# Estimate of cable and connector resistance?



## bart_dood (Sep 2, 2010)

Hi Folks, I'm new here and have a question on cable and connector resistance.
I'm trying to figure out a very inexpensive bug conversion with low cost in mind I have the idea of using 3x brushless DC motors (very large 9.3 horsepower model aircraft motors) in parallel together with 3x water cooled brushless controllers.

As far as mechanics of hooking all this up, I'm a design engineer with extensive experience so it won't be a problem. I also have electronics experience but I'm not an expert.

The problem I see with this is peak currents. The controllers depending on which model I get have a continuous rating of about 200 amps each, but a peak limit of around 360-400 amps.
I planned on running a 48 volt system with 4x lead acid 250ah cells in series.
If the resistance of all the cables plus the internal resistance of the cells is too low if the throttle is floored from a standstill the motors will pull enormous amounts of current and blow the controllers.
I might be able to get around this by designing a microcontroller system that monitors the rpm of the motor and limits the amount of applied throttle to reign in the peak currents, however a far simpler method is simply to make sure there is enough resistance in the electrical system to act as a current limit.

Considering I am running 48 volts nominal, I need to have 48 milliohms to give me a peak current of 1000 amps max. This will be shared by 3 controllers so each controller will only ever see 330 amps max. 

However I need an accurate estimate of how much resistance I will get in a series connection of 4x 250ah lead acid batteries in series with all the cables and connectors.
I used an online calculator for the cables and assuming I put the batteries in the front trunk of the bug and use about 20 feet of 4awg cable gives me a resistance of only 7 milliohms, the cells will have internal resistance of maybe 5-9 milliohms each, so this totals 27-43 milliohms. I'm very close to 48 but I need to add in connector resistance and also make sure I'm over 48 milliohms.

Anyone have experience doing these kind of estimates or have results from measuring resistance in a series of batteries and cables in an EV?

thanks for all the help!


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## dtbaker (Jan 5, 2008)

I can't give you any exact numbers, but do know that if you are pulling 300+ amps you gotta go with 2/0 welding cable. Using that, the internal resistance of a lead battery is the limiting factor.

Your concept of 3x motors is interesting, but in reality gets to be a big pain with alignment and using either belt of chains to connect. A guy out here tried something very similar, and while it did work, torque characteristics of those motors was not nearly as good as plain single brushed motor. You also probably will want to get that voltage up to at least 96 or 120v... The lower voltages just require HUGE amps.


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## bart_dood (Sep 2, 2010)

dtbaker said:


> I can't give you any exact numbers, but do know that if you are pulling 300+ amps you gotta go with 2/0 welding cable. Using that, the internal resistance of a lead battery is the limiting factor.
> 
> Your concept of 3x motors is interesting, but in reality gets to be a big pain with alignment and using either belt of chains to connect. A guy out here tried something very similar, and while it did work, torque characteristics of those motors was not nearly as good as plain single brushed motor. You also probably will want to get that voltage up to at least 96 or 120v... The lower voltages just require HUGE amps.


Yes you are right that the torque of this setup would be considerably different to a brushed DC motor, it would be minimal at low rpms and rise up in a pretty linear way with rpm much like an ICE motor.
My goal for this project is certain very conservative, I'm aiming for a car that is useable for short trips (30 mile range roughly using 30% of the cell capacity) and only has a low top speed (55-60mph) and slow performance. I'm not trying to build a hot rod. I'll only be going on the highway perhaps twice a month for a few miles.

My main challenge is cost, I'm not flush with $$ right now so I want to keep costs low, I don't mind putting more labor and hassle in to keep costs down.


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## DavidDymaxion (Dec 1, 2008)

I'm not clear here, if you are using a controller isn't that the controller's job to limit the current to some value? Or do the model airplane controllers not do that?

I assume you won't be running at 1000 A very much, 48 milliohms * (1000 A )^2 is 48 kW of wasted energy! I'd suggest minimizing resistance as you can, and then have a starting resistor you can switch in briefly to start.

Another random thought: You could put the 3 motors in series to start (a "first gear" of sorts) and switch them to parallel for higher speeds (a "2nd gear" of sorts) -- this would take a number of expensive contactors, though.


bart_dood said:


> Hi Folks, I'm new here and have a question on cable and connector resistance.
> I'm trying to figure out a very inexpensive bug conversion with low cost in mind I have the idea of using 3x brushless DC motors (very large 9.3 horsepower model aircraft motors) in parallel together with 3x water cooled brushless controllers.
> 
> As far as mechanics of hooking all this up, I'm a design engineer with extensive experience so it won't be a problem. I also have electronics experience but I'm not an expert.
> ...


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## bart_dood (Sep 2, 2010)

DavidDymaxion said:


> I'm not clear here, if you are using a controller isn't that the controller's job to limit the current to some value? Or do the model airplane controllers not do that?
> 
> I assume you won't be running at 1000 A very much, 48 milliohms * (1000 A )^2 is 48 kW of wasted energy! I'd suggest minimizing resistance as you can, and then have a starting resistor you can switch in briefly to start.
> 
> Another random thought: You could put the 3 motors in series to start (a "first gear" of sorts) and switch them to parallel for higher speeds (a "2nd gear" of sorts) -- this would take a number of expensive contactors, though.


Hi David, yeah as far as I know most airplane controllers don't control current like this, when you start spinning a prop the current is low and increases with rpm reaching some maximum value. I am doing some more investigation on the endless sphere forum as some of those guys have made heavy bikes and small modeds with these motors and have run into this problem before.
Another option might be to use ebike or emoped controllers which require fitting of hall sensors to the motors, these seem to be able to limit the current to a pre-programmed level.

Even if I find I can use a regular airplane motor control I'm going to add liquid cooling to it, it will help reliablity in the long term and I could even use the heat as a rudimentary cabin heater!

1000amps would be a momentary peak so very very short, but this peak could be enough to blow the fets on the controllers. My reading indicates adding more capacitors to the controllers helps them deal with tiny massive current spikes more effectively too.

Thats a good idea about switching from series to parallel, I thought about a pneumatic contactor that would not require wasting electrical power through an energizing coil but rather use some little pneumatic cylinders to switch them in and out.


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## bart_dood (Sep 2, 2010)

Ok, I have found what I think is a better option instead of these model aircraft motors. I'll do some more search and report back...

In the mean time I'm picking up a used bug transmission that I can work on in my shop, I might recondition it and I can work on designing adapters for the motor(s) etc.

Now I have to figure out how much power I need, I calculated 20 horsepower to do 60mph in a bug on flat ground, no wind. Although in the short to medium term I won't be using the highway much at all, when my first set of lead acid batteries are dead in a number of years I might want to switch out for something more modern, I might want to use the highway more then.

Does 20hp/60mph in a bug sound right?


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