How long will it run with a bad alternator

[eavega]

Hello All

I am still trying to get used to my new-to-me 1988 Stingray 176 SVB (3.0L Mercruiser), and had a situation come up which raised a question:

After going to the fuel dock last Saturday on what I thought was my last boating day for a while (lake has gotten too cold for swimming and water sports). I started the boat back up to get to the ramp for loading and going home. It sounded like the battery was going dead (cranking a little slower than I expected) but it did fire up and run, I figure the battery may have run down a little bit (but I didn't know why). I then did a quick (i.e. less than 5 min) WOT run, followed by about 5 min of no-wake speed to get boat to ramp, maneuvering onto trailer, etc. After the harder-than-usual start at the fuel dock, I looked at the volt-meter on the console and it was reading right at 12 V, which is unusual in that when the boat is running the meter usually reads somewhere closer to 14V, especially at full throttle. When I got home, I put the the battery back on its maintenance charger and forgot about it. I thought about it again last night as I was doing some unrelated work on the boat (new temp sender), and decided to check the battery voltage versus what the gauge was reading to see if maybe I had a faulty gauge. Battery was now reading 12.8V (normal full charge) using a multimeter, and gauge was reading about the same (close to 13V). This got me thinking that perhaps something was wrong with the alternator. Lo and behold, when I took a look, the main power cable coming out of the alternator (the big 10-gauge orange wire) had broken away from its terminal and was just dangling there. No oxidation or damage to the cable, so it looks like it had just been pulled away. Anyway, I re-terminated and re-connected the cable, but now it got me wondering, and this is the question I pose to all of you:

How long will the motor run if the alternator is not running, assuming a fully-charged battery, and the only draw on the battery is for ignition, trim, and running the gauges?

I remember on my '76 70 HP Johnson, the battery is only used to run the starter, and afterwards all the ignition charge comes from the stator and the various magnets and coils under the flywheel. Thus, one could run the motor entirely without the battery as long as you had a way to get it started. I'm not familiar enough with the Mercruiser motor, but I would assume this is not the case with the 3.0. I am really just curious as to how involved the battery and/or alternator are in generating spark, and how a failed alternator would affect your ability to run the boat.

Thanks in advance for any information

Rgds

 

[Silvertip]

Re: How long will it run with a bad alternator

That depends on how fast you run. The faster you run the quicker it dies. It also depends on the capacity of the battery. Batteries are like buckets of electricity -- big buckets hold more juice. It also depends on how often you stop and restart. So your question has too many variables to provide a positive answer. However, a "good" battery should run the engine a couple of hours with plenty of reserve provided there are no other accessories running, no lights, no stopping and restarting and moderate throttle (1/2 or so). Lesson: At the first HINT of trouble -- check the gauges.

 

[eavega]

Re: How long will it run with a bad alternator

Lesson: At the first HINT of trouble -- check the gauges.

Definitely got lucky here, as this was my last, short run of the day. In retrospect I should've immediately believed my gut feeling that something was wrong when the gauge was only showing 12 V while running. I should've also keyed in on the fact that the gauges were jumping around a little when I worked the trim, probably an indication that there was something hinky going on in the electrical system. This is my first boat with full instrument panel, so I am getting used their function and behavior. For next time I will know.

So, to add assumptions; a good group 24 starting battery, with a full charge, running at cruising speeds (lets say 3/4 throttle or less), with nothing drawing down the battery other than the standard instruments instruments (negligible I would assume), and the occasional trim adjustments, Only run the starter once for a few seconds to get motor started, should provide enough energy for spark for a couple of hours? The meat of the question I was asking was whether the battery was providing the initial power which gets stepped up to provide the spark, or does it come from a separate system like on my old Johnson 70 HP?

Thanks for the reply.

 

[Bondo]

Re: How long will it run with a bad alternator

Definitely got lucky here, as this was my last, short run of the day. In retrospect I should've immediately believed my gut feeling that something was wrong when the gauge was only showing 12 V while running. I should've also keyed in on the fact that the gauges were jumping around a little when I worked the trim, probably an indication that there was something hinky going on in the electrical system. This is my first boat with full instrument panel, so I am getting used their function and behavior. For next time I will know.

So, to add assumptions; a good group 24 starting battery, with a full charge, running at cruising speeds (lets say 3/4 throttle or less), with nothing drawing down the battery other than the standard instruments instruments (negligible I would assume), and the occasional trim adjustments, Only run the starter once for a few seconds to get motor started, should provide enough energy for spark for a couple of hours? The meat of the question I was asking was whether the battery was providing the initial power which gets stepped up to provide the spark, or does it come from a separate system like on my old Johnson 70 HP?

Thanks for the reply.

Ayuh,... I/O ignitions are Battery powered...

 

[81 Checkmate]

Re: How long will it run with a bad alternator

Yes - as Bondo pointed out.....The ignition is run buy the alt after the motor is running. The battery is for the inital start ...then the alt takes over and supplies power to your ignition/gauges and acc.

So you were just running off the battery juice.

Just like a car motor! when the alt goes out...its running off the battery untill it runs outa juice.


Thats the way i understand it. I could be wrong also

 

[Don S]

Re: How long will it run with a bad alternator

The ignition is run buy the alt after the motor is running. The battery is for the inital start ...then the alt takes over and supplies power to your ignition/gauges and acc.

Not exactly, the ignition runs off the battery at all times, NOT the alternator. The alternators job is to charge the battery.

 

[Todd4]

Re: How long will it run with a bad alternator

I have run hours and hours off of just the battery after losing the alternator (bearings - what a racket it made) in a car. By the time I stopped (at home) the battery was so low it wouldn't re-start with jumper cables. Then, another time I threw the alternator belt on a different car ('79 280-ZX). Judging by my previous experience, it really didn't bother me too much - besides I was in the middle of nowhere on a Sunday morning, so I just kept driving. I ran maybe an hour before the EFI said, "NOPE! I won't put out for anything less than 10.8 volts" and it died on the side of the road (I suspect the injectors quit opening). The point of the story is - you won't run near as long if your outboard has EFI instead of a good old carburator.

Todd

 

[generator12]

Re: How long will it run with a bad alternator

How long will the motor run if the alternator is not running, assuming a fully-charged battery, and the only draw on the battery is for ignition, trim, and running the gauges?Rgds

Depends on the rating of the battery in terms of RC...

"What is reserve capacity?

Reserve Capacity, (RC) is a battery industry rating, defining a battery's ability to power a vehicle with an inoperative alternator or fan belt. The rating is the number of minutes a battery at 80 degrees F can be discharged at 25 amps and maintain a voltage of 10.5 volts for a 12 volt battery. The higher the reserve rating, the longer your vehicle can operate should your alternator or fan belt fail."

That is from Consumer Reports. The same can be found in any write-up on lead acid batteries.

You would need an idea of the current draw of your boat's system while running, and you'd have to determine the rating of your present battery.

 

[catfishcarl99]

Re: How long will it run with a bad alternator

Yes - as Bondo pointed out.....The ignition is run buy the alt after the motor is running. The battery is for the inital start ...then the alt takes over and supplies power to your ignition/gauges and acc.

So you were just running off the battery juice.

Just like a car motor! when the alt goes out...its running off the battery untill it runs outa juice.


Thats the way i understand it. I could be wrong also

nope. your correct. mine went out on my 98 f 150 a couple months back. i ran on the battery for 2 weeks. i had a spare with me. ide get about 35-40 miles in the day. at night 25 if i was lucky with the headlights on. come home and stick the charger on it. had to swap batteries several times.