Answers for Newbies

[drsolly]

I'm no expert, buit I was a newbie recently enough that I remember some of the questions I had trouble with. So here's a few answers.

Q1. What multimeter should I buy?
A1. Anything. I just did an Ebay search, the cheapest one that came up was about $4, and it looks to me to be just fine for diagnosing e-bike issues or anything else I use a multimeter for.

Q2. What's a C value?
A2. If you buy a 5 amp-hour 20C battery, that means that it can give you 100 amps under load (or so the vendor claims). As in so many things, vendor claims are sometimes optimistic, of course. Another exaple - a 3 amp-hour 40C battery should be able to give you 120 amps.

Q3. What's 4s2p?
A3. 4s means four cells in series - if they're Lipo, that's 3.7 volts each (nominal) so 14.8 volts (nominal). When fully charged, that would be 4.2 volts per cells. The 2p means two of these in parallel, thereby doubling the capacity. So you can now see what, for example, 12s3p would mean.

Q4. How thick to my wires need to be?
A4. That depends on how much current they'll carry. And on other factors, such as how easy is oit for them to lose any heat they get. Here's a table I found:
8AWG 200 amps
10AWG 140 amps
12AWG 90 amps
14AWG 60 amps
16AWG 35 amps
18AWG 20 amps
20AWG 12 amps
22AWG 10 amps

Here's another table I found:

8AWG 95A
10AWG 65A
12AWG 45A
14AWG 32A

Q5. What's the relationship between volts, amps and watts?
A5. Watts = amps times volts

Q6. What's the safe way to handle Lipos?
A6. Read this: http://www.icharger.co.nz/articles/ArticleId/3/Lipo-Lithium-Battery-Safety-Guide.aspx and this http://www.mpoweruk.com/balancing.htm

Q7. Is there an FAQ?
A7. http://endless-sphere.com/forums/viewtopic.php?f=3&t=26488

Q8. Is there a wiki?
A8. Yes, here: http://www.endless-sphere.com/w/index.php/Main_Page

 

[Wishes]

Your first wire gauge to current chart is way off. You will melt those wires if you sustain that kind of current on those guages for any period of time.

The second chart is accurate.

Wishes

 

[wesnewell]

Use the Maximum amps for chassis wiring from the chart here and you can't go wrong.
http://www.powerstream.com/Wire_Size.htm

 

[drsolly]

I agree, it seems to be a bit high. That's why I gave two tables. My source for the first table is here:

http://endless-sphere.com/forums/viewtopic.php?f=35&t=55422&start=50

When it was posted in that thread, no-one seemed to disagree with it. But here:

https://endless-sphere.com/forums/viewtopic.php?f=14&t=57677

When the same table was posted, knowledgable members disputed it strongly, recommending cutting it in half or a third.

There's also this:

http://www.rcgroups.com/forums/showthread.php?t=654870

The poster argues that model wiring isn't the same as house wiring; the run length is a lot less, and there's usually much better cooling. And he comes to 55 amps for 14 AWG wire.

Here's another interesting table:
http://www.viaircorp.com/wiring.html
That is saying that a five foot run to carry 60 amps at 24 volts needs 12AWG, whereas a 20 foot run would need 6AWG. Since an ebike run would be round about two feet (battery to controller), they'd use 12AWG for 60 amps (and 14 AWG for 50 amps). Amps capacity, they're saying, depends on run length, and I would guess that most residential standards are thinking of 60 foot run lengths, not the ebike two foot run length.

This site:

http://www.powerstream.com/Wire_Size.htm

as well as recommendations for maximum load (at 14 AWG it suggests 32 amps) has a handy calculator. Using that, I see that a two foot length of 14AWG run at 24 volts, 32 amps has a voltage drop of 1.38% and is consuming about 10 watts of your power, whereas 10 AWG would be consuming about 4 watts.

Personally, I'm using 4 sq mm wire, which is approximately 11AWG and I'm pulling a peak of 32 amps, safely under the 47 recommended in that table.

I'm left a bit puzzled about the output from the controller to the motor. Apparently, that would be about 2.5 times the amps input, so is you have 32 amps going in, that's 80 amps going to the motor. Yet the three phase wires are not thicker compared to the wires to the battery. I guess the difference is that the current through the phase wires isn't continuous, and it's switched between the three wires, so the relevant thickness would be the combined thickness of the three wires. So, if there's 32 amps at 24 volts from battery to controller, that's 768 watts, and (there's some small losses in the controller and wiring) that means that the motor is getting 750 watts or so, and since that's also getting the same voltage, it must be getting an average of 32 amps through the three phase wires combined, so an average of 10 amps per wire, which is why they can be the same size as the battery-to-controller wires

This is obviously a very important safety issue; maybe this is worth a bit of discussion, and maybe even some experimentation, since an actual test trumps any debated value. Does anyone have the facility for doing a test of wire guages in a realistic e-bike situation?
Meaning, a run length of two feet or so (battery to controller), and wiring that's attached to the bike in a typical ebike way. So you'd take the length of wire, run it alongside the bike in the usual way (perhaps cable-tied to the frame), run X amps though it for five minutes and measure the temperature increase. And make a table for the temperature increase for various values of X, for each wire guage. A nice reproducible scientific experiment.

From that, we could draw up a simple table of "recommended maximum current" (which maybe allows heating of 20 degrees?) and "redline maximum currnet" which would allow a lot more heating without actually melting the insulation (which, I notice, is 200 degrees for silicone insulation).

 

[teslanv]

I come from a construction background, and did my own electrical work on my house. In Residential AC systems, by code, you cannot have more than 20A in a 12ga wire and 30A in a 10ga wire - Until I read up on the forums, and on ratings by the manufacturers, I was apparently being way conservative on my wire sizes. Needless to say, a 12ga appears to be good for <40A systems, and 10ga wire for most <60A systems will be sufficient. I have a 25A controller, and run 12ga. wires "just because"

 

[TroySmith80]

Does voltage have any impact on wire guage requirements? It seems like it would be the amount of power that would be important, not just current. I guess not though, that's why electricity is transmitted long distance at high voltage and low current, and why volting up helps to get more power with less heat in hub motors.

 

[wesnewell]

50A constant will melt the sheathing of most general use 12awg wires in a few minutes. I wouldn't use it on anything more than 40A. Nor would I go over 55A constant on 10awg for the same reason. 20A will melt the plastic coating and turn 24awg telephone wire red in no time. Been there, done that. Just use the rates in the table I posted if you want to be safe. They are within safety limits.
The only impact voltage has is the percentage of voltage drop. The amount of voltage drop remains constant at all voltages. There's a nice voltage drop calculator below the table here.
http://www.powerstream.com/Wire_Size.htm