Effects of Cable Length

[John in CR]

After reading a post about 8" or less cable length necessary for a high powered RC motor, I worry about the length of all of my wire runs. What are the specific effects related to cable length of battery cables, phase wires, and hall sensor wires? Obviously resistance turns into heat and limits the maximum available power, but can it affect performance in other ways? Should we twist or braid our hall sensor wires, does cable length affect the controller's communication with the motor, etc?

I ask because when I rewired my hub motor I installed 2m long cables, because I'm treating my current bike as a test setup and don't know where my motor, controller, and batts will eventually reside, and I don't want to have to splice wires later by cutting them short now. The excess currently resides in large loops. I used straight runs of 24ga for my halls, because twisted pairs of cat5 wouldn't fit through the hole in the hub with the 14ga phase wires I settled on. The result is all 8 wires to the motor run parallel for 2m inside plastic tubing all the way to the controller thru a few loops tied in front of the handlebars (please ignore that obvious increase in wind resistance).

On the battery side, with 72v going into an Ecrazy controller powering a non-geared 500W hub I got from Knuckles, I'm running zip cord about a meter long, 2 conductors for positive and 2 for negative. I realize that I should beef that up a bit, but I bring it up because I wonder about the effect of the several loops I have around the seat post along the way.

Do all these cable no-no's add up to a noticeable affect on performance, or should I not worry about it and shorten the cables and run bigger battery leads later?

John

 

[Link]

If you've got 12ga or better, don't worry about it. R/C motors are pulling way more amps than we do; up into the hundreds.

Obviously the biggest concern is losses to heat. From a few calculations when I was working out what I was losing to heat, I found that (for 10ga) anything less than about 20ft round trip wasn't really that big of a deal for 30A or thereabouts. Something like that, anyway.

 

[docnjoj]

The 2 // 14ga=about 11ga.
otherDoc

 

[ZapPat]

What can kill RC brushless controllers is not the wire's resistance, but it's inductance. As the FETs in your controller turn off and on, they demand high currents very briefly, and this will cause voltage spikes on your DC bus (ie battery input). If there are not enough good capacitors close to the controller's input to cancel the added inductance off long battery cables, the controller may have it's FETs or capacitors blow from these brief overvoltage spikes.

However, RC controllers (ESCs) switch their FETs faster than regular ebike controllers, and thus the ESCs will have this problem much more often than ebike controllers if long battery cables are used. I even wonder if anyone here has ever blown their ebike controllers because of this phenomenon?

Long motor cables I do not think are such a big deal, since the motor already has high inductance and thus will not be affected much by added inductance. You will loose efficiency, but will not blow anything.

One thing to do to lower the inductance of your battery cables is to keep both positive and negative side by side, as close together as posible, and even twist them together if you can. You can also add some good capacitors across the battery input to cancel out part of the added inductance of your cable.

 

[Ypedal]

The 2 meter of wire is not a huge deal, but pay attention to the connectors.

I found that the crystalyte stock of imitation Andersons ( yea.. they use knockoffs.. Knoxie mentioned this a while ago. and i was going to call him a liar until i took a few apart and sure as shit.. )

If after a ride the wires get noticably hot *( feeling by hand ) then it might be the connectors.. but otherwise unless you get into the mucho amp setups...

 

[John in CR]

Thanks guys,

So no worries about "cross-talk" or whatever it's called with the parallel run of all 8 motor wires?

Keep battery +/- cables close to each other...it's good to learn something every day. Exactly why is that recommended, since I'd naturally want to keep them as separate as possible reduce risk of short?

John

 

[johnrobholmes]

It reduces the EMF throwoff of the wires and reduces the inductance too. Twisting DC cables is a good idea for any application, but high current setups really benefit.

 

[wrobinson0413]

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[D-Man]

Shoot, I got at least 12 feet of wire length running from my batteries to a front mounted controller. I'm not shortening anything!

 

[John in CR]

Does thicker battery cable reduce the inductance issue? If not and long runs are unavoidable, what type of cable construction characteristics create greater inductance? I've got 20 and 24ga magnet wire and could make my own power cables with multiple strands and a protective wrap, or would multiple cheap zip cords be fine?

This inductance issue lead to an off topic question. Soon I'll be running 2wd with front and rear hub motors, using 2 controllers and 2 throttles. Should I keep their batteries separate too, so the controllers can't see each other's voltage spikes?

John

 

[wrobinson0413]

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[John in CR]

I'm noticed that the first cell at the + end of my lifepo4 packs is hotter than the rest. Is that because it is bearing the load of clamping these voltage spikes? I'm currently running 72v (actually 69 due a a dead cell), and I have some room near my controller, just not enough for my big packs. Would it make sense to add a 6v booster pack up very close to the controller, say a 4x5 using some nicad D cells that I have? Does the negative side see these spike issues too, ie do I need a pack with a short wire on the negative side too?

