Electrical Help Please

[Sailfree]

My Engine battery has gone flat over the winter and I can't understand how it ever got a charge from the mains battery charger. The charger has provision for more than one bank of batteries but only the service batteries are connected to it.
Adjacent to the engine there is an "isolator" (manual name - looks like a blocking diode to me) allowing the alternator to charge both battery banks yet keeping them separate. Is this isolator clever and lets a 14V charging current from the Service batteries through to also charge the Engine battery yet stop the Engine battery discharging back to the Service batteries or does it only allow the alternator to charge both? The charging circuit diagram in the manual only shows the mains charger connected to the service battery.
At present if it was meant to be clever (and thats the way the engine battery was intended to get its charge) it no longer is! so possibly a burnt out circuit inside the isolator.
The boat is 2 years old and was regularly used last winter so its possible the engine battery got enough charge from the engine so never went flat.

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[halcyon]

Two options
a) the isolator is a bi-directional relay which would charge the engine battery from the mains charger.
b)the engine battery is not charged from the mains, and relies on the alternator.

If it looks like a heatsink it's probably not a), in that case if it's a multi output charger connect a spare output to the engine battery. Charge battery, and check engine is charging service bank.

Brian

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[claudio]

Hi Sailfree,
It sounds to me like the alternater charged both engine and domestic batteries via the alternator and split diode arrangement. The domestic batteries were also charged by the mains charger.

It would be unusual to have the starter battery charged from the domestic battery as in a fault condition (i.e. bad engine battery) the domestic battery would discharge into the faulty battery.

It's difficult to give a definitive answer without seeing the wiring, but if you can scan and PM the circuit diagram I may be able to help



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[VicS]

It sounds as though your system allows the alternator to charge both batteries via a blocking diode system while the mains charger only charges the domestic service battery.

The only variation I can think of is that it's a voltage sensitive relay, rather than blocking diodes, which gives prioriy to the engine starting battery and only switches to charge the domestic battery once the former is charged. If this is what it is it will, I think, also have a -ve connection. That will make it recognisable.

<hr width=100% size=1><font color=purple>Ne te confundant illegitimi.</font color=purple>

 

[Sailfree]

The Isolator (blocking diode?) does have a 4th post top painted green that is not connected to anything.
I could fix the problem by running an extra cable from the charger marked for a separate battery bank but would like to identify how it is meant to work and what components are faulty first.
Certainly have 14v at charger/service batteries and their lead to the blocking diode but only 12v at engine battery side of diode.


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[Gypsy]

My Bavaria is wired in the same fashion. It has a diode splitter so that the alternator charges both the engine and service batteries but the shore paower charger is only connected to the service batteries despite the fact that it has 3 isolated outputs. I questioned the factory about this and they replied that the shore power charger (45A output) can be overloaded if both battery banks were connected and one or both required a lot of charge (this is true but the charger has internal current limiting to stop damage). As the engine battery is seldom discharged very much compared with the service batteries they took the view that the best thing for normal operational purposes was to have the maximum charge go to the service batteries.

I find their logic is correct for normal situations and use. The theory goes wrong when there is some unusual situation whereby the engine battery loses its charge or is drained by the engine being reluctant to start. My solution was to wire up the second output of the charger to the engine battery via a fuse and switch so that I could use the shore power to recharge it when/if necessary. As the shore power charger is current limited to stop internal damage if the load is too grest the only issue is that it may take longer to charge if the service battery is also drawing a lot of charge current. As this only happens for the first stage of the charge process before the batteries themselves limit the amount of current they can take it has not been an issue.
My bet is that your boat manufacturer used the same logic.


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[claudio]

Sounds like blocking diode is OK, engine battery is charged by alternator only.

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[VicS]

Re: 4th post

The 4th post on the diodes is probably for connecting a battery sensed alternator. Yours must be machine sensed. A negative is more likely to be coloured black.

<hr width=100% size=1><font color=purple>Ne te confundant illegitimi.</font color=purple><P ID="edit"><FONT SIZE=-1>Edited by VicS on 15/03/2004 00:24 (server time).</FONT></P>

 

[john_morris_uk]

Re: 4th post

Agree with many others that it sounds like a diode to allow the alternator to charge both the domestic and engine batteries. There is no reason why the shore power charger shouldn't charge just the domestics - in fact it makes a lot of sense because:-

Cranking currents are high, but in Amp-hour terms the loss to the engine battery is very low. A standard alternator regulator is very good at charging whilst not over charging batteries in this situation.

If the shore power charger is charging in any sort of sensible way, it would over charge (and possibly boil) the engine battery whenever it was charging the discharged domestic batteries. (The comment from the Bavaria agent that connecting the engine and domestics batteries to the charger would overload the charger is nonsense: complete drivel in fact)

Lead acid batteries self discharge at varying rates, depending on age, temperature and battery design. If the engine battery was on its last legs towards the end of the season (and you might not have noticed this if the engine always started) it may have just died naturally during the winter.

You could check:

What is the charging voltage (at the battery terminals) when under engine and mains charging?

If the alternator charges through the diode, the batteries without some sort of coltage sensing off the batteries, they will never reach anything near full charge.

If you say what sort of batteries they are (ie Gel or traction, leasure wet acid etc) I can come up with some figues as to what the voltage should be at various stages of the charging cycle.



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[Sailfree]

Spoken with the Dufour Agent today and while the new performance range has the charger connected to the engine batteries the old ones didn't.
I therefore surmise the logic is that with modern marina hopping and the outputs of alternators the engine can keep the battery charged enough for starting (as modern cars that only do short journeys) but the service batteries need some extra help from a shore powered charger and that if the shore power charger was also connected to the engine battery its continuous use would tend to overcharge the engine battery and it would need constant topping up with distilled water.
I will put in a fused switched charging circuit to the engine battery as you did.
Thanks

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[D_Farrugia]

Re: 4th post

The Green coloured 4th post is probably a compensated output for the regulator to counter the (approx 0.7V) voltage drop across the diode. On my Bosch alternator with a remote regulator, the sense wire from D+ terminal which normally goes to the regulator is taken from the 4th post. Your charging light will also have to come off this post if you take your regulator sensing from here.

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