Hi, Ian Watson from TrakPower here, I hope you dont mind me typing out a long boring technical post, but hopefully it will be of some information to you
C rating is a loose and relative number developed to describe the performance of a pack, there is no industry standard to what a cell should hold at this particular rating. it is generally based upon what current draw a pack can handle before the voltage drops to a certain point. Most packs out there have two C-ratings on their packs, constant and burst. Take as an example our 25C 4900 pack, a proven cell used widely in all sorts of setups. now this pack can handle 122.5A constantly, which means that you could put a 122.5A load on that pack, and it will hold a usable voltage for the majority of its discharge before dropping off near its capacity point. At 25C this will take approx. 2 minutes and 30 seconds to flatten the pack completely.
Burst discharge is on the other hand, what a battery can handle over a short period of time and keep above 3.0v per cell. In this case its roughly 250A. No questions asked, you will never reach this current draw! If you would look at a graph of discharge at this rating you would see the voltage drop off within several seconds, and will hit 3v per cell, this is what is known as a "burst", as the pack can handle this current in "burst" of several seconds without hitting its cut-off. As soon as you stop drawing the current the packs voltage will then increase and you carry on as normal.
In all reality with a car, average current draw over a race is very very low, OK it might peak off the line over over a hundred amps for a split second and as you pull out of corners, but In a strait line your car will require little effort to keep at a speed (think back to physics lessons). Even as you are pulling from a start it is unlikely you will hit the constant rating of the pack, far less its burst. Put it this way, if you flatten a 4900 is 10 minutes, that's 6C average, less than 30A average over the race!
But, what happens if you do go to this burst discharge current
with a correctly set up ESC? Will it puff? will it blow up? Will you kill the pack? It's a no.
In theory, IF you hit this burst current, and keep it discharging at that rate, you will hit the 3v mark, and it will keep on going below this save level. If you keep doing this, you WILL damage the pack, it will puff, get very hot, but its unlikely to explode, unless you push it very far! It takes a lot of effort to blow up a pack, and a simple short over-discharge is unlikely to push it to a dangerous point.
99% of controllers have a cut-off in them, and if they don't i would strongly advise to use an external cut-off of some sort. If this works correctly, and you are drawing excessive current from your pack, it will hit 3v, and the ESC will stop, the voltage will go back up, and you can go again (within reason, dont do this when your pack is actually flat!). So what happens here, nothing, your pack will carry on as though nothing had happened.
Technically it is not the current draw which kills a pack, its the relationship between current draw voltage drop and heat. If you draw too much current, the voltage will drop off, below 3v per cell, and damage the pack, end of story.
To put this into real life terms, in a particular type of RC Glider, they draw on
average 75-100C (yes you heard me correct there!) from their packs, for a maximum of 3 to 4 seconds. This is way over any packs "burst" rating, but as it only happens for a short period of time, the packs do not go below 3v per cell, the result, they do not kill their packs at the rate you would image them to! If they flew for more than 3 or 4 seconds at this rate then yes, the packs would get damaged.
Do we reccomend using a pack outside of spec? certainly not! but you would have to try very, very hard to get it to that point
Sorry if half of you have fallen asleep with the length of this post, i didn't intend it to be this long but lipos are a complex topic which isnt always as obvious as it seems. If anyone would like more information about this topic, and would like it in more detail, please feel free to call me on 01763 848404. I sadly dont know much about what fits in what cars (im a RC pilot, not a driver), but im happy to answer any questions regarding lipos and how they work and how they should be treated.
In regards to FlightPower and Trakpower using the same cells, thats correct, we are the same company.
Kind Regards
Ian Watson
TrakPower Lithium Polymer
Technical Support
[email protected]
01763 848404