# Realistically does 7kw work in the real world



## FFMan (Jan 9, 2010)

I'm planning my first EV project, and intend to keep it simple and cheap in the first instance.

The best budget source of motors seems to be forklift and I can find these cheaply to 7 or 8kw. I'll be using lead acid too.

However this power rating seems very low compared to the majority of projects, and I wondered in reality what will performance be like. I only want to commute 20 miles each way, but I do need to manage 60mph for at least half the journey and not be a hazard on a busy road.

Any thoughts ?


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## Duncan (Dec 8, 2008)

Hi
7Kw is continuous at relatively low rpm, - 1200rpm??
You have several things going for you

Torque is proportional to current - and the motor limits are basically a current limit
So a 7Kw motor will be able to sustain 21Kw at three times that rpm

That is continuous - for a short period - a few minutes you will be able to make at least three times that

So a 7Kw motor will be able to produce 21Kw continuous - enough to keep a small car going at 60mph 
And 60+ Kw for acceleration

On the other hand if you need a range of 40 miles lead is not going to cut it
If you can charge at work then you may be able to get 20 miles range out of lead but you will have to replace them every 18 months 
Much better to try and get a Volt or Leaf battery pack


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## FFMan (Jan 9, 2010)

Duncan 

thanks for the reply - most helpful.

So if I want to get more power from a 7kw motor, the way to do this is to over-volt it ?

If that is the case, then am I better starting with a motor rated at 7kw at 48volts rather than at 72v say, as it gives me a more readily accessible route to over-volt ?

Charging at work is an option, so for my 18.5 mile journey, I could charge for probably 7 hours at work on single phase.

I think lead/acid is my starting point (plus I have 80 ex UPS batts to start off with to prototype on). if it works ok, I'll track some lithiums.


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## Sunking (Aug 10, 2009)

Works fine for a golf cart, and many new ones with a LSV option have slightly higher power. Aside from that, no way are you going to get 40 mile range with Pb batteries. Assuming great efficiency of 400 wh/mile will require roughly a 30 to 32 Kwh battery. A 30 Kwh battery is roughly 1700 pounds , takes up a lot of space and yearly or biannual replacement. A 30 Kwh battery is going to cost you roughly $3000 for a 1 year battery and more like $5000 for a 2 year battery.


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## Vanquizor (Nov 17, 2009)

Although I'll echo that there are better budget battery option out there, Sunking greatly under estimates lead batteries and a good chassis.

On my neon conversion I achieved a demonstrated range of over 64.7km (just a bit over 40 miles) on a dozen of the cheapest lead batteries I could find (~$600 total pack cost). My neon uses ~250wh/mile at 60mph, and you could do better in a lighter more aerodynamic package (swift?).

FFMan- your goals are achievable, and if you can put any kind of a charge in at work not at all a stretch. The biggest key will be getting an efficient chassis.

Here is my build log for inspiration:
http://www.diyelectriccar.com/forums/showthread.php/1995-neon-conversion-126154.html

I've been served well with lead batteries for the first year, and enjoy the car so much I'm about to start my 2nd car using some of the same bits but kicking everything up a notch.


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## dcb (Dec 5, 2009)

It is hard to imagine just shipping 12 group 27 batteries for less than $600 

fyi, the dc27s you have pictured are dual purpose (not quite deep-cycle) so it will be interesting to see how they hold up over time/miles.

http://www.crosscountrybattery.com/pdf/BESA_Spec Sheet_2008.pdf


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## Vanquizor (Nov 17, 2009)

I should actually charge the car and do a full range test before dis assembly to see how they held up.

I don't think the batteries I got are anything special, they were just cheaper than the wal~mart ones and I suspect similar quality.

To be clear if I was building a car to daily drive for the next 5 years there is no way I would use lead, but to build a toy to get your feet wet with its REALLY tough to go wrong with bargain basement batteries- the experience is worth way more than the cost of a set of disposable lead batteries (especially if you have 80 of them to play with for free as the OP claims).


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