Building myself some new 2S charge leads, but I'm short on some 18 AWG wire for the balance connections. Primary leads will be 12 AWG silicone jacketed. No one has any more cable locally, and I'd like to build this week if possible. Leads are ~ 3' long, charging at no more than 15A.

My options are:

1) Build the three balance wires with a mixture of 18 AWG and 20 AWG silicone jacketed wire. This makes me nervous because I can't help but think the difference in resistance per unit length of the different wire gauges is going to produce a different voltage drop over the length of the cable, and lead to poor balancing of the cells.

2) Use 20 AWG wire, some silicone jacketed, some regular. I'm not too concerned about the flexibility of the cable, since this is the smaller gauge wire, but see above for concerns re: different voltage drops over length of differently constructed cable.

3) Take the ~+7.4V and 0V connections from the charger end and just run the intermediate connection down the length of the lead. This is the extreme solution, since now two leads see 3' of 12 AWG wire, and the intermediate sees 3' of 18 AWG.

Am I being wreckless? Worrying unnecessarily? Any help appreciated!

Balance currents are much lower than charge currents so any voltage difference will be in the noise. For example, my i306B balances at 300mA. I know some of the newer charges may have an amp or two for balance, but still probably negligible voltage difference. You cam estimate it yourself by looking up the resistance per 1000 feet for the different gauges. Assume 1 foot and 1 amp. V= I*R (ohms law). You will probably have more drop in the balance connectors than in the wire.

Even 20AWG is probably overkill for balance leads. Note that balance leads from progressive are made with 22 AWG.
https://www.progressiverc.com/balance-extensions.html

here, I did for you.
1 foot of 18 AWG is .0063 ohms so the drop with one amp is 6.3mV
1 foot of 22 AWG is .016 ohms so the drop with 1 amp is 16mV

So at max balance current the different would be about 10 mV. At 300mA balance current, the drop difference is only about 3 mV. So as the balance current ramps down, the difference is very small.

You forget the resistance in the solder joints and connectors....

At the end it does not matter.

At 80% of charging the current will go down and at the end it is below 0.5A
At that time the loss in the wire is no factor.

I've been using cheap speaker wire from the hardware store for years on my charge leads, I charge up to 20A on 4S without any issues.

I would stay away from the 3rd option. I used to build my leads like that because it was cleaner looking. Once I started using an icharger and looked at my IR, it was reading around 45 milliohms. Once I ran 16awg all the way to the positive and negative from the balance lead, my IR read under 2 milliohms.

Thanks all. Knew I was overlooking something, just wasn't quite sure which side of cautious I was leaning to!

Got the first lead built up with the 18/22 mixed balance leads & all works fine. I'll have time to buy more cable before my set of bullets come in for the second lead.

Similar situation here. Built a 1s cable for a buddy where the balance leads connected right to the charge leads where they terminated at the charger. Upon first use he was concerned his new Reedy Zapper was crap because his iCharger was showing over 13 milliohms IR. I plugged his battery into my charger, using charging cables that have separate balancing leads all the way down to the battery plugs, and saw a little over 2 milliohms IR. Plugged my cable into his charger and saw the same ~2 milliohm IR. Lesson learned. I always build charging cables with all balance leads terminating at the battery.