Generator and engine alternator at the same time?

[Rivers & creeks]

In a bid to boost our battery charging I hit on the cunning idea of running the generator to power the battery charger whilst we are motoring. The alternator feeds one of those clever boxes that splits the charge between the domestic, engine start and thruster/windlass batteries. The charger just feeds the (currently too small) domestic bank. The potential problem I see is that the battery charger will raise the voltage at the domestic terminals and that will fool the alterenator split charge box into thinking that those batteries are charged and therefore it won't feed any, or not very much juice to them.

Or am I talking rubbish?

 

[david_bagshaw]

I think you are right!

the engine alt will be fooled, however if the battery is well depleted you might get some gain. All depends how fancy the generator fed charger is.

As an example we have a la manche constavolt (old 80s tech, but robust) which pushes out 60A at 28volts,

if the main engines are on this drops to 2 A as the main engines charge at 28.8v

 

[stearman65]

2nd Alternator

When I re-engined our Moody 40 we specified a second alternator. One for the engine battery & one for the house batteries. Belt & braces.
Stearman65

 

[Portofino]

I would not do it,
There is a risk, however small that you could blow something in the charger .( clever- expensive - ball ache to replace box you mention )
Use one at a time either Geny or engines not both .
You should be able to turn the charger off if you run both engines ( alternator)
And the Geny - together if the Geny has been tasked to say boil a kettle or power the A/C , etc etc

 

[Marsupial]

Unless you run the engine for lots of hours, like 50 you will be unlikely to get the batteries fully charged with the engine alternator, 80-90% is the best you will see - unless you have a sterling charge controller or something like it.

The on board battery charger should take the batteries to 100% ish but even then it can take a while, a day on shore power perhaps?

Running the generator and hence the on board charger while the engine is running wont harm anything provided everything is in good order, the engine alternator will as you suggest think the batteries are charged and effectively switch off - unless you have a charge controller in which case the device with the highest charge value will prevail, the one with lowest will switch off.

Think about it, have you ever seen a requirement to disconnect your charger from the shore power when you fire up the engine whilst in a berth? the electronics sorts it all out.

 

[VicS]

I think it will all rather depend upon what you mean by "one of those clever boxes that splits the charge between the domestic, engine start and thruster/windlass batteries."

With no information about it I don't think anyone can offer much in the way of advice except don't do it

 

[Marsupial]

I think it will all rather depend upon what you mean by "one of those clever boxes that splits the charge between the domestic, engine start and thruster/windlass batteries."

With no information about it I don't think anyone can offer much in the way of advice except don't do it

Vic I think he has a splitter diode in which case the on board charger will "win" in a manner of speaking . . .

 

[Rivers & creeks]

Yes, it's split charge diode. The battery charger goes straight to the domestic bank so bypasses the I/2/both and the split diode box.

 

[VicS]

Yes, it's split charge diode. The battery charger goes straight to the domestic bank so bypasses the I/2/both and the split diode box.

If just a straight forward diode splitter you will have no problems.

If the alternator is battery sensed it would normally be sensed at the domestic bank so that could cause problems.
This could could be solved by changing the sensing to the engine start battery.
However
The 1,2, both, off switch complicates the issue. Normally one would not use such a switch and a diode splitter.

How are the batteries wired to the switch and the diode splitter ?

Diagram ??

 

[Rivers & creeks]

I'm not sure where the alternator sensor is, the electrician said that it goes via the alternator warning bulb on the switch panel and that a blown bulb means no alternator charging. But I'm assuming it's via the 1/2/both common live. Each of the three batteries (start, domestic bank, windlass/thruster) have a lead that runs to the diode splitter.

The reason for the 1/2/both switch is that it gives the ability to start using the domestic bank if the starter battery dies, or to run instruments from the starter battery if the domestic bank dies. I know either situation is highly unlikely. What happens in reality is we start on 'starter' and forget to switch over once moored or anchored.

Maybe a good winter plan is to move the starter battery to a point adjacent to the engine starter (they currently have a 6m run via the 1/2/both which is crazy) and fit an isolator switch, running a set of jump cables from the domestic bank to connect in an emergency. Then I can ditch the 1/2/both switch and save a lot of jumbled wiring - at the moment the wires run accross from the domestic bank 4 meters to the 1/2/both, 4 meters back to the bus bar above the domestic bank. Can't be good.

 

[William_H]

Battery charging

A battery will take a charge depending on the state of charge of the battery the size of the battery and the voltage of the charger. You will not increas the charge rate by increasing the capacity of the charger or alternator. The rating of the charger or alternator only relates to the max current it can supply but does not relate to what the batteries will take. Two charging sources will simply share the charge the battery takes.
So most efficent charging is from the engine driven alternator whose output voltage is raised to increse the current the batteries take. ie a smart regulator. good luck olewill

 

[BGW]

In a bid to boost our battery charging I hit on the cunning idea of running the generator to power the battery charger whilst we are motoring. The alternator feeds one of those clever boxes that splits the charge between the domestic, engine start and thruster/windlass batteries. The charger just feeds the (currently too small) domestic bank. The potential problem I see is that the battery charger will raise the voltage at the domestic terminals and that will fool the alterenator split charge box into thinking that those batteries are charged and therefore it won't feed any, or not very much juice to them.

Or am I talking rubbish?

Sterling Power sell an alternator to battery charger - It takes power from both alternators (if you wish) then splits the charge to the engine and domestic banks. - The domestic banks being charged just like a shore power charger - a three stage charge regime. Yesterday I was putting 40A into the domestic bank (24v) at 1200 rpm from two elderly 30A CAV alternators. Before I fitted it they took forever to charge the domestic bank.