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Old 02-01-2001
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Tom Wood is on a distinguished road
Multiple Charging Devices

If you are charging your batteries using shore power and at the same time run your diesel engine, which in turn runs the alternator which also charges the batteries, are you creating any electrical problems?

Tom Wood responds:

Funny that you should ask that question right now. I’ll tell you a story in a minute.  The short answer to your question is no—there shouldn’t be any problems. But this depends on having good quality marine charging equipment with adequate regulation. Some inexpensive automotive-type battery chargers do not meet this requirement. A note here too about regulation—even though we all refer to them as "voltage regulators," these devices are in fact current regulators and maintaining the voltage at a given point is simply a by-product of the maintaining the current flow.

It isn’t uncommon to see cruising boats motoring along with a wind generator turning merrily and a large array of solar panels converting sunlight into electricity. In this case, three charging sources are at work simultaneously. If the regulators on all three sources are adjusted to maintain the same level, each source will share happily in the charging process. If one regulator is set to a slightly higher voltage, that charging source will be asked to provide the lion’s share of the charging.

Now, for the story. Old-fashioned regulators with point contacts rarely had any interaction and each regulator would sense the total line voltage and react to it, regardless of the source. The new "smart," multi-stage regulators have become so intelligent that they sometimes outsmart themselves. We recently had a case where the alternator regulator and battery charger regulator would play a slow motion game of tug-of-war with each other. With both adjusted to 14.2 volts (flooded cells), the alternator would start out at 40 amps output and the battery charger, sensing this high output would shut down to just a few amps. The smart alternator regulator, however, would sense the small output from the charger and taper the alternator output off rapidly. The smart charger, feeling this reduction in output would boost its output. The voltage would stay at 14.2 and the total amperage input to the batteries would stay around 40, but the load would oscillate back and forth between the alternator and the battery charger. Fascinating to watch on the gauges and not in the least bit harmful as long as the total voltage and amperage are under control.


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