howdy. i'm in need of some advise. i have a pretty decent stereo in my boat and the volt guage dips quite a bit. would it be better to run two batteries in parallel, or two batteries with a master switch? we dont listen to it at the beach, only during cruising time. i dont want this to become a debate about noise etiqutte, as there is usually no one around to complain. i'm thinking i should go with 2 batteries in parallel, as there is no load when the boat is not running. let me know what you think. thanks.

How much is "quite a bit"?

Does it still read 13.5 volts or more when underway?

We need a guesstimate of the draw the system puts out, to evaluate all the available options.

d-rod, sometimes if its a deep bass hit, yes, it will dip to about 12.5 volts. never seen it go lower than that unless i'm adjusting the trim. hate to see what would happen if i used the spotlight.
f14, not really sure. i have a kenwood high power head unit and 2 amps with 4 infinity speakers and a sub.
i was thinking that if i wired the batts in parallel i would almost double the capacity at 12 volts and hopefully reduce the voltage drops. might even make life easier on the alternator i just had to rebuild.

What engine are you running?

My suggestion would be to install a second battery. But I would also install a more powerful alternator too. If you give me your engine make & model, as well as year (I'm assuming 1995), I can see what alternator options we have.

Chris

The alternator is designed to keep the battery/batteries topped off, not to fully charge them. I'm willing to bet part of the reason you had to rebuild yours, is because you're asking too much of it.

I wouldn't add another battery for the purposes you outlined. I would, however, add another switched battery OR carry a portable jump pack.

Like F14 said, you need a higher capacity alternator.

Is the voltage drop at idle or at cruising speeds?

I'm sorry if I wasn't clear.. Adding a switch and a battery was what I mean.

Agree F14.

sorry, the engine is a 1995 3.0 lx. i beleive the alternator is 55 amp. just rebuilt about 30 days ago.

I'll check into it tomorrow morning.

Second battery will not reduce the load on the alternator, but will keep the voltage up for big draws.

Have you run adequate wire to and from the amps? Unlike a car, where the ground is generally nearby the device under power, a boat requires a -12V lead as long as the +12V lead. You need to consider the return path of the leads in the circuit length when calculating wire size. If your amplifier is drawing 50 amps of current, and are 10 to 12 feet from the battery, you need #4AWG marine wire for both leads, at a minimum, per amp. That's some serious wire.

Use as big of wire as you can get. #4 as a min I would say for both power and ground to the system. The smaller the wire, or the longer distance traveled, the higher the wire resistance. This will cause a voltage drop at the amplifier. The less voltage the amp gets, the more current it will draw. 55 amp alts are not big at all. Thats where the issue lies. You may need to upgrade the size wire (positive and negative) from the alternator to the battery as well. That is done in auto sound a lot. If you can, maybe add a stiff capacitor to the system, that may help. Not sure about how to do that in a marine enviornment though. Double batts with a switch will provide a means to crank with just in case the regular batt fails.

My system I just installed has a 4 channel amp at 50 watts per channel rms. This is about 70.7 watts peak @ 4 ohms. My speakers can have a min impeadance(resistance) of 3 ohms. This makes it possible in theory to get 86 watts peak out of each channel. That times 4 is 344 watts. If the amp is only 50% efficent, then the total draw in watts to produce these 344 watt peaks would be 344x2 or 688 watts. The total amps to produce those peaks using ohms law would be 53 amps if I had 13 volts at the amplifier terminals, or 55 amps if I had 12.5 volts at the amp, or 50 amps with 13.8volts at the amp. As you can see, there is not anything left when the system is running full bore.

