The True Meaning of the Design Life of a Battery

There is frequently confusion that surrounds the manufacturer’s warranty period and the design life of a battery. With certain batteries that have a “design life” of between 5 to 10 years, it often results in questions when the battery does not last for the maximum design-life period, and when it needs to be replaced earlier than what was expected. It becomes important to avoid confusing the warranty and the design-life as they are somewhat different.

Every battery features a design-life that matches up to the parameters that are required for Eurobat Classification. What this means is that if the standard operating condition has been consistently met, the battery should then last for a period that it was designed for. The service life correlates strongly to the battery working conditions. However, when put into practice, the “normal” operating conditions will not be met consistently.

Here is a list of factors that can affect the service life:

1. Ambient Temperature

Environmental factors that affect the life-span of a VRLA battery the most severely is temperature. Field and laboratory experience has proven that the ideal rule used to calculate temperature effects, involves the life of the battery halving for every 10C rise in an average-operating temperature. For example, if the battery has a life-span of 10 years at 20C, then the life will half to 5 years if operated at 30C continuously. When the temperature goes up and down consistently between the 2 levels, the life of the battery will be based on the time-weighted average of the temperature fluctuations. This is typically the primary failure cause of VRLA batteries not being able to match up to their design life which has been specified regardless of who the manufacturer might be.

2. Float Charge Ripple

When ripple is excessive on the DC supply across the battery it can reduce performance and life. It is suggested for this reason that the voltage regulation inside the system along with the load, but when the battery is not connected, under a steady-state condition, will be more favourable by +/- 1% through 5% to 100% load. Transient along with other types of ripple excursions can also be accommodated as long as the battery remains disconnected with a connected load, and a system peak-to-peak voltage, which includes regulation limits that are falling within +/- 2.5% of the suggested float-voltage for the battery. The current that flows through a battery when operating under float-conditions, should never be allowed to reverse into a discharge mode.

3. Float Stabilisation Ripple

This ripple type comes about when the load demands fall out-of-phase with the rectifier’s capabilities, whereby the battery is utilised to stabilise the battery’s system. A few of the static UPS systems will perform in this way, where this condition can be compared to shallow cycling. In this situation, the standard characteristics of the battery are no longer applicable, and this when the manufacturers need to be providing the optimal operational conditions.

4. Deep Discharging

It is suggested that the low-voltage disconnect features need to be utilised in any connected equipment. However, it is recognised that there can be situations, especially for safety reasons of the system, where requirements for the maximum performance can prevent the usage of low-voltage disconnect features. In these instances, the battery may need to be replaced after this type of discharge.

5. Failed Batteries

Failing to replace a battery that has failed that is still in service will most likely place a significant amount of stress on a full-string of batteries. This could mean that the battery-string is either receiving an undercharge or overcharge from the UPS. Over the long-term, this could shorten the overall service life and have effects that are detrimental to these batteries.

So with all these considerations in mind, with all the conditions and environmental factors that extend beyond the manufacturers or user control that has an effect of the rate that a battery starts to degrade, ultimately means that the “design life” of any battery is only an indication or guide about the how long your battery could last. While it may be reasonable to assume that a 10-year design-life battery should operate for longer than a 5-year design-life battery. The reality is that a service environment that is constantly fluctuating where the battery functions in, actually means that the design life and “true” service life is not the same. UPS battery recycling can help combat this issue, however.