) and it only slightly reduces the battery power requirements, but it drastically reduces the battery energy requirements for long range operation.

It's the requirement of 240km range that's the killer here. Frankly I don't think it's even feasible with lead acid batteries.

9. Sep 1, 2010

### karthik.s

so uart,do you think that we can go for a series hybrid as per report.

10. Sep 1, 2010

### karthik.s

but how much distance do you think that hybrid will last on full charge?

11. Sep 1, 2010

### uart

The hybrid has an indefinite range, so long as you keep up fuel to the internal combustion engine (ICE for short).

In it's most basic operating mode the series hybrid simply runs the ICE continuously, whenever the bus is in motion, constantly charging the batteries. At first it may seem that this would be far less efficient than the simpler option of just powering the drive train directly from the ICE, as per a conventional vehicle. The reason however why the hybrid system really can be efficient is because the ICE only has to provide the average power and not the instantaneous power. This allows a smaller engine to be used and it enables the engine to constantly run at much closer to it's ideal (most efficient) operating point than could the engine in any conventionally powered vehicle. Another large benefit is of course the ability to do regenerative braking.

12. Sep 1, 2010

### karthik.s

how foolish of me to ask range for a hybrid!
i think a 80kw or 90kw motor would suffice. right?

13. Sep 1, 2010

### karthik.s

or would i require a 100 kw motor?

14. Sep 1, 2010

### Staff: Mentor

You're confusing kW and kWh. A kWh is how much total energy you consume by burning a kW of power. So 45 kWh is 45 kW for 1 hour. You need to run for 6 hours, though at an average of only 17.2 kW

And the watt-hours is the capacity, not the watts. So you need 103,000 w-hr / 34 w-hr/kg=3029 kg of batteries based on that energy density.

15. Sep 1, 2010

### Staff: Mentor

If the existing gas engine is 45kW you can use a 45 kW electric.

16. Sep 2, 2010

### karthik.s

for a series hybrid since we can use a smaller auxillary ic engine, will the fuel effeciency be that of the smaller engine.
say for example if we used a car engine then we can expect the mileage to be that of a car??????

17. Sep 6, 2010

### Staff: Mentor

Using a hybrid makes a big difference in overall fuel economy for a city-driving vehicle, yes.