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Author Topic:   Absorbed Glass Mat Battery
rtk posted 12-17-2006 06:51 PM ET (US)   Profile for rtk   Send Email to rtk  
There has been alot of discussion on the pros and cons of AGM (absorbed glass mat) batteries. One of the topics that are frequently discussed is the proper voltage levels to charge this type of battery.

A recent topic warns that an AGM battery charge voltage should not exceed 13.5 volts continuous.

I started a seperate thread because I thought this may be good as a seperate discussion to FrankO's thread, and maybe help him and others make an informed decision on the AGM.

There was a statement made in this thread about the proper voltage level to charge an AGM battery.

The typical reading of most marine charging systems, by the boat's voltmeter and by a seperate handheld volt meter is in excess of 14 volts, and usually less than 15 volts.

My 2003 Mercury 250 EFI electrical system output is 14.2 volts, as per the Smartcraft system.

I switched to AGM batteries a couple of years ago. The fact that the battery requires no service (water level in cells) and the very low self discharge rates were the reason (I hate checking fluid levels if I don't need to).

The label on my MK AGM group 27 battery lists the charging limit at 14.6 volts. The label on my Cabelas group 24 AGM battery lists a float charging level at 13.5-13.8 volts, continuous cycle service at 14.1-14.4 volts.

I have had absolutely no problems with my AGM batteries in my boat charging them at 14.2 volts. The boat actually sits around alot and have never had a situation where I lacked battery power, and the voltage reading is rock steady when running the boat.

Are AGM batteries suitable for use on marine engines that charge in the 14-15 volt range?


davej14 posted 12-18-2006 01:07 AM ET (US)     Profile for davej14  Send Email to davej14     
Ideally a lead acid battery charger will have three stages of charge: bulk, absorption and float. During bulk charge mode, the battery will be charged to about 80% of capacity. As the voltage rises to 14.1-14.8V at 80 deg F(depending upon the battery type) the charger would then switch to absorption mode and regulate the charge voltage until the battery is at 95% to 100% of capacity. At this point the charger should go to float mode. In float mode the voltage applied should only exceed the fully charged open circuit voltage of the battery by a few tenths of a volt, typically about 13.5 volts.

The charger systems on outboards are not very sophisticated and if you are applying 15 volts to a fully charged battery you are creating hydrogen, oxygen and sulfates while gassing the electrolyte. Gassing will start around 14.2 volts in an AGM or flooded cell battery. With an AGM cell the gases are recombined unless there is venting. With a flooded cell these gases can attain an explosive level in a contained space. If you are constantly adding water to a flooded cell then you are overcharging. If your charging voltage is 15V continuously you should consider adding a voltage regulator.

I would prefer an AGM battery to a flooded cell for marine applications.

jimh posted 12-18-2006 10:08 PM ET (US)     Profile for jimh  Send Email to jimh     
The AGM Battery, or the Absorbed Glass Mat battery, or the Absorption Glass Mat Battery, or the Absorbent Glass Mat battery is a battery that sure has a lot of names.
davej14 posted 12-20-2006 10:20 PM ET (US)     Profile for davej14  Send Email to davej14     
An AGM battery, no matter what you call the "A", is constructed using high purity lead plates which are separated by a dielectric fiberglass mat that carries the electrolyte (acid). There are several advantages to this constriction in a marine environment:

1. Because of the supportive glass mat, AGM cells are highly resistant to vibration even though they use high purity lead plates. Some constructions use laminated plates and some use spiral wound plates (ie. Optima).

2. A flooded cell must use a lead alloy for its plates because they are not supported by a fiberglass dielectric and therefore must be more rigid than the cells of an AGM battery. Lead alloy will have higher internal impedance so they are less efficient at producing current or being charged.

3. AGM batteries are sealed so that they do not out-gas potentially explosive hydrogen and oxygen or leak corrosive sulfuric acid.

4. AGM batteries have much lower internal leakage so they will maintain their charged state for extended periods. A flooded cell should be charged every month.

The only advantage I can think of for a flooded cell is that they have a lower in initial cost. You get what you pay for.

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