Electronic – Why battery charge C rating is usually lower than discharge

batteriesbattery-chargingbattery-chemistrylithium ion

Why most (or all?) batteries can be discharged at an higher C rating than the charging C rating?

For example I have the following battery for my quad-copter:
https://hobbyking.com/en_us/turnigy-4000mah-3s-30c-lipo-pack.html

If you look at the specs tabs, you will see that the C rating for Discharge is 30C which means I can draw at max 120 AMPS continuously (which will last me about 2 minutes) and the C rating for Charging is 2C, which means I can charge it at 8 AMPS continuously (which will take about half an hour to charge fully).
All of the Lithium based batteries I found has significantly higher C rating for discharge than for charge.

What is the reason for this big difference? Is it limited to Lithium based batteries only or is it a common trend across most types of batteries?

Best Answer

This is done by design, this type of battery is designed to have a low internal resistance in order to be efficient when driving high current loads. But it usually can't charge as fast without reducing the battery life because the electrolysis effects will decompose some molecules in the electrolyte and usually release a gas hydrogen, CO2 or other, which can't be recovered.

This also depends on the application, it is actually recommended to use a lower discharge for some application in order to increase the delivered power, by minimizing the dissipation in the battery due to the internal resistance. This is the case in IoT application where batteries with higher power density are used. Those batteries usually have a very high internal resistance (1 to 300 ohm) and will dissipate internally if the curren tis high, in this case the battery is used in conjunction with a super capacitor which is slowly charged by a current source providing simultaneously a high current in an intermittent basis and a low current voltage source for maintaining the sate (RAM RETENTION) in the device. in those application it is beneficial to use a higher power density because IoT devices usually need to run for years and may also use power harvesting, but only need to use high power for a very short time when transmitting data.

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Electronic – Battery Ampere-hour rating vs Battery Amps (not an experienced user)

Your hunch that batteries have a current limitation is correct. In general, it's hard to tell the current rating [A] from capacity [A·h]. You have to look it up in the datasheet. A lot depends on the design of the battery.

For example: coin cells with 500mAh capacity may have only 3mA max current.
Another (opposite) example: automotive starter battery with 40Ah capacity may have 500A max current.

Lead-acid batteries are interesting in this respect, because there are two distinct types.

Starter lead-acid batteries are designed specifically to deliver high peak current for a short period of time. Deep discharge, however, dramatically shortens the life of a starter battery. So, it's not suited for routine operation at high depths of discharge. Your typical starter battery in the automobile works at very shallow depth of discharge.
Deep cycle lead-acid batteries are designed (as name suggests) to discharge further. But they can not provide as much instantaneous current.
Here's an example datasheet for a deep cycle battery. Have a look at the nominal capacity on p.1. Notice that capacity depends on discharge current (i.e. the rate of discharge).

Depth of Discharge    Starter Battery    Deep-cycle Battery

100%                  12–15 cycles       150–200 cycles

50%                   100–120 cycles     400–500 cycles

30%                   130–150 cycles     1,000 and more cycles

(Source.)

p.s. If you want to read-up, here's an excellent web site on batteries - Battery University.

Why are the charge and discharge rate of Lihium Polymer Batteries different

My best "educated guess" is that if you are discharging a battery, the "heating effect" is decreasing with time, whereas when you are charging the battery, the heating effect is increasing with time. So, as time passes, nothing happens to the battery when discharging. But in the second case, most likely the battery would explode, or at the very least, burn up! This still can happen, even at the lower charging rate if the battery can't dissipate the heat generated at this current.

Best Answer

Related Solutions

Electronic – Battery Ampere-hour rating vs Battery Amps (not an experienced user)

Why are the charge and discharge rate of Lihium Polymer Batteries different

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