'89 OMC - Adding Sound - current draw concern

[Liquid_force]

Hi -

We have a pair of tower speakers and an amp capable of 300-400w rms at full output -- of course it is rarely driven at full output. It has a pair of 15A fuses.

The concern is I am planning to add a couple subs in the foot well area along with a 2nd amp. Total theoretical power would be approaching 1000w rms. Again, most of the time running at 1/2 volume or less.

In a car I've always been told alternators are sized w/about 40% head room. It is common to run 1000w rms sound systems w/o significant impact on the charging system. I'm wondering if the same principle would apply in a boat?

I'm not sure what size the stock alt is. I want to think I saw 51A somewhere. DB electrical sells 65A replacements.

Basically I'd just like to accumulate some info on my current alt's specs, and the average current draw of the boat's standard electrical system.

 

[Silvertip]

Re: '89 OMC - Adding Sound - current draw concern

Yup -- prime candidate for a dual battery switch or ACR and a very large group 29/31 deep cycle to power the system.

 

[HT32BSX115]

Re: '89 OMC - Adding Sound - current draw concern

Howdy,

First of all this is really not a Mercruiser. OMC, Volvo, issue. It would be better placed in Electrics, Electronics and Trolling Motors

2nd, I'll "second" the second battery..

3rd, There's no such thing as watts "RMS" it's just meaningless.

There's peak watts and there's average watts.

Peak watts (like peak voltage or current) could be measured using a peak-reading watt-meter or scope.

You are probably interested in average current in this case when you talk about ALT/GEN power capability.

You'll also be interested in peak current only in that the power supply (the battery or alt) is capable of the peak currents required so there is no severe drop in supply voltage at the audio amp......as it would cause additional distortion at peak output.

Batteries are capable of fairly high momentary peak currents (several hundred amps in some cases) Alternators are not.

At currents not much higher than rated, an alternator will "fold-back" a little and will not provide rated voltages.....such that the voltage output of an alternator will drop to battery voltage ....... Some regulators have what's called "Fold-Back Protection" which will protect the power supply during "Faults" (shorts)..... Then it's all battery baby!

In automotive applications (including marine and aviation) the generator/alternator is designed to provide enough current to power everything on a continuous basis........

..........AND at the same time, provide charging current to the primary battery, which is used to start the engine.... (and in aviation the battery is usually considered ONLY an emergency power source to be used ONLY when the primary and/or secondary GEN fails.......in "light" planes it's also a "starting" power source like in a marine application.......)

.......And still be some amount below rated maximum current output. (you called that "head-room")



400W average would require approx 30A (additional) from the power supply. If you're running audio levels at that power the peak power can be much higher and the peak currents can also be much higher (which is why some use HUGE capacitors in the circuit ......they provide the short duration, peak currents that no battery (or alt) can provide)


Your best bet will be to install an additional BIG deep cycle battery (like the BIG marine Optima etc) to run the high current draw "stuff"



(jump in here Bruce!!)



Cheers,


Rick

 

[Liquid_force]

Re: '89 OMC - Adding Sound - current draw concern

I don't need a lecture in 12v audio terminology. The 12v audio industry (and informed consumers) are well aware RMS power is a term specific to the industry. It IS the standard by which amps are compared and it's the most relavent spec of a 12v amplifier. It's average continuous power -- we call it RMS.
Peak ratings on car amps mean almost nothing. Companies that define the term all define it differently. Most just use it as an attention getter to lure ignorant buyers. It is little more than a random number in most cases.

If you'd like me to give you a summary of actual power requirements of a 1000w (RMS!) sound system as it relates to music and crest factor I'd be happy to.
It is A LOT lower than most people would expect.
RMS ratings of an amp are derived using 0dB sine waves.
1000w of sine wave power from an amp that's 60% efficient (pretty typical) would be about 120A at 14v. Thankfully we don't listen to sine waves. Extremely high demand music would require no more than 1/3 as much - 40A. At something less than full volume it's going to be in the 20-30A range over a few minutes of use.

I'm not opposed to an audio only battery, but if I'm playing at 1/2 volume it would seem reasonable to expect at least a half hour of play out of a decent fully charged starting battery.

I'd simply like to determine the current capability of the std charging system because locating a higher output alt and installing it would be a much bigger hurdle than a 2nd battery.

 

[Yepblaze]

Re: '89 OMC - Adding Sound - current draw concern

I don't need a lecture in 12v audio terminology. The 12v audio industry (and informed consumers) are well aware RMS power is a term specific to the industry. It IS the standard by which amps are compared and it's the most relavent spec of a 12v amplifier. It's average continuous power -- we call it RMS.

If you'd like me to give you a summary of actual power requirements of a 1000w (RMS!) sound system as it relates to music and crest factor I'd be happy to.
It is A LOT lower than most people would expect.
RMS ratings of an amp are derived using 0dB sine waves.
1000w of sine wave power from an amp that's 60% efficient (pretty typical) would be about 120A at 14v. Thankfully we don't listen to sine waves. Extremely high demand music would require no more than 1/3 as much - 40A. At something less than full volume it's going to be in the 20-30A range over a few minutes of use.

I'm not opposed to an audio only battery, but if I'm playing at 1/2 volume it would seem reasonable to expect at least a half hour of play out of a decent fully charged starting battery.

I'd simply like to determine the current capability of the std charging system because locating a higher output alt and installing it would be a much bigger hurdle than a 2nd battery.

So what was the questrion?

 

[Liquid_force]

Re: '89 OMC - Adding Sound - current draw concern

So what was the questrion?

forget it. I'll figure it out myself.

 

[BRG25]

Re: '89 OMC - Adding Sound - current draw concern

What engine do you have in your boat? Take a look at the Sticky thread at the top and find the service manual for yours. My 4.3 alternator is rated at either 55 or 65 amps.

 

[Liquid_force]

Re: '89 OMC - Adding Sound - current draw concern

it's the 4.3.
P/N's 985466. I've found 65A replacements but I was thinking I saw a rating on the alt itself of 50 something.
I'd feel a lot better if it's the 65 option since I don't think the standard electrical system requires nearly that much current.

I don't know -- I guess I can just give it a try and see how things run.

Edit:
Thx again bubba.
I guess I'll just continue as planned w/the sound system. First couple trips out I'll haul along a back up battery and if I have a problem I'll incorporate it into the charging system.