Douglas Krantz - Technical Writer - Describing How It Works
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How Long does a Sealed Lead/Acid Battery Last?

Sealed lead/acid batteries are commonly rated to last 5 years, but that's the best case scenario. The lifetime of a battery is shortened by shelf life, gradual loss of capacity, the temperature that the battery is stored at and used at, and the actual current used from the battery.

The manufacturer draws arbitrary lines to indicate the lifetime for a battery

By Douglas Krantz

The common rule-of-thumb is that a lead/acid battery will last about five years from the date of manufacture. There are, however, several factors that shorten up that lifetime.

Purchase Date

Between the time that the battery was manufactured and the time the battery was available for sale, you can expect one to three months to have passed. If you're going to resell the battery for installation, the time it sits in your shop until it is installed also has to be added to this delay. So, the time between manufacture and installation of a battery has to be subtracted from that 5 years of life.

Gradual Capacity Loss

Unless something catastrophic happens to cause the battery to suddenly die, the capacity of a battery fades, or declines over time. For the manufacturer to list the average lifetime for a battery, because the battery's capacity fades over time, the manufacturer has to draw an arbitrary line in the decreasing capacity. It's when the battery crosses that line, so no longer has enough capacity, that the manufacturer uses for the batter's lifetime.

Half Shorter; Half Longer

Keep in mind that half of the batteries last a greater time than the average lifetime, and half of the batteries last a lessor time than the average lifetime. When you hear that the manufacturer say that the average battery lasts 5 years, that means that half of the batteries don't last that long.

You may be lucky and have one of the batteries that lasts longer than 5 years, but don't count on it. When you buy a lot of batteries, about half of them last less than 5 years.

Storage Temperature and In-Use Temperature

The temperature that the battery is exposed to affects the capacity lifetime of the battery. If the battery is stored or used at a temperature that is higher than normal room temperature, the battery's lifetime decreases. If the temperature is high enough, that decreased life can be drastically shorter.

Even cold temperatures can affect the lifetime of a battery. Remember that batteries are chemical based storage systems. As the temperature goes down lower than room temperature, the chemical reactions to produce the electricity are reduced. These slowed down chemical reactions reduce the immediate capacity of the battery, and sometimes the low temperatures can reduce the long-term capacity of the battery.

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The chapter headed Conventional Fire Alarm Supervision - Checking Continuity

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Amp/Hour Rating versus Rate of Use

Batteries are rated in Amps of Current per Hour (Amp/Hours or AH). How fast you're using the current does affect the capacity of the battery.

For instance: one amp of current used from an 8 AH battery might last the full 8 hours of use, but 8 amps of current used from the same 8 AH battery won't last anywhere near 1 hour. The higher current cuts the capacity of the battery. If this high current use is repeated over and over again, the long-term lifetime of the battery will also be reduced.

Often, when testing batteries, the manufacturer measures the capacity of a battery over 24 hours. For an 8 AH battery, the current that the manufacturer uses for the measurements is more like 1/3 of an amp.

Unless you are planning on keeping the current down so the battery will last 24 hours, don't plan on the battery having the full Amp/Hour capacity stamped on the side. The manufacturer doesn't know how you are going to use the battery, so the manufacturer can only provide a general capacity for the battery.

Repeated fast-draining of a battery shortens the lifetime of a battery.

Imprecise Science

The lifetime of a battery is affected by the remaining shelf life, its gradual loss of capacity, the temperature that the battery is stored at and used at, and the actual current used from the battery.

Other less common factors are also involved, so the Amp/Hour Rating on a battery is not a hard and precise science.
Post this by your fire alarm panel -- It shows the in-house fire alarm system and how it calls the fire department.


facpdoug@douglaskrantz.com
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