6s lipo: 4500-30c or 5000-25c?
 

I am looking at two different lipo's from the same brand for my X2-CRT. The truck has the cc-neu 1518.

These are the specs of the lipo's I'm looking at:
6s30c4500mah: 135a cont - 270a burst, 784gr., max 4c charge
6s25c5000mah: 125a cont - 250a burst, 773gr., max 4c charge

Based on the c-rating the 4500mah pack would be the better choice. But I have no idea how many amps my setup will pull (geared for 39mph without ballooning calculated).

I only bash and wonder if either lipo would make any difference compared to the other?

30C is better. If you bash 30C=more stand backflips

I don't think it will make much of a difference performance wise. Using a 6S setup should help keep the current reasonable and either pack should be up to the job. I might go for the 5000mah to try and squeeze in a little extra runtime.

Many people seem to think that higher C-rating is always better, and this isn't necessarily true. Yes, you need a battery capable of producing enough current for a given setup, but once you go past that there really aren't any benefits e.g. I could use a 5000mah 50C pack, but if my system is only drawing 80A then I won't see any benefits over using a 5000mah 20C pack. I guess the exception would be voltage drop. If you are running a certain pack close to it's limits, then bumping up to the next higher C-rating would help reduce voltage drop under load. But it's always good practice to use packs that are capable of producing current beyond what your setup will draw to create a margin of safety and not overwork the packs.

The last thing I would say, and I think most will agree, is that C-ratings can vary greatly. Some people say brand X overrates their packs extremely, and others swear brand Y packs can produce far more than their rated current without extreme voltage drop. Again, the best bet is to assume any pack is slightly overrated, and plan for it buy using packs that can perform above and beyond what your setup will draw from them.

Here are some discharge graphs of the packs I'm looking at. It compares the same capacity packs of different C-ratings. The only real difference I see with the bigger packs, is that voltage under load(25c) of the 30c packs is higher at the beginning of discharge, and halfway the 25 and 30c packs perform similar. In the end the 25c packs last just a little longer until the cells reach 3v/cel. The 25c cells do get a bit warmer, but nothing to worry about.

http://www.elektromodellflug.de/akku...power-line.htm