When shopping around for lipo batteries, especially lipo batteries for fpv quadcopters and drones, a number thrown around a lot(a lot) by manufacturers is the C rating.
So what exactly is the lipo C rating?
In plain English, the C rating is how much current the battery can deliver.
Usually, the C rating will be expressed as two numbers separated by a slash, like 45/90 or 65/130. The first number denotes the constant current it can deliver. This is current that the lipo battery can push out consistently without dropping voltage significantly(and damaging your battery).
The second number is the burst current, or the current the lipo battery can push out for a short time – usually, this is a much higher current, and the voltage will start to drop rapidly, too.
How to calculate current using the C rating
The formula for calculating current is fairly simple:
Current = C rating x Amps
So if you have a 1300 mAh lipo battery with a 65C rating, the current your lipo battery can steadily deliver is:
Current = 1.3 x 65
Current = 84.5 amps
If the burst rating on said battery is 130C, then the burst current your lipo is capable of is:
Current = 1.3 x 130
Current = 169 amps
C rating reliability
At this point, you should be raising some eyebrows. 169 amps is a LOT. It’s no small number. To put things into perspective, an average 2205/2300 kV motor draws about 25-30 amps on decent 5″ tri-blade props at full throttle.
Multiply that by 4 and the max current you’re looking to draw on a normal set up is 100-125 amps.
In regular flying, though, you’re rarely going to manage to pull the whole 125 amps, and for three reasons:
- Most of your flying is going to be under 60-70% throttle, and you’ll constantly be moving the throttle stick up and down
- Even if you do a full throttle punchout, the current draw on the testing bench is usually a little higher than in actual flight
- Finally, current draw is not proportional at all – if a motor draws 25 amps at 100% throttle, that does not mean it will draw 12.5 amps at 50% – the progression is more exponential.
Here’s some DVR footage that shows current draw and voltage sag on an old copter of mine running DYS 2205/2300 motors and 5040×3 propellers using a 4S 1300 mah 65ish C pack(I don’t remember exactly which one it was). Note how the voltage sags and the amp draw rises and falls, at roughly what speed the copter is going at. I didn’t have sticks overlay, otherwise you could have directly seen the throttle stick and compared it against the amp draw. If I get a chance, I’ll record another video soon:
Coming back to lipos, however, most lipo manufacturers like to inflate the C rating, a lot.
Unfortunately there is no reliable way to measure how accurate the C rating actually is unless you go and fly the lipo and see how well it performs.
Still, that’s not to say that C ratings are completely garbage.
I have one 45C Turnigy Graphene 4S 1300 mAh pack, and I noticed that it sags noticeably more than my other 65+ C packs on a typical 5″ setup running 2205/2300 kV motors and tri blade 5″ props.
So the lower C rating batteries do in fact(or did in fact for me) perform less than the higher C rating packs.
However, did they perform proportionately less? I don’t think so.
By that logic, a 100C pack such as the ones available from China Hobby Line should perform twice as better as the 45C packs – but that’s just not the case.
There is definitely improvement, but it does not progress proportionately.
The burst ratings are certainly inflated, though, so really the best advice to take away here is rely on experience, and take manufacturer ratings with a pinch of salt.
Determining how much C rating you need
If you don’t want to read too much into the details, here’s the short answer:
For more power and current delivery – especially on a miniquad – stick to a higher C rating battery, at least 60 or above for the continuous rating.
If you want to be exact, or are planning on building a monster setup with 23xx or 24xx motors and plan to put heavy propellers on them, you’ll want to check first on Mini Quad Test Bench to see if they have any testing data for your motor and prop combination.
You’ll want to get a battery that can, on paper at least, handle the 100% throttle current well under the full burst rating, because remember, the numbers are inflated.
Remember that C rating is not the only indicator of current – if a battery falls short of your expected current draw, use a bigger battery instead!
For 5″ builds, you can use a 1300 or 1500 mah battery comfortably, and if you’re using monster motors and monster props, then you can get away with 1800 mah as well.
Doing some quick math, here’s how the three batteries would perform with a hypothetical 65C rating:
1.3 x 65 = 84.5
1.5 x 65 = 97.5
1.8 x 65 = 117
Decent lipos to pick up
High C lipos are plenty nowadays and in a variety of capacities, suited for micros and minis.
Tattu Lipos are solid, as are Thunder Power, Turnigy Graphene, and Bonka lipos too.
Budget yet well performing(enough for most of our flying, unless you are a competitive racer) lipos include China Hobby Line and Infinity.