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.
- Charge to full as often as possible, this point cannot be over emphasized.
- 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.
- Don’t regularly discharge your bank below 50 percent state of charge.
- 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.
- Use smart chargers. Not all chargers that claim to be smart are in fact smart.
- Use temperature compensated charging for all charging sources.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- Avoid installations in engine rooms or hot areas of the boat. Heat shortens battery life.