Power from alternator - how much is 'spare?'

[jameshoward]

I understand that the output of the alternator on a 230SL (1966, manual, no a/c, with p/s) is 490 watts. I would like to install a small discreet amplifier under the drivers seat. I have found a 100w amp but would like to know whether such an amplifier would draw too much current through my alternator and so damage it. In other words, is there enough juice left after that required by the car to charge the battery, run the lights, etc to power a 100 watt amplifier?

Are there any electrical ninjas out there would can tell me how to work out what the car needs and what's left over? In fact, I'm not even sure that I understand my own question...but hopefully someone understands what I mean.

Grateful for any advice.

JH

[Peter van Es]

I'll give you a view and then another view. You can then hopefully determine the answer...

 

• yes, you can do that, as long as you do not use the amplifier at the same time as your headlights  
• I have two 55W high-beam accessory lights and have found no problems whatsoever driving the car with these, and normal lights on.

In other words, joking aside, I think you'd have no problem getting the amp, and even using it in the dark. I could not begin to answer a question about how much draw of current is too much.

When driving the car, the alternator needs to support the power consumption of all of the accessories. The battery is indeed another load on the alternator (because some current is being used to charge it).

Only when idling, or when the demands of the accessories exceeds the capacity of the alternator, will the battery start supplying current also.

The alternator, if rated at 490W, outputs a voltage of some 14 volts, and can supply around 35 Amps. The battery, if original and rated at 55Ah, can supply 55 Amps for an hour... then it is dead. Some of us have 70Ah batteries installed.

If the current drain exceeds that which the battery and the alternator can provide, your battery will deplete during use and you will need to put it on a charger in between uses (a trickle charger is ideal).

Since I do not know what the fan, lights, wipers, heater, powersteering etc consume, it's hard to say what the limit it. However, my experience with my lights shows you can safely add a 100W amplifier. Oh, and by the way unlike with my additional high-beam headlights, the 100W rating of the amplifier is peak performance... it will only be used when you've turned the volume up to boombox levels with a very strong bass line. So, your lights might dim in tune with the music. Has some appeal too!

Peter

1970 280SL. Also known as 'admin@sl113.org' and organiser of the Technical Manual.

[JimVillers]

James ... As Van says, an amplifier is rated at RMS power output.  I doubt that in the reality of "normal" use that it would draw enough amprage to be a problem.    

If the amplifier actually consumed 100 watts, that is only a little over 8 amps.

Jim Villers
190SL, 230SL 5-Speed, MGB 5-Speed

[Paddy_Crow]

You should never wire any load directly to the alternator. There should always be a fuse in the circuit to protect the alternator, the wiring, and the component itself. But it is unlikely that you would use a heavy enough wire to damage the alternator.

If you wire the amp off an interior circuit, it's not going to damage the alternator. I won't promise, however, that it won't overload the circuit and blow the fuse...

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