Wiring loom resistance vs battery internal resistance?

[jimmyhackers]

I get that a lower resistance value is better in both scenarios, but are they equal in detriment to performance?

So i ask, is 2mohm of resistance in the main battery wiring loom going to generate the equvalent detriment (voltage sag) as 2mohm of internal resistance inside that lithium battery?

just wondering if upping my cable AWG again might eek out a bit more performance/efficiency/range?

i previously went from 14awg and xt60s to 10awg and xt90s. but i could get some 8awg or 6awg even.

im running 3x 6s 20ah eig cell packs in series for 72v. no bms.
the loom attaches to the controller with one main xt90 and branches off 3 ways to three xt90 connectors for each of the three 6s batts
there is roughly about 0.5m of paired loom wire. and roughly 6-7cm of paired wire on each battery.

Also, Im using the xt90 anti-spark variants now, does anyone know if the reduced contact area of one pin effects thier performance compared to thier vanilla variety?

 

[larsb]

is 2mohm of resistance in the main battery wiring loom going to generate the equvalent detriment (voltage sag) as 2mohm of internal resistance inside that lithium battery?

Yes, but make a calculation of the possible improvements before doing anything, if it’s 10awg already then probably the difference to 8awg isn’t large.

The connectors are normally negligible in the total resistance if sized properly, i would probably go to xt150 or even hxt 8mm bullets if you’re running up to a hundred amps

 

[Wattman]

Personally, I would be more concerned about heat and safety than performance difference, the reason wire gauge matters. Performance delta as larsb alluded is modest, mostly manifests in heat emission.
Controller and motor size matter when it comes to wire gauge. Battery spec's only aren't the full determinant of wire gauge.
You didn't post your motor and controller spec's.
But let's say you have even a mid to upper range strength ebike with your 72 volt battery that can pull 30 amps. which is over 2000watts.
You can even extrapolate from household applications if you have ever pulled any wire in your house. Length of wire matters as well.
But standard code is, 10 gauge wire can accommodate a 30 amp breaker until you reach 150 feet. At 150 feet or more, you should use 8AWG or 6AWG wire.
Length of wire will be much shorter on an ebike for example which reduces resistance. But for a powerful battery and powerful motor and 30+amp controller, thicker wire, than 10AWG is best. 10AWG is ok for more common 30 amps.

 

[DogDipstick]

Yeah. But its negliegible. The pack on my bike is 23 mOh, measured, from + to -. The whole thing, form controller (past the fuses , contacts, lugs, ) it is 25 mOh.

My cells are .. 1mOh dead on each.

the bus and wire and lugs and contactors are .. less than that combined. Contactor is 0.2mOh ( two tenths ofg a milliohm)... ( not alot of dissapate waste) ( pack heats up first).



Good luck chasing tenths of milliohms. I bought a 1000$ camera just to do this. Dissapate power is very easy to measure given the V and the Vdrop over the current. This is converted tow BTU in my hed by the formula 3.18 btu/hr is 1 w.

Wires get warm. Just dont go full stupid and push 250A thorug a ... 12g wire. Lol.

My pack heats up pretty good.. 180A out from the battery . Main leads are 6awg. Somewhere round 12kW. The other big Kelly I have run, did 18kW. 280A out. That one I used 4awg. Just built a Sabvoton 5kW kit motor Super73 with a Sab 72150 and it did.. 150A out 6awg.



Yeah they get a lil warm. Battery gets hotter tho.. even thoug its a 10C + pack with high quality new cells.. like a 10p of a Molicell p42a pack IR... 1mOh per group.,

 

[Wattman]

You build some really cool high power ebikes DDS. Very interesting post.
Thanks for sharing your experience from the big power ebike crowd.

 

[docw009]

That 25 milliohms in a battery is a model though. Voltage sag is electrochemical,, at some nonlinear rate as the cell IR goes up.

I made a 10S-5P ebike pack with Panasonic 22P cells that came in at a tired out 42 milliohms. You would figure 82 milliohms for that, and experimentally I get 93 millohms, using a YR1035 AC impedance meter. So far so good.

At 20 amps, the IR voltage sag should be less than 2 volts, In reality, I see 6-8 volts show on my handlebar meter. The 30V LVC kicks in if I pull throttle at 35V or lower. That's OK, as I can still pedal home without it, Probably should have scrapped those old cells though.





.

 

[DogDipstick]

You build some really cool high power ebikes DDS. Very interesting post.
Thanks for sharing your experience from the big power ebike crowd.

Thankyou. Here is a pic of the Super73. I am going to post a thread about this winter project bike soon with lots of pics, it was a fun build. I do like building bikes. I have a few more in the works too. Stay tuned if you like those kinds of things.

This bike was right around 90 lbs. Alotta fun. Lol.

 

[jimmyhackers]

my bikes hub motor is a 9c chinese clone hub motor, was sold originally rated at 1000w 48v, 26" wheel.
the controller is a 1500w hksunwin 48-72v controller.

my amp meter was a bladed fuse holder (when i got it over 7ish years ago), if i remember correctly ive managed to pop a 35amp fuse but not a 40amp..... so its somewhere in that ball park, weirdly close to double its rating.

i have had a look at awg resistance and got fed up most charts were in ohms per 1000 feet and not meters
after doing many conversions ive guesstemated i could loose between 0.6mohms and 2mohms from my loom if i went to 6awg and shortened it as much as i could.

i couldnt find any relevant resistance figures for xt60 90s or 150s.

when i went from screw terminals to soldered connections in my batteries...i dropped 1-2mohms per cell and the volatge sag reduction was quite noiticebale along with the performance increase. but thats 1-2mohms per each of the 18 cells so a 18-36mohms drop in total.

i guess going 6awg will make a difference.....but wether i will notice it as much or at all.....i probably wont.

 

[larsb]

Nope, you won’t notice that as the total resistance is a lot higher

 

[jimmyhackers]

true
rule of thumb 5% and below is not percievable on the "butt dyno". my 18s battery with about 5-6ohms per cell works outs to 90 - 108 mohms total batt.
this is one of the three 6s 20ah on my isdt charger, 34mohms total... the other 2 are similar.

34 multiplied by 3 is 102mohms total. so yeah....id have to drop over 5mohms for it to be noticeable.

i am bored and have way to much free time on my hands though