[Nougatti]

How the heck do people keep weight down in these EVs?
We're a project group of 7 students studying machine engineering, at Gjøvik University College in Norway, and we're building a 1974 VW Beetle EV from scratch, using a 48V forklift motor with the forklift controllers and eight 12V deep-cycle marine batteries with 110Ah each. (We plan to have a separate small 12V battery to power the onboard lighting, windshield-wipers and such)

The batteries are of the Gel-type, meaning they don't leak and can be put at an angle. This was a requirement from our project supervisor, since they're supposed to be so much more environmentally friendly.
We're connecting them in two packs of 4 batteries each. So that each pack gives out 48V and has 110Ah, giving us 220Ah in total to run with.
However. These batteries weigh 32kg each, thus making the vehicle very heavy.
I know the American "car authorities" don't care much about how you customize your vehicle, but here in Norway the weight cannot exceed the original weight of the vehicle pr. axle, , due to brake dimensioning, suspension and so on.

Also, we'll be putting one pack of 4 batteries in the back, behind where the back seat would be, and one pack of 4 in the front to even the load. However, the battery-guy we're purchasing these batteries from, says that long wires will make us lose a lot of current, and that between the batteries, the wires need to be the same length, too, to avoid losses. What are you guys' views on these things?

 

[Duncan]

Hi Nougatti,

48 volts is too low

I am building with a 48v forklift motor and I am planning on using 150v+

Increasing the voltage reduces the current and enables the motor to reach higher revs

You will probably find that 48v and VW gearing will give a top speed of 50 Kph

Look up the conductivity of copper and run some numbers on your proposed cables,
you can't lose current - you can (and do) lose voltage down your cables
You also need to check that you are not going to overheat the cables

The flip side of increased voltage is that the forklift controller becomes useless
I am going to build an "OpenRevolt" controller

How much are you paying for your batteries?
You should check on Lithiums - the costs are now much closer especially as 110Ah Lead acid only give the same range as 60Ah lithiums

As far as weight is concerned my target weight is 650Kg - but I am building a two seater

 

[Woodsmith]

Agreed that 48v will be too low, I run my little tractor at 48v.

The other issue with a low voltage is that for the same power you will have higher current. That leads to needing cables with a larger cross-sectional area, adding to more cost and more weight.
The problem with the cable losses is due to the I²R losses.
Due to the resistance of the cables the losses increase with the square of the current. So you really want to keep current low and that means a higher voltage.
The higher voltage will reduce current, for the same power, and allow you to reduce the size of the cables and redcue the losses in the cables. It will mean that you can run longer cables with a less significant volt drop.

Given you have 8 x 12 v batteries you might as well go for 96v. This saves the issues of having parallel packs and matching cable lengths.
It will mean getting a suitable controller. There are many available, some better then others so do search here for opinions, and there is also the Paul and Sabrina open source controller, the Open Revolt. That seem to be the controller of choice for cost saving and quality and is the one I will be using.

If you can budget for it using lithium batteries will save a lot of weight and give you a more usable depth of discharge (DoD).
With Lead you need to keep your DoD to no more then 50% but that can be 80% with lithium. That means either more range or smaller batteries.


If you calculate the cost difference you may find it worth while considering lithium.

 

[tomofreno]

Look at vehicles in the "garage" (tab at upper right) here, as well as on http://www.evalbum.com to see different approaches, components, and performance. Pay special attention to ones who report actual data on performance, rather than an estimate or wild ass guess they made of what it might be when the car is complete. See how the performance compares with your goals to see if your planned approach makes sense. Search for build threads ("Builds and Conversions" thread on this forum) or web sites for the ones you are particularly interested in to get more detailed info. Email the owners with questions. Look up prices of components used (Google them or click on suppliers adds to the right of the screen here) to compare costs. The weight of cables is negligible. Estimate all masses as a percentage of the total vehicle mass to see if it is worth trying to reduce them. If you will only reduce the total mass, and hence the rolling resistance and required tractive effort by 1% or less, so what? Some will say, yeah, but those add up. Yes, they add up to maybe a few percent or less, unless you are racing, so what? You won't even notice this difference when driving. Add one passenger to the car and you add much more weight than this. Don't sweat the small stuff, there is enough to do without that. In a VW your longest cable might be what, 3 meters? That will have negligible power loss unless you plan to run many hundreds of amps or more through it, and even then the loss will be a very small percentage of power used to move the vehicle. Getting good, low resistance, connections is much more important. Like Duncan said, look up the resistivity and do the calculation. Also as said, use higher voltage to reduce current and attendant losses, unless you only plan to use it to putter around the campus at 35 kph or less. The batteries are the main source of weight, and as already said, LiFePO4 cells are the best way to reduce that if it is within your budget. Spend some time reviewing, and then ask specific questions. It takes much less time, effort, and money to read about others mistakes than to go through them yourself.

 

[crap]

Sounds like you should add an electrical engineering student to your group.

Saving weight is difficult in an old car, it pretty much only has the bare essentials to begin with. Really the only option to save a lot of weight is to use other components. A lighter motor, lithium batteries, and so on.

If you're desperate, you could save some weight using a more powerful dc-dc converter and throwing out the 12v battery. Also, those forklift controllers can be quite heavy if it is an old one. Replacing it may save you some weight (and allow you to increase the voltage, you really should have at least 72v)

 

[GerhardRP]

Have you already bought the batteries, or selected them? If so, post the model details and specifications. I will do a Peukert calculation so you can see what lithium pack would match the performance of your chosen pack.
Gerhard

How the heck do people keep weight down in these EVs?
We're a project group of 7 students studying machine engineering, at Gjøvik University College in Norway, and we're building a 1974 VW Beetle EV from scratch, using a 48V forklift motor with the forklift controllers and eight 12V deep-cycle marine batteries with 110Ah each. (We plan to have a separate small 12V battery to power the onboard lighting, windshield-wipers and such)

The batteries are of the Gel-type, meaning they don't leak and can be put at an angle. This was a requirement from our project supervisor, since they're supposed to be so much more environmentally friendly.
We're connecting them in two packs of 4 batteries each. So that each pack gives out 48V and has 110Ah, giving us 220Ah in total to run with.
However. These batteries weigh 32kg each, thus making the vehicle very heavy.
I know the American "car authorities" don't care much about how you customize your vehicle, but here in Norway the weight cannot exceed the original weight of the vehicle pr. axle, , due to brake dimensioning, suspension and so on.

Also, we'll be putting one pack of 4 batteries in the back, behind where the back seat would be, and one pack of 4 in the front to even the load. However, the battery-guy we're purchasing these batteries from, says that long wires will make us lose a lot of current, and that between the batteries, the wires need to be the same length, too, to avoid losses. What are you guys' views on these things?