Alternator/Charging Question

[kmongar]

Hi,
I am hoping someone can help me figure this out. I have a 1995 Volvo Penta stern drive, ford 302, with 2 batteries and a Perko switch. The batteries are a couple years old but have had limited use and have always been kept fully charged. The alternator was rebuilt recently by a very reputable local company. A few days ago I was fishing in the bay/ocean and the motor was running all day and a bait live well pump was also on most of the day. There was also a fish finder on. I had the Perko switch on battery 1 all day. That battery went dead and the motor died after about 12 hours on the water. I switched to battery 2 and everything was fine. My question: Why wasn't the alternator keeping the the battery charged?
any help would be appreciated.

 

[boat1010]

Re: Alternator/Charging Question

I guess the first thing you need to do is figure out if the charger is working. Just take a volt meter and see what the voltage is when the motor isn't running, then again with the motor running. It should be over 13 volts when running.. Test the battery and see if it is good. All things go bad sometime or another. Batteries do not last for ever.

 

[Silvertip]

Re: Alternator/Charging Question

You very likely have a voltmeter on the console of your boat. It tells you whether or not the system is charging but you do need to look at it periodically. Just like oil pressure, fuel, water temp. Unless you check, Battery #2 may not be charging either and your reputable alternator builder may not be as reputable as you think. And it is certainly possible that it hasn't worked since you reinstalled it because of something you did or didn't do.

 

[kmongar]

Re: Alternator/Charging Question

OK thanks. The voltmeter on the console reads 12 at idle and about 13.5 under throttle. The majority of the day was at idle.

 

[Silvertip]

Re: Alternator/Charging Question

There your are! 12 volts is not charging -- in fact it is "discharging". 13.5 or more is charging. The ignitiion system on a boat draws a fair amount of juice and will kill the battery. Alternators need some revs before they produce enough power to actually charge the battery -- especially if there are other high current draw items being run at the time.

 

[kmongar]

Re: Alternator/Charging Question

OK thanks....I wondered about that. I guess I will take the alternator in to the shop for check out. It may be that I just need to upgrade to a higher output alternator.
thanks again.

 

[boat1010]

Re: Alternator/Charging Question

Something else you can look at is maybe a different size pulley on the alternator so it will spin faster when the boat is at idle. If you run it that low of RPM a lot it might be something to look at.

 

[Silvertip]

Re: Alternator/Charging Question

You totally missed the point. There likely is NOTHING wrong with the alternator. I said it takes some revs for some alternators to produce charging voltage. Since you were at idle most of the day, the system was discharging. Your car on the other hand operates totally different than a boat so the pulley size is such that they do produce power at low rpm. If the voltmeter on the boat shows 13.5 volts or higher at 1500 rpm or higher then the alternator is ok.

 

[hrdwrkingacguy]

Re: Alternator/Charging Question

If he is sitting fishing with accessories on and the engine idling, how long would you have to rev it to 2500 or 3000 to get one battery back up to comfortably charged? I realize there are different battery sizes and different levels of discharge, but how long should it take a 60 or 70amp alternator to refresh a battery(roughly)...Could it be a few seconds or does it take a few minutes or longer???

PS I realize the complete arbitrariness of the question, but i am positive almost everyone on this board has sat anchored listening to music, enjoying the day, and wondered what their battery is going to do...

 

[Silvertip]

Re: Alternator/Charging Question

If he is sitting fishing with accessories on and the engine idling, how long would you have to rev it to 2500 or 3000 to get one battery back up to comfortably charged? I realize there are different battery sizes and different levels of discharge, but how long should it take a 60 or 70amp alternator to refresh a battery(roughly)...Could it be a few seconds or does it take a few minutes or longer???

PS I realize the complete arbitrariness of the question, but i am positive almost everyone on this board has sat anchored listening to music, enjoying the day, and wondered what their battery is going to do...

thnk about it. A 60 amp alternator puts out 60 amps maximum. A deeply discharged 60 or 70 AHr battery would take on hour or more to become fully charged. And just so you know, you do not run an engine at 2000 or 3000 rpm in neutral unless you want all the internal parts to escape through the hole left in the block by those pieces. The object here is to make use on-board electronics only within the means of the on-board electrical system. There is no free lunch here. If you use more electricity than you are generating the battery goes dead. Simple as that.

 

[fishrdan]

Re: Alternator/Charging Question

I think the alternator is suspect, or at least, it's not operating like he needs it to.

An alternator won't start charging until it gets over it's cut-in RPM, then it will keep charge until the engine is turned off (or maybe until the alternator's RPM drops too low). I was having issues with my old alternator not charging enough at slow speeds so I bought a 100 amp high output alternator. The new alternator has a smaller pulley on it, so it spins faster. Once this alternator gets over 1200 engine RPM it kicks in and will keep charging until the engine is turned off.

