Originally Posted by
beemerfan
Not trying to drum up a new debate here, just adding to the general knowlege base. Since the C rating thing has come up so many times in the past, I bought one of
these gizmos from HobbyPartz that measures wattage and current draw. I believe it is fairly accurate and I wanted to measure the actual peak draws of a VTA car. We've figured in the past that the average is about 10 - 12 amps but the peak draw is where the C rating comes in to play. So the results...
About 20 amps!
I started by doing several burnouts to simulate a race start or when you get going after a mishap. The meter actually maxed out at 29.9 amps for that.
Then I did 3 runs with smooth rolling starts to avoid a big draw and ran about 5 to 10 laps per run.
Run 1: 19.5 amps
Run 2: 20.4 amps
Run 3. 19.6 amps
This was on a big track using a Novak GTB speed control, a Ballistic 25.5 motor set to 45 degrees of timing and geared at 3.32 FDR.
This means you need a 6C 5000 mAh battery to cover those peak draws of hard acceleration and a little more than 4C everywhere else. Combine this with Darkside's recent test of his Reedy 25C Wolfpack outperforming a much higher rated pack, and it's a pretty strong argument that C ratings don't matter in VTA. 20 to 25 C packs are more than good enough for this class.
One key thing missing from your assessment is the voltage under load. Based on what you said, yes, a 6C pack would survive being run in a VTA car. However, a pack with a higher C rating (assuming the rating is accurate and not just marketing fluff)will maintain a higher voltage under the peak loads. The motors rpm is based on the voltage input to them. So the pack that can maintain the higher voltage under load will give faster acceleration and higher top speeds. It may be less noticeable if the amp draw is fairly low but there will still be a difference.
A pack with a higher true C rating will always have an advantage over a lesser pack. The gap narrows in the lower current applications like VTA but its still better to have a stronger pack.
It looks like that device you are using can measure peak watts as well as peak amps. Watts are V x A, so at the same amp draw a pack that holds better voltage would output more watts(power). Thus faster acceleration and higher top speeds. It would be interesting to compare and old 20C pack to one of the new 60+C packs. I'm sure you would be able to measure the difference.