Tom:
An AGM (absorbed glass mat) battery is a lead acid battery, and therefore is subject to sulfation. As the electrolyte is fully contained in the mat, the electrolyte is not subject to stratification, which is the other reason for equalizing a battery.
Regardless, of wet cell or AGM, it is preferable to minimize sulfation in the first place, rather than equalize the battery to fix sulfation, after it has occurred. The best way for you to do this is to have a smart marine type battery charger that is plugged into �shore power� whenever the boat is not in use. This will bring the batteries to full charge after each trip, keep them at 100 percent between trips, and shut the charging completely off when not needed. Whether you keep your boat on a trailer or in a slip, this is the best thing you can do to prolong battery life, regardless of battery type.
If you start the trip with 100% charged batteries, the fact that you bulk charge only for a few cycles will be essentially negated by bringing the battery back to 100 percent, and keeping it there, immediately after the trip.
Leaving the battery in a partial discharge state between trips (which is common practice of many boat owners), is a large contributor to sulfation, requiring more frequent equalization.
One of the battery guru�s once wrote that an AGM is not necessarily �a plug and play� replacement for a wet cell. He meant that you need to look at the entire battery / charging system(s) when making the changeover. Older marine shore power chargers may not have the correct voltage settings for the bulk and float charge stages and may need replacement. AGM batteries can withstand a much higher bulk charging rate than wet cells. If you replace the engine alternator with a higher output alternator and regulator, you would be able to accomplish your bulk charge on the water in less time. Or you could achieve a fuller capacity charge in the same amount of engine run time.
Although most (if not all) AGM manufacturers tell you not to equalize the AGMs, if you have a problem with sulfation and explain the symptoms to them, they will usually instruct you on how to equalize the battery and what voltage settings to use.
By the way your use of the phrase �it was never a problem to equalize them every 30 days to bring them back up to speed� bothers me. While equalization every 30 days may be prudent on a large alt energy battery bank (especially for people that cannot or will not take hygrometer readings), it is most probably overkill in a boat or RV that has a smaller battery bank and does not experience as many discharge/charge cycles within the 30 day period.
I view equalizing as a controlled frying or overcharge of the battery. While it will fix the problems of sulfation or stratification, it is not something that should be done too frequently. There is a down side to the equalization cycle, and too many of them will shorten the overall life of the battery.
Oso
