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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?

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    \$\begingroup\$ I believe this is due to the "charged" state having metallic lithium on the electrode, while "discharged" has it in ionic solution; the charging process needs to deposit it cleanly on the electrode in order to be effective. I think temperature also matters. \$\endgroup\$
    – pjc50
    Jan 4, 2017 at 11:13
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    \$\begingroup\$ Title does not correspond with the actual question! \$\endgroup\$
    – Leon Heller
    Jan 4, 2017 at 11:13
  • \$\begingroup\$ Ditto - fix your title. \$\endgroup\$
    – Andy aka
    Jan 4, 2017 at 11:14
  • \$\begingroup\$ 30C discharge? Doesn't that mean that a 20 Ah powerbank could be used to jump-start a vehicle? (in theory) \$\endgroup\$
    – neverMind9
    Oct 21, 2018 at 0:52

1 Answer 1

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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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    \$\begingroup\$ Back of memory suggests that there are also ion migration effects that differ in each 'direction'. \$\endgroup\$
    – Russell McMahon ♦
    Jan 4, 2017 at 13:04
  • \$\begingroup\$ As a side note, stated discharge C ratings of LiPo batteries are often 100% pure marketing BS, not dissimilar to the traditionally inflated capacity ratings of Li-ion batteries. tasteyourjuice.com/wordpress/archives/14657 \$\endgroup\$
    – Dampmaskin
    Jan 4, 2017 at 15:03
  • \$\begingroup\$ @Dampmaskin Isn't Li-ion and LiPo batteries are the same? I thought that the "LiPo" name is just for marketing reasons. If I'm not mistaken actual Lithium polymer batteries are still a new technology and are not available in the market. \$\endgroup\$
    – UnTraDe
    Jan 5, 2017 at 9:04
  • \$\begingroup\$ @555 So charging the battery at a slower rate than the maximum specified can extend the battery overall life? And is this effect occurs in different types of batteries (with different chemistry)? \$\endgroup\$
    – UnTraDe
    Jan 5, 2017 at 9:06
  • \$\begingroup\$ Yes. Also for certain types of batteries like most LiIon/LiPo keeping the battery undercharged to about 80% will significantly increase the battery life. And the recommended storage charge is usually between 30% and 50%. So if you use this type of battery in a system that is always ON, like an intruder alarm, the battery should not be charged more than 50% to increase the system maintenance cycle. \$\endgroup\$
    – 555
    Jan 5, 2017 at 10:50

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