Don’t Kill That New AGM Battery

An effective charging regimen is essential to prolonging AGM battery life.

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By now, many sailors have enjoyed their first long weekend on the boat this year, and some may have noticed that the expensive absorbed glass mat (AGM) battery they bought last season or the year before isn’t holding a charge like it did when new. There are many things that can lead to reduced battery capacity in an AGM battery, but most often the cause is due to poor maintenance and charging regimen.

Battery manufacturers want their batteries recharged to 100-percent state of charge after each discharge. In reality, few cruising boats (or any boats kept on a mooring) return their batteries to 100-percent state of charge after each cycle. This typical battery use, or abuse, is called partial state of charge operation, and if it continues, a sailor can see his AGM battery perform noticeably worse than a less costly, deep-cycle flooded battery bank. This isn’t the battery’s fault; it is just a lack of understanding of how to properly care for an AGM battery.

In 2015, we carried out a comprehensive update of our 2006 test of sealed valve-regulated lead acid batteries also known as either gel or absorbed glass mat batteries. For the more recent test of partially charged AGM batteries, Rod Collins of Compass Marine and marinehowto.com  cycled five different batteries through 30 deep cycles (to below just 50 percent state of charge) and then put them on the charger for one hour to demonstrate just how quickly a cruising sailor can ruin a good battery. Even after just thirty of these cycles, some of the new absorbed glass batteries in our test never fully recovered to their pre-test capacity.

Here are some tips that came out of our test that can help you prolong the life of your AGM battery.

  1. Charge to full as often as possible, this point cannot be over emphasized.
  2. Get back to at least 80-85 percent state of charge (full capacity) with each charge cycle and get to 100 percent state of charge as soon as you can thereafter.
  3. Don’t regularly discharge your bank below 50 percent state of charge.
  4. Size your most powerful charge source, usually an alternator or inverter-charger, for a minimum of 20 percent of bank capacity. Odyssey TPPL AGMs prefer 40 percent of amp-hour capacity as minimum charge current.
  5. Use smart chargers. Not all chargers that claim to be smart are in fact smart.
  6. Use temperature compensated charging for all charging sources.
  7. Use smart solar controllers. Some solar controllers start each new day at a new absorption voltage charging cycle. This is not healthy for AGM batteries that have low self-discharge and minimal parasitic loads when left unattended on-the-hook. Smarter controllers have a voltage trigger to pop them out of float mode. If they don’t drop to the trigger voltage they remain in float.
  8. Using the correct float voltages are a critical aspect of AGM batteries. Chargers that use dip switches for programming often lack the correct voltages for both absorption and float settings.
  9. Use an alternator temperature sensor and external regulator if possible. AGM batteries can demand a lot from an alternator and the heat created can shorten its life or cause premature failure.
  10. For the best charging performance minimize the voltage drop in system wiring. Even a 3 percent voltage drop at 14.4 volts means just 13.96 volts at the battery terminals. Incorrect voltage sensing robs you of the fastest charging potential, especially during short duration, high current charging events.
  11. Know your correct state of charge at all times. If this means investing in a battery monitoring device it will help in overall cycle life. If you are using voltage to determine state of charge be sure you are getting it as accurate as possible.
  12. Although wet cell batteries can benefit from equalization—carefully monitored charging at a higher-than-normal voltage to recover lost capacity—it is generally NOT recommended for AGM batteries. However, some makers such as Lifeline, do state that careful equalization is possible with their batteries. This process must be monitored closely, and the optimal process can vary by boat. Lifeline can help develop a custom equalization program for your boat.
  13. Avoid installations in engine rooms or hot areas of the boat. Heat shortens battery life.
Practical Sailor has been independently testing and reporting on marine products for serious sailors for more than 45 years. Supported entirely by subscribers, Practical Sailor accepts no advertising or any form of compensation from manufacturers whose products we test. Testing is carried out by a team of experts from a wide range of fields including marine electronics, marine safety, marine surveying, sailboat rigging, sailmaking, engineering, ocean sailing, sailboat racing, and sailboat construction and design. This diversity of expertise allows us to carry out in-depth, objective evaluation of virtually every product available to serious sailors. Practical Sailor is edited by Darrell Nicholson, a long-time liveaboard sailor and trans-Pacific cruiser with more than three decades of experience as a marine writer, photographer, boat captain, and product tester. Before taking on the editor’s position at Practical Sailor, Darrell was the editor of Offshore magazine, a boating-lifestyle magazine serving the New England area. Darrell has won multiple awards from Boating Writer’s International, including the Monk Farnham award for editorial excellence. He holds a U.S. Coast Guard 100-ton Master license and has worked as a harbor pilot and skippered a variety of commercial charter boats.

6 COMMENTS

  1. Hi, I had a couple of gels, 360 Amp Hour then I tried two same capacity. Bad idea. They lasted under a year. Ultimately, I ditched the gels and AGM and stuck with golf cart 6 V. Four provided 440 Amp hours at 12 Volts at much less cost and longer lasting then either AGMs or gels.

  2. You can’t beat flooded batteries. I like Trojan. They have a watering system that uses a squeeze bulb to water all cells at once and it doesn’t allow over watering

  3. Some AGM batteries tolerate equalization, Lifeline actually recommends it. This will help de-sulfate the plate surface area lost from sitting at long periods of partial discharge. On my house bank I’ve gone from apparent capacity loss of 35% to fully recovered after an equalization cycle.
    Bob Williams
    Owner of SALT (Sea Air Land Technologies Inc.)

  4. Wakespeed has made it much easier to charge properly while underway with its WS500 alternator regulator. Unlike typical voltage-only single stage and multi stage regulators, the WS500 offers the ability to connect to a current shunt at the battery and much more closely follow battery manufactures’ recommendations for charging. The WS500 also offers CANbus connectivity, and is integratable with a variety of LiFeP04 battery and BMS brands. It’s a far more logical approach than the outdated voltage, time and PWM approach that’s been the standard for so long.

    Rick Jones
    Thomason Jones Company

  5. This is crazy. AGMs are advertised for their ability to be abused. You can run them tipped upside down, their low discharge, and faster charging, not to say a supposed improvement over gel batteries. Now you say when they’re discharge, they have to be topped up every time? Firstly this is the first time I have ever heard of this over West advisories, Boat US reports and even with people like Don Casey. Where do yo get your information. Certainly not from their manufactures? BTW I just replace all of mine AGMs after 6 to 8 years of rugged misuse including keeping the 80 lb guests on board over temperate Michigan winters in outside storage. They work well and went their advertised limit.

    • Thanks for your comments. If you follow the links to our actual tests you should find the most complete answers to your questions. Or you can send questions directly to practicalsailor@belvoir.com and I’ll try to help. Much of this guidance does echo that given by the manufacturers.

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