Does this make any sense... Essentially a very short wire to the + side of my series of batteries, and then a long cable involved in some of that series connection? Would it even help at all, other than giving me a bit of extra voltage?

John

 

[ZapPat]

ZapPat is correct about the inductance being an issue for the extended cables, though the solutions are not quite correct. [...]

Hummm... :? I don't see where we differ in solutions, except I did not talk about the hall cables (I've never hear of anyone having problems with induced noise in these).

 

[ZapPat]

I'm noticed that the first cell at the + end of my lifepo4 packs is hotter than the rest. Is that because it is bearing the load of clamping these voltage spikes? I'm currently running 72v (actually 69 due a a dead cell), and I have some room near my controller, just not enough for my big packs. Would it make sense to add a 6v booster pack up very close to the controller, say a 4x5 using some nicad D cells that I have? Does the negative side see these spike issues too, ie do I need a pack with a short wire on the negative side too?

Does this make any sense... Essentially a very short wire to the + side of my series of batteries, and then a long cable involved in some of that series connection? Would it even help at all, other than giving me a bit of extra voltage?

John

For your cell heating issue, I could not tell you what is causing this, but there was a thread not long ago in the battery section where a number of people also observed this phenomonon. However, it is almost assuredly not the voltage spikes/current pulses causing this (to my knowledge at least).

Adding small booster packs as you suggest would not be a solution, and yes, this would only give you extra voltage. The total length (actually the area) of the cable loop from controller negative through the batt(s) back to the controller positive is what counts. And I guess in theory any ferro-magnetic metal right by your cable might also raise your inductance (like a steel bike frame), but I have never measured this, so in practice I couldn't say if this would have a notable effect or not.

 

[Pete]

I would find a wire that is rated for the max input of the controller then downsize it by one gauge. For example, if you find that 14 gauge meets your rating, go to a 12 gauge wire instead.

Wouldn't that be upsize then?

 

[johnrobholmes]

Smaller AWG numbers (8 vs 9) are larger diameter wires.

 

[deardancer3]

Another fellow and I were working on an ebike project together a while back.

One problem that we addressed was his blowing up voltage regulators for running ebike accessories.

Since I had a similar ebike system, I put an oscilloscope on the various wires. To my surprise I found wlld voltage swings, fairly high frequency spikes on the Battyery plus line at the brushless controller; on a "constant" 26v input we had + spikes reaching over 50V and negatives going below ground. about 20 microseconds in width if I remember, but dont quote.

This was a 24v Cyclone internal controller, but am suspicious that this might also be true of other controllers.

Anyway, adding noise suppresion caps of sufficient size and voltage right at the controller incoming voltage line, and taking his accessory voltage tap closer to the pack, solved his problem.

I am curious if adding these caps might also help with longer term reliability problems on other ebikes systems, and, might be a clue as to why the first cell in a pack might be reunning warmer.

And of course help with this discussion about wiring and noise rejection.

D

 

[John in CR]

I'm noticed that the first cell at the + end of my lifepo4 packs is hotter than the rest. Is that because it is bearing the load of clamping these voltage spikes? I'm currently running 72v (actually 69 due a a dead cell), and I have some room near my controller, just not enough for my big packs. Would it make sense to add a 6v booster pack up very close to the controller, say a 4x5 using some nicad D cells that I have? Does the negative side see these spike issues too, ie do I need a pack with a short wire on the negative side too?

Does this make any sense... Essentially a very short wire to the + side of my series of batteries, and then a long cable involved in some of that series connection? Would it even help at all, other than giving me a bit of extra voltage?

John

For your cell heating issue, I could not tell you what is causing this, but there was a thread not long ago in the battery section where a number of people also observed this phenomonon. However, it is almost assuredly not the voltage spikes/current pulses causing this (to my knowledge at least).

Adding small booster packs as you suggest would not be a solution, and yes, this would only give you extra voltage. The total length (actually the area) of the cable loop from controller negative through the batt(s) back to the controller positive is what counts. And I guess in theory any ferro-magnetic metal right by your cable might also raise your inductance (like a steel bike frame), but I have never measured this, so in practice I couldn't say if this would have a notable effect or not.

Though I don't understand, I do follow the "wire loop". 2 things
1. Is making the frame the "-" side cable a good idea?
2. If I make two 3v booster packs to go in series with my main packs, and use very short cables to attach the boosters to the controller, this would essentially put the entire area of the "loop" inside my battery. Does this accomplish anything wrt the inductance issue or are caps the only solution with my given controller and pack placement? Batts I have, but no caps in the cupboard, plus a little boost means more fun and will give my front wheel drive a bit more traction.

