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Understanding C Rating and Current Output

Updated: Apr 28, 2023

Drone batteries can be purchased in all types of configurations of voltages and capacities. If it can’t, we at Upgrade Energy can make it happen! It is the combination of voltage and capacity that make up the total power of the battery.

The ability for the battery to provide current is also a factor, which we will discuss in another blog later. If we multiply the capacity of the battery in amp hours, take mAh and divide it by 1000, and multiply that number by its average (nominal) voltage, you get the lithium batteries watt hours. It is possible to have a battery with a high voltage and low mAh rating, and a low voltage with a high mAh rating that have the exact same amount of energy available. For example, a 6s battery has a nominal (~average) voltage of 22.2V. Say it has a capacity of 5,000mAh. 22.2 * 5 = 111 watt hours. Another battery, say a 3s LiPo may have a nominal voltage of 11.1V and a capacity of 10,000mAh. 11.1 * 10,000 = 111 watt hours.


So both batteries have the same energy available!


C Rating is a factor of power output in relation to the capacity of the battery. Say a battery has a C rating of 10 with a capacity of 10,000mAh or 10AH. First convert to AH (divide mAh by 1000). Then multiply by the C rating. Thus, 10 x 10 = 100 amps. Most batteries will feature a C rating, but some just a watt hour figure. This can be helpful for determining how much current in relation to both capacity and weight. Thus it is important to keep in mind that a battery with 2000mAh with the same C rating as a 4000mAh battery will only output half the current as the 4000mAh.


Exaggeration of C rating in the battery industry

Unfortunately, C rating has become a bloated number in the battery industry. This has become especially egregious in the drone racing world where every ounce of power counts. It is not uncommon to see C ratings of 100 and higher. This translates to the battery outputting say 1.4AH * 100 = 140 amps. From testing, we've found these batteries to hardly even deliver half this current before voltage sags below safe levels. Furthermore, these batteries are fitted with an XT60 and 12 gauge wire, these would be well undiersuited if the battery was to actually perform with this C rating. At Upgrade Energy, to combat this fraud, we offer our ratings in two form factors, one is the manufacture cell rating, which is lab tested and validated, and an equivalent C rating to that of a common battery on the market, not to be actually used for any scientific purpose as those days are long gone.


Selecting a battery for your application

Unfortunately one cannot have everything in life. Thus, you are typically choosing between capacity and current output. A battery with substantial current output will often have a low energy to weight ratio, and vice versa. Consequently, a battery with too low of a current rating will have unusable voltage sag. This can be seen in some 18650 3000mAh and above cells. While in theory they deliver more capacity, in reality the voltage sags too low for use and the pilot must land far before they reach the 3000mAh rating. This is why we offer 2600 and 2800mAh 18650 cells as these cells provide far more current without having any real world capacity difference than the 3000mAh cells.


Recapping

To wrap things up, keep in mind C rating = current divided by capacity in AH. Or current * capacity in AH to get C rating. Keep in mind C ratings are often overblown and not to be trusted, and that voltage sag plays a real part in the real world useably of the battery.



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