No battery is actually rated for 850A, C ratings are mostly bogus. When speccing a system, usually the ESC will specify the motor limit. It's rare but some ESCs like Castle specify a minimum C rating on the battery too but that's not often a concern.

True. Do not rely on the C-rating of batteries for this! 850 A is an enormous current.

If you want to know the actual current going through the motor under load, you'll need either a dyno with some current measuring gear (eg: "Load Master") or a data logger in the vehicle when you drive it around. Some, not all, ESCs have some data logging built in, so check the specs/manual/etc of whatever ESC you have on hand. Eagle Tree used to sell a data logging suite, but they're no longer around.


OT: 12AWG wire is often only recommended for 25-30A (just do some internet searching) There is a huuuuuge YMMV when it comes to this (aka caveats and considerations, eg: sexy multicore wire is less current capable than solid copper, and brushless motors are not inherently "continuous current" devices).

FWIW: Hobbywing used to publish some basic data, but again, a number of assumptions went into that. For example, an XeRun V10 G3-3.5T was rated for a maximum power of 462W and current of 126A. The latter for more than a short period (seconds, most likely) would result in the stator windings melting. (ref: https://hobbywing.com/en/products/xerun-v10-g327.html)

Electrical ratings are a rabbit hole of nuance and stipulations and are never as simple as a single amp/power/voltage value. None of it is helped by the BS electrical ratings used in RC where ratings are not standardized and the manufacturers never explain how they arrive at their ratings.

At the end of the day, ratings are there as guidelines to prevent you from melting equipment. You'll be perfectly fine if you use a motor that is within the ESC's guidelines and an appropriately sized battery (and by that I mean a hardcase 4s lipo meant for 1/8 vehicles if you're using a 1/8 motor/esc and not a small airplane soft shell 4s pack with 16awg leads). Trying to use RC ratings for much else is a fool's errand.

I agree, i know no lipo is rated for close to that many amps 850... but I want an accurate guesstimate at least of the power being used under full load. I dont want to see a single cell ever begin to puff on my ~$170 4s. Knowing the power load will greatly help to assess proper gearing range without too much strain on battery.

No I wouldnt ever rely on the numbers. 850a in our application is outrageous even for max burst. I mean do they think we are idiots? Even for a huge 1/5 setup wouldnt see close to that from a lipo I reckon. But they can be used to get into a proper guesstimate range of whats going on. The numbers can be trusted to a degree with the very best companies, which is what I run only.

And if it turned out the motor was drawing 200A, would you consider that within the rating? 200 is well under 850 but pretty much any battery will puff with 200A loads.

Just read the manuals. ESC manuals tend to tell you the upper limit of what motor is suitable. And motor manuals often have gearing guidelines for different vehicle weights. And if you're still really keen on an amperage number after doing that, look at the amp rating on the ESC.

Here is some fun with trying to draw some current with a 13.5. Well, towards the end of the video...

https://www.youtube.com/watch?v=9uR9pnE21XY

if you really want amp info (which tbh you don't need, just follow mfg recommendations and you'll be fine) you can either measure it with some kind of data logging esc or separate current clamp, or you could use some basic physics equations to calculate how much power is required to accelerate a specific mass to a specific velocity, apply some power factor and efficiency correction factors, and calculate amps from that.