Question about alternator output

[jhaynie]

This is actually for my boat, not my car, but I would think its the same thing (4 stroke engine).

The voltage at the battery is only reading 13.4V at 2500rpm. I have always seen 14.6 volts minimum. Could a bad battery (it would have to be two batteries in this case) cause this, or would it have to be an alternator/regulator problem.

John

Unique 289FIA #9367
351W Fuel Injected

[dmiller]

if one of the batteries is internally shorted, your alternator would not be able to put out the normal voltage. can you separate the batteries, and watch the voltage while only charging one at a time?

[Jim Harding]

John,

In a word.... maybe. A charged battery "at rest" will read something on the order of 12.6 volts. If you then start the engine, and spin it up enough so that the alternator outputs something higher than 12.6 volts, the alternator will then supply all the voltage/currnt needs of the car (or boat) and also supply a recharge current to the battery. If the car (or boat) puts demands on the alternator, the output voltage will drop. Normally, with very little load on the alternator, the 14.6 volts you mention is probably the MAXIMUM voltage you should see, and not the minimum. Voltage readings over this can actually over charge the battery and damage it over time.
The size of the alternator, usually rated in watts (which is voltage times current), will be the determining factor on what the voltage reading will be under different loads supplied by it.
A bad battery that has a very low internal resistance will put an abnormal load on the alternator and cause the voltage to be lower than what you have previously seen. This is providing you have not adden any electrical devices that may now be taxing the alternator. To check this, turn off everything electrical so that the only thing the alternator is feeding is the engine electrics and the battery. If the voltage is still reading 13.4 volts, shut down the engine and try running the engine on just one battery at a time. If you get the same exact voltage reading with each battery, you may have an alternator that is beginning to crap out. If one battery reads something closer to 14.6 than the other, then you might have a battery going south.
This test is the easiest one I can think of to eliminate one or the other. Let me know what you find.

Jim Harding
La Plata, Maryland

1982 #3004

[COBRAGAR]

JIM ,HOW EVER YOU TEST IS UP TO YOU AND THE INFO YOU GATHERED HERE. BUT PLEASE GO BACK W/A MARINE ATLERNATOR MARINE ATL. HAVE BUILT IN SPARK ARRESTOR SO NOT TO IGNITE THE GAS VAPOR IF YOU WHERE TO HAVE ANY . GOOD LUCK