Does anybody have a link to a runtime calculator? I am working on a highspeed 8ight 2.0 project and I am trying to select the batteries for it. I am thinking since this will be used for max top end speed I could go with smaller bateries for lighter weight but I know runtime will suffer as a result. I just want to know by how much. Running 2 3s 5000 mah batteries would be nice but they are very heavy (about 30 ounces) so I was thinking of getting a couple 2500- 3000 mah batteries(18-20 ounces) and saving a considerable aount of weight. but If I would only get 5 mins of runtime i won't do it. Is there such a calculator for cars?
Thanks

runtime is capacity / current = time. Even though we don't know a lot of actual numbers for your set-up we can figure that the 2500s would run just about half the time as the 5000s.

Also, consider the amount of current you will be pulling. Since this is a high speed project, chances are you are geared quite high and tall gearing will pull more current. Those ~3000mAh cells better have one heck of a high true C rating to be able to deliver the high currents. As current=mAh*Crate, the lower in mAh you go, the higher the C rate needs to be.

Drive Calc. It's a free online performance predictor. Takes a bit of learning to use it, but it is extremely accurate. Just google it.
Walt

Unless I'm mistaken, Drive Calc is mainly intended for aircraft. Of course, a motor is a motor, but aircraft tend to be much more predictable because the current draw is more even with very few bursts. When dealing with land-based vehicles, there are a LOT of variables to consider; drive style, terrain, vehicle weight, tire style, etc, etc. And cars are much more "peaky" for current draw.

The easiest way to get a rough approximation is to run whatever battery you have right now while timing the run, and use this (http://www.scriptasylum.com/rc_speed/power.html) to re-figure it using different battery configurations.

For instance, if you get 20 minutes of runtime on a 6s 5000mAh battery, your estimated power usage is 15A average with 90A peaks according to the link above. If you go down to a 6s 3000mAh battery, the runtime will be reduced to 12 minutes, and you would need to make sure the battery has a true C rate of 30C or more for those 90A estimated bursts. Note that since the 3000mAh pack weighs less than the 5000mAh pack, your runtime will be slightly higher than 12 minutes, but it at least gives you a rough idea.

Thanks for the replys. I downloaded the drive calculator but there are too many variable that I wouldn't know how to convert to a surface application so I don't think that will work much for what I am trying to figure out. also I did check out the http://www.scriptasylum.com link and they have some good info there. not just the power output calculator. I will have to bookmark them! but it sounds like I will just have to get a few different batteries and do my own testing. If anybody knows of a calculator that is ment specifically for surfacevehicke or know how I can convert some of the variables to a surface application that would be awesome! Thanks

The scriptasylum site is mine, so if you want to know what variables to use, let me know...