The C rating on LiFePO4 and RC LiPo batteries and what it means

I’ve had a few questions from fellow flyers at my flying field (and seen questions also on forums), regarding the “C rating” on a LiFePO4 and RC Lipo battery,  and in particular what it means for us radio control pilots, so here’s an explanation to get it all wrapped up.

For us electric RC fans, the C rating is simply the measure of maximum safe dischange a battery can take over a sustained period of time.

As you’re probably aware, RC planes and helicopters draw significant amounts of current when we’re using them so this is an important concept to be aware of.

In simple terms, “C” acts as a multiplier of the mAh capacity of the battery. Here’s a quick table I’ve put together:

Discharge Rate    Time to Discharge   
     1C                1 hour
     2C                30 mins
     4C                15 mins     
     8C                7.5 mins
     20C               3mins
     etc...

As you can see, the higher the discharge rate (or current, or throttle position), the quicker it takes to discharge a battery… common sense really

A Park Flyer example

When on WOT (wide open throttle) our DHC Beaver rc plane draws approx 18A*. Depending on the battery capacity and “C” rating will depend on whether we are causing damage to the battery discharging at these Amps.

Radio Controlled Beaver

In this case, I use a 1500mAh LiPo battery, rated to 20C (just like this one).

So let’s work out if we’re causing too much stress on the battery:

• We know that this airplane draws 18A on full throttle (with the 3 bladed 8×6 prop I’ve fitted)
• We know my battery has a 1500mAh capacity (or 1.5Ah)
• Dividing 18A by 1.5A gives us 12…  this is the discharge rate “C”

Maximum current draw at WOT    /     mAh battery capacity    =   actual discharge rate in “C”

As the battery is rated to 20C, if I’m only discharging at 60% of it’s maximum (12C) when at full throttle, we can be sure I won’t be causing damage to my battery. And that’s assuming I fly around the flying field  at full throttle the whole time (which I don’t given it’s such a light model).

At a glance – determining the maximum discharge rate of a battery

“C” rating    x     mAh capacity    =    max Amp draw

A simple way to determine the maximum Amp draw a battery can take is to multiply its capacity by its C rating. So using my 1500mAh LiPo as an example again, discharging at 20C would draw 30A of current. So that is the maximum safe amount of draw the battery can take, assuming the manufacturer isn’t overstating anything!

Again, 18A is only 60% of 30A so we’re not stressing the battery. In reality it comes down cool after most flights, generally not even warm, so all is good here!

What’s the Burst figure?

You may see that some LiPo’s actually have two C ratings – the “burst” and the “constant”. Often the burst figure is not labelled on the battery, but is rather in the sales literature (example).

The manufacturer is essentially stating that for periods of no longer than 30 seconds, the battery can discharge at the “burst” C rating. Personally I don’t like to discharge any more than approx 60% of the stated “constant” figure, as doing high rates of discharge repeatedly tends to shorten the life of your battery.

Furthermore, by buying getting a higher voltage battery, you can drop the Amps required to achieve the same wattage, which is generally a more cost-effective solution (a lesson for another day perhaps – for now just remember from your high school lessons that Amps x Voltage = Watts).

So – even though you may see burst figures, my preference is to ignore them, and do all your calculations based on the “constant” maximum discharge rate – you’ll be financially better off in the long run.

And finally, what’s the difference between LiFePO4 and LiPo Batteries?

Without going into the chemistry, LiFePO4 batteries, also known as A123, allow much higher discharge rates than lithium-polymer batteries. RC heli pilots who require serious amounts of discharge (40C and higher) are starting to look at these, and as the price comes down it is likely these will become more mainstream.

I hope this all makes sense – and please feel free to comment on this post, or drop in some examples of your own for us to work with!

* we measured this with this watt meter, connected it between the battery and ESC .

Zippy Flightmax LiPo Battery