Batteries Self Discharge as they Sit on the SHELF!
Batteries, an electro chemical system, isn't quite stable, and so any battery will self discharge, or go dead, all by itself, on the shelf. Bottom line, the type of battery you choose dictates the rate at which the battery decays, rots, or self discharges while sitting on the shelf.
2% self discharge for AGM Batteries vs. 10% Flooded Batteries.
How does that effect storage of your boat?
The simple version of battery self discharge:
Every 60 days, your boat battery could either die, or be just fine. Your choice.
The more complex version of battery self discharge:
A battery is an electrochemical system, that uses a chemical reaction between materials to store electrical power. The chemistry involved inside the battery literally moves electrons through a chemical reaction, both directions, as needed to charge and discharge the power. Over time, these chemicals degrade, losing their ability to hold, transfer, and receive the electrons. The end user sees this degradation as a loss of capacity inside their marine battery.
Depending on the construction technique used to manufacture the marine battery, the rate at which it battery self discharges dramatically differs. For instance, most flooded lead acid marine batteries will self discharge at a rate of 10% in 24 hours after charging, and another 10% each subsequent 30 days of storage. On the other hand, an AGM, sealed lead acid marine battery will self discharge 2% in 24 hours, and 2% each subsequent 30 days.
Fifty months is not the shelf life of an AGM battery.
Alright, the wise guy in me tried this one too. As a marine battery reaches the 50% charged point, it self discharges more rapidly. Self discharge on a battery left to rot actually looks like a geometric or log rhythmic graph after 6 months. To avoid this simple charge your batteries, and exercise them at least 3-4 times a year. This exercising, along with nominal use, will help your AGM batteries stay in tip top shape.