Alternator charging engine battery but not the domestics

[Ric]

I have a Volvo MD2020. Recently I had to fit a new alternator after the last failed.

I have now found that the new alternator is charging the engine battery, but not the domestics. When running motor, engine battery voltage is around 13.8 as expected, but the domestics are not showing any charge on the battery monitors.

I have solar and wind, and they are both charging the domestics as shown by the battery monitors.

What to check first?

 

[[70521]]

How are batteries wired up? If VSR you may not be tripping it on. Mine will trip over at 13.8 volts and trip off at 13.0 volts

13.8 volts from a new alternator sounds low. My recent one charges at 14.4 volts.

 

[Ric]

They are wired as originally fitted by Jeanneau. All has worked fine until I had to change the alternator.

When I check the engine battery while engine is running, that is as you say 14+ V.

The domestics are no way near that. I have a Sterling monitor and so am used to seeing 14+ v on it when the engine is running. It is actually discharging when the engine is running, even with just a light load, so I am sure there is no charge reaching the batteries from the alternator.

I only get charge on domestics from solar/wind at the moment

 

[Rhylsailer99]

Maybe try blocking the solar and stopping the fan from spinning then start the engine to see if the domestics are charging . Or test in the dark with no wind .

 

[halcyon]

Depending on age, Volvo fitted an alternator that had two outputs, one for engine, one for service battery, used blocking diodes internal with the alternator monitoring voltage on output.

Brian

 

[Ric]

The Sterling indicator lights show that everything is working normally btw.

Depending on age, Volvo fitted an alternator that had two outputs, one for engine, one for service battery, used blocking diodes internal with the alternator monitoring voltage on output.

Brian

Possible - the motor dates from 2001. The alternator failed last year and I replaced it with an aftermarket alternator which the vendor assured me was exactly the same as the original. It certainly had all the same terminals in exactly the same place.

I will try running the engine in a few minutes (onboard at anchor currently) and see if there is any charge.

 

[Ric]

OK, its now dark, no wind. Started engine - absolutely no change to domestic volts (currently 13v) and no charge according to Sterling AH counter. But engine battery volts immediately leapt to about 14.5v

I have a separate voltmeter that can be switched between engine and domestic battery - when engine is running engine battery is clearly at alternator V (about 14.5) but domestics remain at 13v.

 

[Daydream believer]

Do you have a decent VSR or just a simple 4/5 pin relay.
It may be the problem
test relay
I am not sure if taping it to see if it is sticking will work - others may comment

I have an MD 2020 from 2003 & when having it serviced I have been told that it is a pretty standard alternator used in other applications.

 

[Ric]

Yes, as you say it seems like a standard alternator.

I’ve found the splitter now - tucked away behind the switchboard. It is just a simple 4/5 pin relay. At least it will be easy to change.

 

[BabaYaga]

Yes, as you say it seems like a standard alternator.

I’ve found the splitter now - tucked away behind the switchboard. It is just a simple 4/5 pin relay. At least it will be easy to change.

If the relay is controlled by alternator D+ (which is often the case) could it be that this connection was never made on the new alternator?

 

[Ric]

Thank you, yes I definitely reconnected the D+ terminal.

But in any case when I delved into my old-fashioned relay controlled by D+ I found that the two cables from splitter going to Engine and Domestic terminals were totally fried - plastic completely melted and wires fused together by plastic. Seems the wires were not actually touching - otherwise I would have seen same voltage on both engine and domestic batteries. Obviously the relay was not closing so that must have failed too. So that would explain the symptoms.

I have now bought and fitted (with brand new connecting cables) a Victron Cytex VSR relay and so will see over the next few days if that fixes the problem.

However, I am still somewhat baffled how the wires got melted in the first place. If the relay had failed open there would have been no issue. If it had failed closed, that could have created some moderate equalising currents - but surely not enough to totally fry the cables? And if it had failed closed, why did the two circuits then get dsconnected again? Would be interested to hear of scenarios that could have caused such a series of events, as perhaps I am sitting on a disaster waiting to happen again…