I can troll of my 24V transom mount trolling motor for 6-7 hours before the batteries need charging. At that point I change over to trolling off the main engine, Mercruiser 140HP, and charge the batteries while idling off the main engine, 700RPM or so. It takes a couple hours to charge the trolling batteries back up, but then I can go back to trolling off the electric trolling motor again. I'm guessing the 100 amp alternator is kicking out 50 amps +- at idle as it drags down the engine while charging heavily.

The high output alternator is my free lunch D) as I can drain down my trolling motor batteries and charge them back up while on the water.

 

[Silvertip]

Re: Alternator/Charging Question

Two 12 volt deep cycles recharged -- fully -- in two hours is a "free lunch"?? Don't think so. Numbers don't lie but "estimates" are generally wrong. Your 24 volt troller pulls 42 - 46 amps. 7 hours run time says that equates to 294 - 322 amps. At 50 amp charge rate (1/2 of that 100A alternator output) that means full charge takes between 5.8 hours to 6.4 hours or 1/2 those numbers if you run the engine fast enough to deliver 100A. Does the bigger alternator "extend" run time? Certainly. But your batteries are not "fully charged" in two hours at idle.

 

[hrdwrkingacguy]

Re: Alternator/Charging Question

reving it to 2500-3000 means cruising the 2 1/2 miles back to the ramp...takes < 15 minutes...The only reason I ask is a mechanic told me when my battery was dead due to a loose connection on the alternator, that I should rev it in neutral for 30 seconds and the battery would be fully charged...

 

[fishrdan]

Re: Alternator/Charging Question

Two 12 volt deep cycles recharged -- fully -- in two hours is a "free lunch"?? Don't think so. Numbers don't lie but "estimates" are generally wrong. Your 24 volt troller pulls 42 - 46 amps. 7 hours run time says that equates to 294 - 322 amps. At 50 amp charge rate (1/2 of that 100A alternator output) that means full charge takes between 5.8 hours to 6.4 hours or 1/2 those numbers if you run the engine fast enough to deliver 100A. Does the bigger alternator "extend" run time? Certainly. But your batteries are not "fully charged" in two hours at idle.

I didn't say the batteries were completely discharged, they just needed charging (for max power on the TM). When I notice the power dropping off on the trolling motor, I recharge the batteries. The TM is a Vector 70# (digital) that has a max amp draw of 42 amps at stall speed, so running it 1/2 speed is maybe 15-20 amps @ 24V. 6-7 hours of trolling x 15 amps is 90-105 amp hours / battery. I can idle the main engine for a couple (2-3) hours while trolling and it tops off the TM batteries, no problem. I've done it many times and it works great. This is a high output alternator like I have, idle output is 65 amps... This is the alternator I have, but they don't list it's idle output, their 140 amp alternator puts out 85 amps at idle though.

I prefer the quietness of a TM and with the high output alternator charging the TM batteries back up, I'm not strapped to the capacity of the batteries to determine how long I can run the TM, or the length of my fishing trip. I can charge the TM batteries on the water, while trolling of the main engine for awhile. So yeah, it's a free lunch.

For the OP's situation, running a bait tank pump and a couple pieces of electronics should be a piece of cake, compared to charging up batteries.

 

[boat1010]

Re: Alternator/Charging Question

You said that you have two batteries. While you are running the boat the switch should be on both. When you stop then switch the battery switch to 1 or 2. Then the draw is just on the battery that is selected. When you start the boat switch it to both again to charge both of the batteries. But you have to keep an eye on the volt meter. if it is not above 13 volts then you will have to run it faster.

 

[kmongar]

Re: Alternator/Charging Question

Once this alternator gets over 1200 engine RPM it kicks in and will keep charging until the engine is turned off.

I don't quite get the "kick-in" rpm. Are you saying that when the motor is turned on it must be revved to 1200 before the alternator begins to work but after that the rpm can be dropped to idle (700 to 800 rpm) and the alternator will charge? Or does the rpm have to be maintained at 1200 or higher "until the engine is turned off"?

 

[fishrdan]

Re: Alternator/Charging Question

when the motor is turned on it must be revved to 1200 before the alternator begins to work but after that the rpm can be dropped to idle (700 to 800 rpm) and the alternator will charge?

Yup, it's actually called the "cut-in" RPM or speed, 1200 is just an approximate speed. It's kind of the switch that turns on the alternator, then the alt continues to charge if the RPMs drop. When I'm trolling/charging, I have to blip the throttle to kick in the alternator, then back to idle speed.

DB electric (a link I posted) has Ford/Volvo marine alternators, they might have one for your engine. My high output alternator is a china knock-off but it works great, and for $60-70...