I have also read that you can take the fuse rating of the amp and that would give a pretty good ballpark of the current needed at full tilt. My amp uses 2 25 amp fuses. total of 50 amps. pretty close. That amp, when I used it in my toyota truck with a 65 amp alt also made the lights dim or blink to the music when played hard. As a side note, I do not know if the voltmeter dips when I play the system hard. I just have not noticed yet. Also, I dont run a sub. The sub can cause a large amp draw due to its impeadance or resistance lowering total system resistance and causing an increased current draw. Resistance is just that in DC circuits, but when talking about speakers and music, the voltage at a speaker is AC and the resistance will change with frequency thus you have impeadance which is resistance in a frequency changing ac circuit. If a 4 ohm sub is bridged across 2 channels of the amp, then those 2 channels each see 2 ohms which doubles the current draw. So an otherwise 30 amp draw could become 60 amps. Something to think about.

Just some thoughts I had. Others can chime in that have done these things as well or to correct any mistakes I may have made in my thoughts or calculations.

I agree with the wire advice, and your dipping voltmeter is telling me the problem is your wiring, probably factory, is just too small for your increased current demands for the stereo.

A fact that a lot of folks don't realize is that with DC wiring, there can be a tremendous voltage drop in just a few feet of wire. The amount of voltage drop is a function of distance, required current, and the wire's AWG.

One concept that may be difficult to understand is that the wire may very well be sufficient to handle the current, but not sufficient to keep from having an excessive voltage drop.

Electron flow (the flow of electricity), is actually a mechanical process, wherein electrons flow from one atom to another. This causes friction. In layman's terms, using a larger AWG wire is sort of like reducing that friction. As friction increases, resistance increases, and the wire begins behaving as a series resistor in effect.

The confusing part then is that the wire may be way oversized in terms of current carrying capacity, but not at all oversized due to the resistance factor.

So, the advice of using the large AWG wire that is being suggested is due to the voltage drop, not the current requirements.

To put it into perspective, using williemon's figures of 50Amps of current, a 4AWG wire, marine grade with a 105 Degree Centigrade jacket, can actually handle 160Amps of current, far more than this expected 50Amp load. However, if you look at the voltage drop, assuming a 3% voltage drop between the battery and amp (0.36V assuming a nominal 12V supply), then at 10ft, the formula indicates you need a wire having 29,861 mils. One mil is a wire having a thickness of 0.001, so 29,861 mils falls between 4AWG and 6AWG.

So, a 4AWG wire, 20ft total round trip (+ and - leads), can handle 50Amps with no more than a 3% voltage drop, or a maximum of 160Amps (with an exceedingly significant voltage drop).

However, in williemon's example, he is putting a lot of emphasis on peak current - with all channels simultaneously at peak - which probably is not going to happen. In that case, you can probably use something less than 4AWG. How much less depends a lot on your type of music, its dynamic range, and the instantaneous power demands of the system.

The stereo system on my boat has 4x40Watt and 1x150Watt amplifiers (CES2006 RMS). The amps are only about 3ft from the DC distribution panel, and I have both wired with independant 12AWG wires. The circuit breaker at the distribution panel is a 20Amp breaker - powering both amps. By the way, most breakers trip at 125% or more of average - not peak current (fuses on the other hand are much faster acting).

I have no power issues at all, no voltage drops, no overheating of the wire, no flickering of lights; even the amplifiers don't get warm on their heat sinks.

But the main reason I can get away with the 12AWG wire is the amps are only 3ft from the power source. If the amps were, say 20ft away from the distribution panel, I'd have to use at least 8AWG just to maintain the low voltage drop on the wiring.

There are calculators on-line that can help you in determining the size of wire vs. voltage drop. One such calculator is at www.genuinedealz.com

I would caution about using a capacitor in a marine environment. A very large capacitor is typically used in an auto stereo environment to somewhat mimic a short-charge battery at the source of the amplifier. However, in a marine environment, with the requirement for ignition proof engine room components, the installation of a capacitor, even if it is outside of the engine room, could cause a spark on the battery when it is being serviced (removing battery wire), loose battery wire, or so on.

wow, thanks for the detail. that helped out alot. i'm going to upgrade the wiring before i do anythng. with any luck that should take care of most of it. you guys really helped alot.
thanks