posted 12-02-2007 12:04 AM ET (US)            

My lessons with AGMs have taught me that a new alternator/charger may be required because AGMs really need a higher amperage charge that standard lead acid or gels. Additionally AGMs require a special charging cycle that a lot of standard chargers do not perform, another thing to check out.

posted 12-02-2007 11:10 AM ET (US)            

The fundamental difference in charging an AGM battery as compared to a conventional lead-acid flooded cell battery is that the AGM battery is a sealed battery. When a lead-acid battery is charged with excessive voltage, the chemical reaction causes gas to form. In a conventional flooded-cell lead-acid battery, the battery is vented to the atmosphere, so any gas released is not contained in the battery. In an AGM battery, there is no open vent, and the gas accumulates in the battery container, building an internal pressure.

The typical AGM battery is a valve-regulated battery. Its enclosure contains a pressure relief valve. If the internal pressure builds to an excessive level, the valve opens and releases the pressure.

In a conventional flooded cell battery, electrolyte which is lost due to out-gassing can be replaced. This is just a matter of adding more water to the battery and recharging it. In a valve-regulated lead-acid absorbent glass mat battery (VRLA AGM) battery, electrolyte lost in gassing which is vented cannot be replaced. An AGM battery which is chronically over-charged will lose electrolyte, and this can lead to very poor battery life.

So we see that there is justifiable concern for the voltage used to charge an AGM battery. Now we turn to the specific problem of Edward's small boat electrical system: his Yamaha 90-HP motor produces 15.1-volts of charging output.

First, I question the nature of the voltage regulation on the Yamaha motor. Edward says that is is "regulated" at 15.1-volts. This seems like an odd voltage to select for regulation. Is there any confirmation that the Yamaha charging system is regulated to 15.1-volts? Or does is just have a simple unregulated output which tends to peak at 15.1-volts?

Second, I question the notion that an AGM battery cannot be charged with a charging voltage greater than 14.0-volts. The restriction on charging voltage for a lead-acid battery with a gelled electrolyte is often cited as being 14.0-volts, but the charging voltage for VRLA-AGM battery is generally permitted to be as high as 14.5-volts without damage. The exact recommendation varies with the battery manufacturer. A typical value, which varies with temperature, is about 2.4 to 2.43 volts per cell, and thus for a six cell ("12 volt") battery gives a charging voltage of to 14.4 to 14.6 volts.

Third, I am not fond of the proposed design in which a constant voltage drop is inserted into the charging circuit output. This is based to two problems. First, I doubt that the charging output is as well regulated as suspected, so that there will be plenty of times when the charging output voltage will be lower than 15.1 volts. If a constant voltage drop is inserted, and the charging voltage is lower than expected, the final output could be too low to effectively charge the battery. Secondly, the voltage drop across the diode will create heat which will have to be dissipated. This will require some sort of heat sink arrangement. Mounting all of this under the cowling of the outboard motor could be a problem.