John

 

[ZapPat]

Adding small booster packs as you suggest would not be a solution, and yes, this would only give you extra voltage.
Though I don't understand, I do follow the "wire loop". 2 things
1. Is making the frame the "-" side cable a good idea?

I would not do this, two main reasons come to mind why not: Iron is a bad conductor (high resistance), so it wouldn't help much. Also I think it's safer if your frame is floating in terms of the electrical circuit of your ebike, so that if you touch any positive wires by accident to the bike while working on it, you won't blow anything.

2. If I make two 3v booster packs to go in series with my main packs, and use very short cables to attach the boosters to the controller, this would essentially put the entire area of the "loop" inside my battery. Does this accomplish anything wrt the inductance issue or are caps the only solution with my given controller and pack placement? Batts I have, but no caps in the cupboard, plus a little boost means more fun and will give my front wheel drive a bit more traction.
John

I'll quote myself here... :wink:

Adding small booster packs as you suggest would not be a solution, and yes, this would only give you extra voltage.

The loop is everything that is in the series connection of the battery bus, including any internal wires in your pack no matter how long or short they are. Hey, even the cells have inductance! You could imagine inductance as stored magnetic energy, both around your conductor (ie battery wires) and additionnaly in any nearby ferro-magnetic objects adjacent to the wires (like the bike frame, unless it's aluminum).

Capacitor suggestion if needed:

These caps are easy to get from digikey, and are cheaper than equivalent low-ESR panasonics.

 

[John in CR]

Thanks Zap and everyone else. This is exactly why when I rewired my motor that I installed 2m wires and didn't cut them...just in case. Time to move my controller back next to my batteries. Am I correct in thinking that short battery cables instead of long ones that loop around the seat post a few times will reduce my battery sag due to decreased resistance and inductance. My sag seems excessive though I haven't found quantifiable info on how much to expect. I'm running Ping packs totaling 69v and 15ah, and at rest voltage is typically 76 volts. Even with a fairly fresh charge, high loads occasionally trip the 61v LVC of my 72v Ecrazy controller. I realize I need to mod that LVC to run my packs down anywhere near full capacity, but I want to get most things right before trying any long distance runs.

John

 

[wrobinson0413]

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[Link]

I probably wouldn't wrap the cables around the post because you are adding to the inductance by doing that. Just look up the formula for the inductance of a solenoid.

I wouldn't worry about it. The cable on my BMX is the one I used to use for the MTB. It was way too long, so I had to wrap it around the frame at one part. I haven't noticed any problems.

 

[wrobinson0413]

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[ZapPat]

Thanks Zap and everyone else. This is exactly why when I rewired my motor that I installed 2m wires and didn't cut them...just in case. Time to move my controller back next to my batteries. Am I correct in thinking that short battery cables instead of long ones that loop around the seat post a few times will reduce my battery sag due to decreased resistance and inductance. My sag seems excessive though I haven't found quantifiable info on how much to expect. I'm running Ping packs totaling 69v and 15ah, and at rest voltage is typically 76 volts. Even with a fairly fresh charge, high loads occasionally trip the 61v LVC of my 72v Ecrazy controller. I realize I need to mod that LVC to run my packs down anywhere near full capacity, but I want to get most things right before trying any long distance runs.

John

Reducing your cable will not reduce your sag problem much, since most of it comes from your battery likely. You might get an extra volt at maximum current...maybe! As for the inductance reduction, it will help your controller live a bit longer, maybe heat a little less, and slightly reduce the chance of a blown FET. I am curious to see how wrobinson's test with added caps gives...

As for your voltage sag, I'm sure it's mostly your battery, since these cells are not A123's or power tool cells either. I do not know how much DC internal resistance version 1 Ping cells have (he now uses better ones supposedly, and I'll post about them when I receive them soon), but as an example I'll use the headway cells that have been tested. The latest test done by RLT gives about 17 milli-ohms DC resistance (not too good). So lets say you run your pack at 1.5C - for a 15Ah pack that's 22.5 amps. Now 17mohm times 22.5A gives a voltage drop of 0.38V per cell. 24 cells in series would then be a drop of about... 9 volts. But since your pack is 15Ah, and headway cells are each 10Ah, then it would be more like around a 6 volt drop, so that looks kinda like what you are getting here. Of course some people seemes to have headway cells that only have an 9mohm DC resistance (I could not confirm this), so this would cut the drop by about half down to about 3 volts. But I have yet to see tests done on a few PingVer1 packs to determine their internal DC resistance, so I don't know if your own V sag is "normal". One thought is you might have weaker cell(s) in your pack - you would have to check each cell's voltage under load to see this.

 

[wrobinson0413]

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