What size/type of relay for charging system and where to buy it

[sailorman]

I realise this is an old thread, but I have read all the comments with interest and have pretty much decided to fit a VSR (Merlin VSR140) per the circuit on an earlier post. One of the reasons I had for fitting one is because I thought that it would enable some of the capacity of the engine battery to be used for domestic when the engine battery is fully charged. Am I correct in this thinking or is a VSR purely to ensure that the engine battery always has power?

Look at Victron Cyrix it works both ways.
alternator charging will charge engine start first battery charger will charge domestic first

 

[nigelmercier]

I .. have pretty much decided to fit a VSR... I thought that it would enable some of the capacity of the engine battery to be used for domestic when the engine battery is fully charged...

Unless your batteries get so discharged that the charging system cannot instantly bring them up to the charging voltage, there is no concept of waiting for one battery to charge. A VSR simply parallels the banks when it senses a charge voltage. With this in mind, the switched and permanent batteries are therefore interchangeable. This is useful to know, because you can size the VSR on the expected current of the lower of the two.

 

[halcyon]

The Cyrix looks a good choice although it hasn't got the indicator lights that tell you what it's doing like the BEP unit. This is a feature that I like but I suppose is not absolutely necessary. I don't like the idea of combining batteries in an emergency if the starter battery has packed up, I would prefer to have it set up so that you could use the 'house' battery on it's own. I'm not an authority on electrics but it would seem to me that if you combine a 10.1 volt battery with a 12.7 volt battery, you'll only get 11.4 volts which may not be enough to spin the engine fast enough.

To charge a flat battery needs 13.0 volt, though 10.1 is below flat, that is why a VSR has a drop out voltage of 13.0 volt or there abouts as it means the batteries are not on charge.

If the engine battery is flat just turn the engine isolator switch off, the VSR is connected to switched side of isolator, so if VSR has link start option it will connect the engine to the service battery only, remember to size VSR for starter current.


Brian

 

[Esmerelda]

Look at Victron Cyrix it works both ways.
alternator charging will charge engine start first battery charger will charge domestic first

I was more thinking about when the batteries are not being recharged (and assuming all batteries are pretty much fully charged), is it the case that for a while at least the engine battery is parallelled with the domestic bank, so that some (domestic) use is made of the charge on the engine battery?

 

[halcyon]

I was more thinking about when the batteries are not being recharged (and assuming all batteries are pretty much fully charged), is it the case that for a while at least the engine battery is parallelled with the domestic bank, so that some (domestic) use is made of the charge on the engine battery?

Do you mean when you stop the engine ?

If you are drawing no power,yes they will remain connected for a period, but then both are charged and nothing will happen.

If the service battery has a load on it voltage will fall rapidly and batteries will be disconnected, isolating batteries.

Batteries have three voltage ranges, as a rough guide,

rested/no load 11.8 - 12.8 volt, on load, varies with current drawn but sat 5-10% battery capacity 10.8 - 12.5 volt, on charge 13.0-15.5 volt.

So if the engine is running the alternator supplies power, if you stop charging you move from the on charge range to the on load range, voltage drops and the relay drops out. Okay you may loose a fraction on one percent of engine battery capacity in the short time before drop out.

Brian

 

[Graham_Wright]

Funny nobody has mentioned two alternators.

I have the luxury of a standard 40A for the starter battery and a 120A for the house batteries both permanently connected.

And, because I like complication, I have a multitude of switches to cross connect everything.

The fear of an open circuit alternator output has vanished. Also, if one of the alternators fails, I can jury rig.

 

[nigelmercier]

I was more thinking about when the batteries are not being recharged (and assuming all batteries are pretty much fully charged), is it the case that for a while at least the engine battery is parallelled with the domestic bank, so that some (domestic) use is made of the charge on the engine battery?

When you stop charging, a VSR will drop out. Certainly within a few minutes.

If I were doing this again, I would make my own unit. A heavy-duty relay turned on a few minutes after the alternator starts energising. I've had to modify my VSR so it doesn't turn on when the ignition is off.

 

[xeitosaphil]

I've had to modify my VSR so it doesn't turn on when the ignition is off.

That's interesting Nigel, for what reason?