Engine RPM's for Charging

seahawk

Would anyone be able to help with a fairly "vague" question.....I understand that theres lots of variables in the question         We recently bought a 2008 280 EC with the 377MAG Merc....320 HP.     As part of the 'delivery trip' home in the spring, we're going to be 'poking' along through one of the local canal/waterway systems. The trip is around 200 KM's ....there are 40-some locks to go through where the engine will be stopped and re-started each time.....and not many places to go faster than 5 Kts,....This is a new boat to us so I'm not sure what RPM I'll be running to maintain 5-8 kts.  And in that distance there are only 4 places that have shore power ( if there is docking space available )...so potentially 2 or 3 days between Shore power charges that could bring the batteries back to 100%. The question is, at what RPM does this engine have to be turning for the alternator to begin generating sufficient power to charge the batteries. This is a single engine boat with no on-board genset.    Thanks very much for any input. 

Aqua_Aura

To get 5-8 it's you will probably be 1500 RPM which should be plenty of RPM to charge. As long as your volt gauge is showing over 12V then it should be charging the battery. If you question the reliability of the batteries I would take then out and have them tested by a professional. Make sure whoever does it has a computer tester and not just taking it down to a local parts store and using a mechanical type tester. 

You should have two batteries as well so one to start the engine and one to run the house power so if you did run down your battery watching TV or whatever you should still be able to start the engine and charge back up. 

69fastback

If your charging system and batteries are in proper shape, you should see a full 14V at idle. 

Grahamu

Sounds like the  Rideau Canal and there are many places where you will be able to cruise at 3500 RPM south of Perth and, as above, probably around 1500 in no wake zones.  If it is June when you travel it won't be that busy. I agree with @Aqua_Aura take the batteries out and check them. If they are more than 5 years old buy new ones. Also check your alternator and battery charger voltage outputs once the batteries are re-installed.

seahawk

Thanx for the input. The boat is new to me and I'm not familiar with the the accuracy of any of the gauges. I'm buying all new batteries in April so they should be good.
              
I was previously assuming that if the engine was running then the alternator would be charging....(and to a certain extent I guess thats right.)      But....if I have a 250 AHr battery House bank, and the alternator is 'capable' of 50-60 amps output (when cold and running at full power) , I'd realistically have to assume that the hotter the alternator got, and, the lower the RPM's,... the less power is being delivered, (never mind the electrical loss getting from the alt to the batteries.) So I might be actually getting 15 amps to the batteries...???.... And I'm probably only going to be running for a couple of hours a day. And thats my dilemma ....are the batteries going to be charging up enough .....or do I need to buy a little portable Honda genset to bring with us for those days between shore power. BTW, Graham, its the Trent-Severn....but your point is well taken.   Thanks everyone......:-)
 

Grahamu

Trent Severn was my original guess. If you don't intend to stay at marinas along the way but stay at the locks a portable generator as backup would be a good idea. Another point is that your engine alternator is probably sensing the start battery voltage which if properly isolated will be much higher that your house bank. If possible have the mechanic change the sense wire to your house bank, The isolator will deliver a charge to the engine battery when voltages rise but deliver higher amps to the house.

LaRea

As we discussed in your other thread, the digitally controlled battery isolator (e.g., ProIsoCharge) will be your friend here.  Besides eliminating the diode voltage drop, it also optimizes charging between the batteries.  

I'm not a fan of using portable generators aboard boats.  Some do it and aren't dead yet , but I don't like the risks.  

Grahamu

I don't recommend running it on the boat either but I have seen boats tied up on the locks where the generator is on shore with a shore power cord connected, just have to be aware of other boaters re noise and fumes.

seahawk

If I do decide to buy a generator, it'll be used away from the boat....hopefully I'll choose spots on a lock away from others and I'll run the generator as far as i can from our boat and other people. I dont like having to listen to other people gensets, and I'm going to try the best I can to be respectful of who is around me. BTW LaRea....a ProIsocharge is on my wish list.....and possibly a new charger, but I'd rather get the boat back to home port if I can, and have the work done by someone I know.........Thanx again for all the ideas.....

seahawk

Grahamu...I'll have to check and see where the sense wire is but thats a really good idea to have the V sensed at the House rather than the Start. That sounds smart....;-)

Handymans342

I’m not sure what is connected to your batteries and running while you are under way. Hopefully it’s less than your alternator output. 

Grahamu

You will probably need an oil pressure switch installed to turn off the sense wire voltage when engine not running which the ignition key would normally do.

seahawk

Graham...never thought of that. Thanx everyone.  much appreciated.