Don’t Kill That New AGM Battery


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  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. Dont 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. Avoid installations in engine rooms or hot areas of the boat. Heat shortens battery life.
Darrell Nicholson, editor of Practical Sailor, grew up boating on Miami’s Biscayne Bay on everything from prams to Morgan ketches. Two years out of Emory University, after a brief stint as a sportswriter, he set out from Miami aboard a 60-year-old wooden William Atkin ketch named Tosca. For 10 years, he and writer-photographer Theresa Gibbons explored the Caribbean, crossed the Pacific, and cruised Southeast Asia aboard Tosca, working along the way as journalists and documenting their adventures for various travel and sailing publications, including Cruising World, Sail, Sailing, Cruising Helmsman, and Sailing World. Upon his return to land life, Darrell became the associate editor, then senior editor at Cruising World magazine, where he worked for five years. 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.


  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


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