BU-808c: Coulombic and Energy Efficiency with the Battery

All batteries have losses. The energy retrieved after a charge is always less than what had been put in. Parasitic reaction that occurs within the electrochemistry of the cell prevents the efficiency from reaching 100 percent. Ultra-fast charging and heavy loading also reduces the energy efficiency. This also contributes to battery strain by reducing cycle life.

Battery efficiency is gaining interest. This is especially critical with large battery systems in electric vehicles, energy storage systems (ESS) and satellites. The efficiency factor is commonly measured by coulombic efficiency. A coulomb is a unit of electric charge. One coulomb equals one ampere-second (1As).

Coulombic Efficiency

Coulombic efficiency (CE), also called faradaic efficiency or current efficiency, describes the charge efficiency by which electrons are transferred in batteries. CE is the ratio of the total charge extracted from the battery to the total charge put into the battery over a full cycle.

Li-ion has one of the highest CE ratings in rechargeable batteries. It offers an efficiency that exceeds 99 percent. This, however, is only possible when charged at a moderate current and at cool temperatures. Ultra-fast charging lowers the CE because of losses due to charge acceptance and heat, so also does a very slow charge in which self-discharge comes into play(See BU-808b: What Causes Li-ion to Die)

The coulombic efficiency of Li-ion improves with cycling. To prove this, Panasonic, E-one Moli, Sony, LG and Samsung Li-ion batteries in 18650 cell format where cycled. Some cells began with a coulombic efficiency of 99.1 percent and improved to 99.5 percent with 15 cycles. Some started at 99.5 percent and reached 99.9 percent with 30 cycles. The consistency on repeat tests was high, reflecting in Li-ion being a very stable battery system.

Lead acid comes in lower at a CE of about 90 percent, and nickel-based batteries are generally lower yet. With fast charge, NiCd and NiMH may reach 90 percent but a slow charge reduces this to about 70 percent. Lower charge acceptance when above 70 percent state-of-charge and self-discharge that increases when the battery gets warm toward the end of charge are contributing factors for the low CE. Best efficiencies of all batteries are attained in mid-range state-of-charge of 30 to 70 percent. All battery systems provide unique CE values that vary with charge rates and temperature. Age also plays a role.

Voltaic efficiency

Voltaic efficiency is another way to measure battery efficiency, which represents the ratio of the average discharge voltage to the average charge voltage. Losses occur because the charging voltage is always higher than the rated voltage to activate the chemical reaction within the battery.

Energy Efficiency

While the coulombic efficiency of lithium-ion is normally better than 99 percent, the energy efficiency of the same battery has a lower number and relates to the charge and discharge C-rate. With a 20-hour charge rate of 0.05C, the energy efficiency is a high 99 percent. This drops to about 97 percent at 0.5C and decreases further at 1C. In the real world, the Tesla Roadster is said to have an energy efficiency of 86 percent. Ultra-fast charging on newer EVs will have a negative effect on energy efficiency, as well as the battery life.

Last Updated: 4-Nov-2021
Batteries In A Portable World
Batteries In A Portable World

The material on Battery University is based on the indispensable new 4th edition of "Batteries in a Portable World - A Handbook on Rechargeable Batteries for Non-Engineers" which is available for order through Amazon.com.

Comments

Comments are intended for "commenting," an open discussion amongst site visitors. Battery University monitors the comments and understands the importance of expressing perspectives and opinions in a shared forum. However, all communication must be done with the use of appropriate language and the avoidance of spam and discrimination.

If you have a suggestion or would like to report an error, please use the "contact us" form or email us at: BatteryU@cadex.com. We like to hear from you but we cannot answer all inquiries. We recommend posting your question in the comment sections for the Battery University Group (BUG) to share.

I understand. Hide this message.
Gregory Free

Do you ever do any comparisons or analysis of specific LI ESS systems on the market now - Nov 2021 time period? Just curious what you are benchmarking to and from. Thank you for your response.

Looking for comments from the previous website?

Comments from the previous website are not compatible with our new commenting system but we have preserved them so our users can still reference and make use the information in them.

Show Old Comments
On January 30, 2019, Ian Benton wrote:
Do you have any graphs of charge (coulombic) efficiency vs. cell voltage for lead-acid? Obviously, coulombic efficiency reduces as water dissociation starts to take place, but are there any figures out there?
On March 20, 2018, Jamie wrote:
Andrew, do you mean energy efficiency on particular? As the page states, there are other measures of discharge efficiency such as coulombic efficiency or voltage efficiency... Energy efficiency would be my preferred general metric, but it depends on charge and discharge rate, and so might be a hard thing to define. Your best bet may be too try and measure it-measure how long it takes to charge a cell with a constant current and monitor the change on voltage... Then do the same for discharge and calculate the energy in and energy out...
On January 24, 2018, Andrew wrote:
A request. I am having trouble finding real life discharge efficiency numbers for Lithium Ion batteries. For example - if I require X KWh of energy supplied I need a battery with X / "discharge efficiency" capacity which will be greater than X. I would then use this new figure to calculate a required weight of batteries for my application (I am planning on using 200 wh/kg for this density). Any real life examples appreciated.
On October 23, 2017, Nelson wrote:
Hi, There is a mistake in your article at the end of the second paragraph : "One coulomb equals one ampere (1A) per second." One coulomb is actually equal to one ampere second (1As). Or one ampere is equal to one coulomb per second (C/s). Best regards, Nelson
On October 16, 2017, Tas wrote:
Can you show us some diagrams about the Energy efficiency vs. SOC and Energy efficiency vs. electric power? This is a bit machanical engineer approach but still I hope there are some "ideal" or "specific" diagram to the Li-Ion batteries used in HEVs. Thanks for Your help.