Preferred Way to Charge Batteries?

Windjammer

Well-Known Member
Nov 10, 2007
724
Acworth, GA
Boat Info
340 Sundancer 2006
Engines
8.1 Mercruiser V-Drives
Due to the flooding, the marina has temporarily disconnected AC power to the dock I am on. Planning to goe up this weekend to check things over...to recharge/top-off the batteries, which is the "preferred" (or "best") way to do this?
1. Crank both engines and charge from the alternators?
2. Fire up the generator and charge from the on board charger?
3. Both?
4. Does it really matter?

I will confess that I don't fully understand the charging relationship when the engines and generator are running with the A/C converter ON...
 
Kind of depends. Most engines only add a small net positive charge to the batteries at idle because of the parasitic load that they use themselves. If you have a high amperage battery charger, the generator is probably your better choice. If you decide to run the engines, bump them up above 1700 RPM, not idle. That will get the alternator up to its higher output.
 
You want to run your generator periodically anyway so my vote would be to run the generator with the on board battery charger turned on. Uses much less fuel too and depending on whether your fuel pumps at the marina are operational, you may want to conserve fuel.
 
Due to the flooding, the marina has temporarily disconnected AC power to the dock I am on. Planning to goe up this weekend to check things over...to recharge/top-off the batteries, which is the "preferred" (or "best") way to do this?
1. Crank both engines and charge from the alternators?
2. Fire up the generator and charge from the on board charger?
3. Both?
4. Does it really matter?

I will confess that I don't fully understand the charging relationship when the engines and generator are running with the A/C converter ON...

The engine start batteries are separated when charged by alternator, so you’d have to run both. At 1500-1700 you’d be burning about 20 gph total.

The better choice is to run gen and use converter charger. Less fuel consumption and more efficient conversion of mechanical energy to electricity.
 
[QUOTE="Henry Boyd,
The engine start batteries are separated when charged by alternator, so you’d have to run both. At 1500-1700 you’d be burning about 20 gph total.

A lot of searay boats were equipped with battery combiners which allow one motor to charge all battery banks at same time with either motor running.
 
Ok I can’t help but jump in here.

Your on-board charger is most likely 30amps or less. So, if you have all night then yes running the generator and charger is your better option but I’m guessing you want to only stay up a couple hours.

The alternators are probably 60-100 amp each so that seems the obvious solution but... you will draw down the state of charge significantly simply by using the batteries to start each engine.

the answer is this: if you are going to start your engines anyway just to run them, then use the alternators and let them run for a couple hours. If not then simply start the generator but it will take 3-4 hours probably to add anything significant.

Josh
 
How long do you think you might be without power - or without taking the boat away from the dock? Are you using any power other than the typical ECM/CO Detectors/Radio Memory/etc? If you're just talking a few weeks and aren't using any other high-draw 12V items, don't worry about it. The batteries will be just fine.
 
How long do you think you might be without power - or without taking the boat away from the dock? Are you using any power other than the typical ECM/CO Detectors/Radio Memory/etc? If you're just talking a few weeks and aren't using any other high-draw 12V items, don't worry about it. The batteries will be just fine.

Best guess on getting power restored is 2 to 3 weeks and there are no high-draw items turned on..I turned the cockpit refrigerator off on Saturday, and flipped the 12vdc breaker on the galley refrigerator to off as well...nothing in either of these other than a few cans of soft drinks...I replaced batteries back in April with Duracell AGM's.
 
Probably more efficient charge with generator. Depends on the charger and current state of the batteries. If they are at 70-80% a couple hours on a 30 amp charger
Will probably be sufficient. You could also use the engines for a while to bulk charge and finish of with the generator. AGM's can handle that pretty well I think.
 
If it was me, I wouldn't sweat it. Batteries last a lot longer without being charged than most give them credit for. A lot longer. And you've got really good batteries in there, to boot. Switch your battery switches off just to be sure - even disconnect the CO monitors if you want (leave them disconnected on the galley countertop so you see them and reconnect the next time you're in the cabin). But the CO's pull a very miniscule amount of power - honestly, I wouldn't even worry about that. I've gone "uncharged" for longer periods of time than you're talking about with no problems.

But, if it's convenient for you to get down to the boat and you don't mind doing it... I'd vote for the genny (as you're really just topping them off). :) But in the end, there's probably really no "one best" way, either.
 
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You want to run your generator periodically anyway so my vote would be to run the generator with the on board battery charger turned on. Uses much less fuel too and depending on whether your fuel pumps at the marina are operational, you may want to conserve fuel.

That’s what I’d do!
 

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