Lead-acid battery capacity variation during life.

This is what the IEEE-485-2010 standard says about why an ageing margin of 1.25 is nearly always included in lead-acid battery sizing calculations.

Understanding the concept of an end-of-life 'knee' and how it may differ for different types of battery helped us negotiate an agreement with a vendor to replace batteries that had failed capacity discharge acceptance testing on an Australian LNG project.

Although the below relates to lead-acid batteries, it is also useful more generally when thinking about why, or how much, ageing margin is added to other battery types; lithium iron phosphate (LiFePo4), for example, which may have a different end-of-life 'knee' shape.

Nevertheless, the accepted practice is still to over-size lithium batteries by a factor of 1.25 (1 / 0.8), even though they may or may not be due for replacement when they reach 80% of their original rated capacity; lithium batteries may or may not have a much slower rate of degradation. Lithium batteries have not been used in standby power applications long enough to know the exact shape of their aging curve.

As a rule, for long-duration discharges of a vented lead-acid battery, the capacity is relatively stable throughout most of its life, but it begins to decrease rapidly in the latter stages, with the 'knee' of its life versus capacity curve occurring at approximately 80% of its rated capacity. This characteristic is well documented for discharges at the one hour rate or longer.

For high-rate, short-duration discharges of vented lead-acid batteries and all discharges of VRLA batteries, there are too many variables to state definitively where the 'knee' occurs. Because a battery with a certain resistance increase will show a larger voltage drop during a high-rate discharge than during a low-rate one, it is reasonable to expect that its short-duration performance may drop significantly below 80% of its rating before it reaches the 'knee' at that rate.

Most battery manufacturers warrant their batteries for 80% of the published capacities. While some batteries may be delivered with 100% initial capacity, others may be delivered with as low as 90% initial capacity that may or may not reach I 00% capacity over time. At some point in time, the battery capacity will start to decrease. Unless the user has extensive knowledge/history of the battery model being utilised and/or periodic testing (IEEE Std 450-2002/IEEE Std I 188-2005) is utilised, the user will not know the time frame when the battery is approaching 80% capacity. In lieu of this knowledge or testing, the user should always include a 1.25 ageing factor to account for battery aging.

Due to the aforementioned reasons, IEEE Std 450-2002 and IEEE Std 1188-2005 recommend that a battery be replaced when its actual capacity drops to 80% of its rated capacity. As previously stated, to ensure that the battery is capable of meeting its design loads throughout its service life, the battery's rated capacity should be at least 125% (1.25 ageing factor) of the load expected at the end of its service life. Rare exceptions to this rule exist. For example, some manufacturers for specific products (e.g., Plante) expect their cells to maintain I 00% of the published rates for their entire service life, and therefore, a 1.00 ageing factor could be used. If a 1.00 ageing factor is used, then the battery should be replaced whenever the capacity drops below I 00%.

As mentioned previously, batteries may have less than rated capacity when delivered. Unless I 00% capacity upon delivery is specified, the initial capacity of every cell should be at least 90% of rated capacity. This may rise to rated capacity in normal service after several charge-discharge cycles or after several years of float operation. If the designer has provided a 1.25 ageing factor as recommended, the battery will still meet the duty cycle so as long as the initial capacity is above 80% of the published capacity. While specifying I 00% initial capacity provides the user some level of confidence, it is not recommended that a 1.00 ageing factor be used.

I would welcome feedback and constructive comments on this article, especially if others have found data relating to how lithium iron phosphate batteries age.

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