AGM BATTERIES

While fitting out my new boat, I went looking for a better battery. With today's technology I thought there had to be an alternative to corrosive acid spills, dangerous gas emissions, loss of full charge when a boat is left standing for a few weeks, and lengthy recharges after using an electric motor. Fortunately, I didn't have to look far because, at long last, technology has made life with boat batteries easy.

Absorbed glass mat (AGM) batteries were originally developed for military use. They keep their electrolyte captive within fibrous glass mats between cells mounted inside a sealed case. Other technological advantages aside, the ruggedness of AGM batteries makes them perfect for boats and 4WDs.

AGM batteries are spill-proof to a point where they'll literally still function when standing on their heads — or for that matter upside down — or even when submerged. Acid spills and corrosive emissions during charging are a thing of the past, as is constant checking of electrolyte.

AGM batteries are both a deep cycle and a cranking battery in one, so they can be used to power an electric motor and be used as a cranking battery to start the outboard. They have very low rates of self-discharge, so don't go flat if left for a while without use.

AGM batteries can also be charged at much faster rates than normal wet cell batteries. The 55 pound Minn Kota bow mount electric on my boat gets used a lot for slow trolling and moving about while lure casting — which of course does eventually run the battery down.

By using a special charger (supplied by Absorbed Power, the people who distribute the AGM battery I use,) I can bring the battery back to full charge in no time. Thanks to an auto cutout function in the charger, even in the couple of hours between early morning and evening sessions there's ample time for a full recharge without worrying about over-charging.

One thing that is quite important with these AGM batteries is to avoid over-charging because it's one of the few things that can reduce the extended service life they offer. Switch Mode AGM chargers supplied by Absorbed Power use a 3-stage system that initially fast charges at 14.6 V while charge levels are low. As the battery nears full charge, the charger switches down to ‘absorption’ mode before going into ‘float’ mode to maintain a full charge.

It means I can leave the battery connected to the charger overnight and get a good night's sleep secure in knowing we'll have a fully charged battery the next day, without worrying about reducing battery life by over-charging.

Something I discovered while investigating better batteries is that standard wet cell lead acid batteries can commonly only be charged to 65-70 per cent of capacity by normal boat and vehicle alternators. On the other hand, AGM batteries can be fully charged by a boat or vehicle charging system.

Standard wet cell lead acid batteries can also be damaged if regularly discharged to beyond 50 per cent of their capacity — which you often do while fishing with electric motors. It doesn't compromise an AGM battery's service life though.

In a nutshell, after many dramas with boat batteries powering electric motors, an AGM battery has changed the experience from a constant hassle to no problem.