It is fairly common that a new hobbyist to RC may think too high of a C rating would be bad for their RC. In fact the higher the C rating, the better off your entire system will be. Here is a quick review of how we use the C rating to calculate the maximum continuous discharge current the battery can output. C ratings can come in the form of either continuous or peak ratings. Be certain that you are using the continuous ratings.
Continuous Discharge Current = Continuous C rating x Battery capacity (Ah)
Discharge Current = 35C x 4.5Ah
Discharge Current = 157.5A
There are three primary reasons that you will want to get the highest C rating possible for your RC, let’s go through them.
Get the Highest Performance Possible
When a LiPo battery is under load a voltage drop occurs. It is this voltage drop that can hurt performance. If your power system is experiencing a significant voltage drop under load, the motor will not be able to hit maximum RPM. Lost RPM is exactly where we lose performance. Your RC vehicle will have a reduced top speed. However, batteries with a higher C rating will be able to maintain voltage under load. This will mean your power system will be able to produce more power output in watts leading to a higher top speed.
Another area where you will see the greatest benefit is under heavy acceleration. A higher C rated battery will be able to sustain higher voltages but also at higher current output. This will allow an RC car to accelerate out of a corner more aggressively, or for an RC boat to get out of the hole faster.
For these reasons, it is a great idea to select a pack with the highest C ratings that you can.
Reduce the Temperatures on your LiPo Battery
Temperatures effect all electrical components and it is in our best interest to keep them as cool as possible. For a LiPo battery, temperatures below 140F are acceptable. Above 140F should not be acceptable and will cause permanent damage. To maximize the lifespan of the battery reducing temps much below 140F will help.
Moving up to the highest C rating that you can will reduce the temperatures of the LiPo battery. This is due to the fact that the battery will be able to sustain a higher amount of output. For example, consider a load that you need to satisfy at 70A. Two battery options for simplicity allow for a 100A and 200A maximum continuous draw. The 100A pack would be loaded at 70% of its maximum capability. The second battery pack would be loaded at 35% of its maximum capability. The reduced amount of stress on the second battery is going to result in lower temperatures and longer life.
Reduce Ripple Voltage within Your ESC
Ripple voltage is described in detail within this article. A higher C rating will be able to reduce the amount of ripple voltage that the ESC will see. This is due to the fact that the voltage under load would be reduced. Furthermore, if an adequate C rated battery is not selected for the power system, ripple voltage can be excessively high. If the amount of ripple voltage present in the ESC is too high, the capacitors on the ESC can fail. Once this happens the ESC will ultimately fail.
It’s quite obvious that we would want to avoid this scenario at all cost. To reduce the ripple voltage within the ESC and maximize reliability of the ESC, select the highest C rated batteries that you can.
In the image above, ripple voltage is produced by a 4s 4000mAh 35C battery. The current draw is around 80A where the battery is rated for 140A. Ripple voltage is over 10% of the batteries nominal voltage which is very concerning. This must be addressed before it takes out the ESC. This LiPo battery is no longer performing at 35C due to many possibilities. Read more about LiPo batteries and what ages them here.
What can Limit C rating?
Size of the LiPo Battery
A major factor that can limit the C rating that you can use in your RC, is the size of the battery pack. As you increase the C rating of an RC LiPo battery, the physical size tends to increase. In addition to the increase in size is also the increase in overall weight. I have a battery pack that has one of the highest, accurate C ratings, that you can buy. However, it is far to heavy to place in to the RC plane that I bought it for. As a result, I have decided to use another pack instead. The new pack is lighter, improving the low speed flight characteristics of the plane. **A lighter plane will allow a less stressful landing for the pilot.** The pack is also smaller allowing an easier fitment upon strapping the pack within the planes battery bay.
Cost of the Higher C rating
Another factor that comes in to play is of course the almighty dollar. (Or which ever currency you have in your country – we use the dollar in Canada) Higher C rating batteries tend to get very pricey very quickly. Biggest thing here is that your budget should be met. If you can’t grab the 65C LiPo battery but the 45C fits within budget and your RC’s requirement, then buy it!