Trolling Motor Batteries Lifespan... Or Not

For most of us, the common sealed lead-acid battery is the go-to for trolling motors, with the AGM (absorbent glass mat) being the hands-down choice.
Nowadays, those with big budgets are switching to LiPo batteries - aka "lithium" - but thats a different technology and it has its own problems. So, for a long time to come, the sealed, lead-acid AGM battery will remain the choice for a great many anglers.

For those who dont know the difference, or the advantages and disadvantages of either one, I wrote and in-depth, two-part feature on them. You can find that here:

Batteries For Trolling Motors

As a follow up to that, I offer the following, because anyone who claims to know how long a trolling motor battery will last is only guessing. 

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So...How Long DOES a Sealed Lead Acid Battery Last? adatped from an article by Douglas Krantz

Sealed lead/acid batteries - including the common AGM batteries used with trolling motors - are generally 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 how the actual current is used from the battery.

However, several factors combine to shorten that lifetime. Purchase Date Between the time that the battery was manufactured and the time the battery was available for sale, you can expect AT LEAST one to three months to have passed. Then, the battery will sit on the shelves of distributors and retailers for-who-knows- how long. A year is not uncommon. So that "sitting ime has to be added to this delay. Therefore, right from the start, 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. Thats just how it is. For the manufacturer to list the average lifetime for a battery, because the battery's capacity fades over time, the manufacturer draws an arbitrary line in the decreasing capacity, and its aline that favors their predictions. When the battery crosses that line, and no longer has enough capacity, the manufacturer will say the battery has exceeded its lifetime. Half Shorter; Half Longer On the positive side, half of the batteries last a greater time than the "average" lifetime - and half of the batteries last a lesser 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 battery, you can expect that it may not last 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. Cold temperatures can affect the lifetime of a battery, too. 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. To make matters worse, sometimes the low temperatures can also reduce the long-term capacity of the battery. 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 also affects 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 draw cuts the delivery capacity of the battery. If this high current use is repeated over and over again, as it is in trolling motors, 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 might last, say, 24 hours...., don't plan on the battery having the full Amp/Hour capacity stamped on the side. The manufacturer sinply doesn't know how the battery will be used, so he can only provide a general capacity rating for the battery. POINT TO REMEMBER: Repeated fast-draining of a battery shortens the lifetime of a battery. Imprecise Science In summary... - The lifetime of a battery is affected by the remaining shelf life. - Gradual loss of capacity - The temperature that the battery is both stored and used at  - The way the actual current is used from the battery. Still other less common factors, like lack of maintenance, connector issues and more are also involved, so the Amp/Hour Rating on a battery is not a hard and precise science. ++++++++++ What does all this mean? You must do all you can to extend the useable life of your batteries. You must keep an eye on them, and, you must understand them and their needs. For all that, I encourage you to read the feature article linked in the introduction. Many thanks to Douglas Krantz

Douglas Krantz helps those who install, service, and test fire alarm systems. He has worked in the electronics and fire alarm industries for more than 40 years....and he knows what he's talking about on the subject of batteries.  www.douglaskrantz.com