Direct Soldering your batteries
OK, here we go again. I try every once and a while to dispel this myth, but no matter what I do, this one won't die.
The resistance of 12AWG copper wire is about 0.0002 ohms per foot. 14AWG is about 0.0003 ohms. The resistance from the end of the wire to the end of the cell, thru the battery bar and it's 2 joints, is about 0.00025 ohms, mainly because only about 20% of the racers I see can solder properly, but that's a different story. A perfectly soldered hardwire connection will have about 0.0015-0.0002 ohms of resistance, end to end. About the same as an inch of 12AWG wire.
The resistance from a properly terminated Deans or Corally style connector is about 0.00002 ohms.Yes, they're both LOWER than an inch of 12AWG wire (0.000025 ohms).
Let's do some math.
At 20A (stock class here, with a fast track), tells us we'll get about 0.005V lost across the average (poorly soldered) direct wire method, the same as a foot of 12AWG wire. That's 5 thousandths of a volt. with 7.2V (give or take) coming out of your pack, it's 0.0694% of your total voltage. Seven hundreths of a single percent of your energy.
The loss in the connectors will be a lot lower, on the order of 0.0004V. Including solder joints. 0.0055% of your nominal pack voltage. Fifty five thousandths of one percent.
In engineering terms, it's nothing. it's less than your speed control. It's less than a Tamiya connector by several orders of magnitude. It's less than the bad solder joint the guy in the next pit just made while hardwiring his pack. It's absolutely lost when you take into account the inefficiency of your motor.
You'll lose more track position by driving badly or moving an inch and a half outside the perfect line than you will because of the loss of your connectors. If you can't solder, or use a foot and a half of wire in your car, it's your own fault. The only conceivable reason to hardwire your packs is so that in the event of a crash, you won't get disconnected. Funny thing, I'm a really bad driver from time to time, and the only time I had a pack get disconnected was when I lost a retainer clip and the pack came out of the car.
Regarding soldering, if the joing isn't shiny and smooth, like a little drop of polished silver, it's wrong. If it looks dull, its wrong. If it looks crystalized, it's wrong, If it's lumpy it's wrong. If you can't see yourself in it, it's wrong. If it's wrong, you're losing more voltage than you ever would with good connectors.
This applies to battery bars, ESC conenctions, motor connections, and every other place you put solder in your car. Think about it.
Oh, and when you solder batteries, if it takes more than 3 seconds to get the solder on the end of the cell to flow and the joint to be happy (battery bars, wire, anything) you need a bigger iron, because you're just killing your batteries. I use a 350W gun to make packs. It takes 2 seconds to solder a bar, and my cells are cool to the touch when I pull them out of the jig.
My $0.02.
-dave