UNderstanding Lipo "C" Ratings for RC Cars (plus voltage and Capacity)

Lithium Polymer (Lipo) battery technology has been perhaps the best leap of technology in the RC world. The relatively high energy density and absurdly high continuous current capability has meant that quadcopters have become a viable hobby and electric RC cars can now outperform their Nitro cousins without most of the fuss.

Voltage as measured in volts (v)
A single lipo cells has a maximum charge voltage of 4.2v which is usually reported as 3.7v for real-world purposes. Voltage can be increased by connecting these cells in series (s for series) and is reported as 1s (1 cell in series), 2s (2 cells in series) 3s (3 cells in series and so on). Voltage is additive when cells are placed in series i.e. 1s = 3.7v, 2s = 7.4v, 3s = 11.1v and so on.

Capacity as measured in milli Amp hours (mAh)
Lipo cells can vary in capacity, these are measured in mah and signify the amount of 'fuel' the lipo battery can output before it is depleted. All else being equal the larger the capacity they larger the physical size and mass (weight).

C - Rating
Where voltage and capacity are simple physical parameters, C-rating is slightly more complex. It is closely tied to capacity and has no relationship to voltage. In short the C rating signifies the maximum continuous output that the battery can maintain. Strictly speaking the C value multiplied by the capacity is the maximum continuous current output which we measure in Amps. The best way to describe this is to give examples:

• 1500mah battery x 30C = 45,000mA = 45 Amps maximum continuous output.
• 1500mah battery x 60C = 90,000mA = 90 Amps maximum continuous output.
• 3000mah battery x 30C = 90,000mA = 90 Amps maximum continuous output

So it's interesting to see from above that increase Capacity has a similar effect to sticking with the same size and increase C-rating.

The stock 1500mah 25C battery from the WL toys 144001 can theoretically allow 37.5 Amps

This 144001 upgrade battery with larger capacity is also 25C but the larger 2600mah capacity means it can theoretically allow up to 65 Amps

Quadcopter batteries tend to have higher C ratings. Although this CNHL Black 4s 1500mah battery has 4 cells, I have split it to 2 x 2s batteries that I use with my 144001. 1500mah @ 100C will theoretically allow up to 150 Amps continuous which I have seen on these.

Will a battery with a larger c-rating push more amps to my ESC?
This is a misunderstanding I see a lot. Batteries don't push current, current is demanded from them by the circuit in question, in our case the ESC/motor (much less so for the servo). The only way a larger C-rated battery will make your car go faster or put too much current to your ESC is if the current battery is a bottle-neck. Without measuring equipment the easiest way to see if your battery is the bottleneck is if it is warm or hot after running. As you can probably figure the battery in this case is being pushed to its limits and this is where the performance of your battery can start to diminish. If this is the case and the new larger or higher-C battery can supply more power to your motor/ESC then you should get more power and hopefully not overload the ESC (or motor). Personally I'd rather have an over specced battery and limit throttle/punch on my ESC to have more consistent control and operate within system capability.

The stock ESC for the 144001 does not have a published rating but my best guess is around 40 Amps. Update: It's actually 60 Amps - thanks Richard Dzidek. This means that realistically a battery with a higher c rating and or capacity than stock should give you a little more punch on heavy acceleration

The ESC on my favourite 2838 brushless combo for the 144001 (see upgrade guide here) is rated for 35 Amps only meaning the stock battery will also perform just fine. Another reason I like this combo for upgrading so much

Soon I'll be pairing this big Flycolor 80A ESC with a large, high KV motor for speed runs. The stock battery with 37.5 Amps max continuous current is well below this. To avoid being the bottleneck I'll need to go to a battery with a larger c-rating and/or larger capacity. In reality I'll go both to give me a big overhead.

Hopefully this helps you understand things a little better and at least gives you a good starting point. One thing to be wary of is that C-ratings can be used as a bit of a marketing tool so since the concept is a little more obtuse - I'd recommend comparing within a brand but not so much between brands. Overall RC cars are not as demanding on lipo batteries as quadcopters so you can see there is more wiggle room. If you must have the best batteries Gaoneng (GNB) batteries are probably the best commonly available ones from the demanding quadcopter world with CNHL in second.

If you found this helpful also check out my article on upgrading your charger for RC car batteries

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