Elevating Self-discharge

All batteries are affected by self-discharge. Self-discharge is not a manufacturing defect but a battery characteristic, although poor manufacturing practices and improper handling can increase the problem. Figure 1 illustrates self-discharge in the form of leaking fluid.

The amount of electrical self-discharge varies with battery type and chemistry. Primary cells such as lithium and alkaline retain the stored energy best and can be kept in storage for several years. Among rechargeable batteries, lead acid has the lowest self-discharge and loses only about five percent per month. With age and usage, however, the flooded lead acid builds up sludge in the sediment trap, which causes a soft short when this semi-conductive substance reaches the plates.

Nickel-based rechargeable batteries leak the most and need recharging before use when placing them on the shelf for a few weeks. High-performance nickel-based batteries have a higher self-discharge than the standard versions. Furthermore, self-discharge increases with use and age, and the contributing factors are crystalline formation (memory), permitting the battery to “cook” in the charger or exposing it to repeated harsh deep discharge cycles.

Lithium-ion self-discharges about five percent in the first 24 hours and then loses 1 to 2 percent per month; the protection circuit adds another three percent per month. A faulty separator can lead to a high self-discharge and if critical, the electrical current will generate enough heat that can in extreme cases lead to a thermal breakdown.Table 2 shows the typical self-discharge of battery systems.

Table 2: Percentage of self-discharge in years and month.Primary batteries have considerably less self-discharge than secondary (rechargeable) batteries.

The energy loss is asymptotical, meaning that the self-discharge is highest right after charge and then tapers off. Nickel-based batteries lose 10 to 15 percent of their capacity in the first 24 hours after charge, then 10 to 15 percent per month. Figure 3 shows the typical loss of a nickel-based battery while in storage.

The self-discharge on all battery chemistries increases at higher temperature and the rate typically doubles with every 10°C (18°F). A noticeable energy loss occurs if a battery is left in a hot vehicle. High cycle count and aging also increase self-discharge. Nickel-metal-hydride is good for 300-400 cycles, whereas the standard nickel-cadmium lasts for over 1,000 cycles before elevated self-discharge starts interfering with performance. The self-discharge on an older nickel-based battery can get so high that the pack loses its energy through leakage rather than normal use.

Under normal circumstances the self-discharge of Li-ion is reasonably steady throughout its service life; however a full state-of-charge and elevated temperature increase the self-discharge. These very same factors also affect longevity, a phenomenon that applies to most batteries. Table 4 shows the self-discharge per month of Li-ion at various temperatures and state-of-charge. The high self-discharge at full state-of-charge may come as a surprise to many. This explains in part the asymptotical self-discharge characteristic when removing a battery from the charger.

Table 4: Self-discharge per month of Li-ion at various temperatures and state-of-charge
Self-discharge increases with rising temperature and higher SoC. 

Comments

On December 3, 2011 at 7:26am

John Fetter wrote:

Self discharge is generally caused by impurities. In the case of lead-acid, it is feasible to keep impurities from causing excessive self discharge. This is done by using a suitable chemical substance that prevents the impurities, that inevitably migrate via the electrolyte, reaching the negative plates.

Impurities are present in the negative plates at the time of manufacture, however, they get buried in the negative active mass over time. The impurities in the positives and that are put in the electrolyte with the filling water just keep electroplating out onto the negatives.

These “electroplating” impurities can be stopped by that chemical substance. After about three months in service, the battery ends up with near-zero self discharge.

On December 18, 2011 at 4:56am

Tom Tercek wrote:

This is the best website on batteries, thank you so much..
I have been wondering what the difference is between “precharged”
NIMH vs standard NIMH batteries.
What are the pros and cons of these 2 types??
Thanks

On February 1, 2012 at 8:03pm

Kenny wrote:

So, do you charge your phone to 100percent or charge it only to 80 percent and use it down to 20 percent and recharge again ?

On February 14, 2012 at 12:59pm

tytower wrote:

Comment 1 -John Fetter - So what is that suitable chemical substance then ?

Tom Tercek- I notice precharged is marked on more recent rechargeables because the have lower self discharge . Standard rechargeables seem to be flat in 1 to 3 months max. The precharged seem to last many months of non use before needing recharge.

On February 14, 2012 at 1:30pm

John Fetter wrote:

tytower:  The source and preparation of this material was revealed on, “How to Prolong Lead-acid Batteries”, Dec 8, 2011 at 3:12 am, (a parallel page on this website). You can find more info. scattered about on various pages.

On May 9, 2012 at 9:22pm

Richard Britton wrote:

Thanks for the wealth of Info on self discherge od secondary batteries.  Much of it, I’ve not seen before, and I’ve been working with batteries about 65 years. 
I would like to find a very low self discharge battery, 6 volt, Group 1 for use in antique cars dating back to the teens. Often they sit unused for years.  They should also have a long useful life (eg. 10 to 30 years). Use of a trickle charger is no problem.

On June 6, 2012 at 3:51am

Valentin Lecuyer wrote:

Thanks for all the infos.

I am facing a big problem of Self-Discharge in a CR2 (Lithium Prinary) battery : When I use it for 75% of its capacity, in less than a day, the 25% left disapear by Self-Discharge.
Does anyone knows about this problem?

Thanks

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