What input voltage can a Voltage Sensitive Relay tolerate?

[Mike Adams]

I seem to have blown up my 140A VSR. I started to the engine with the leisure battery isolator off by mistake. I have two alternators with their outputs connected together and connected to the leisure battery bank. The VSR supplies the starter battery. I assumed the VSR would cut in as soon as the alternator voltage rose above the cut in value of 13.3v. Could it be that the alternator voltage went too high and damaged the electronics in the relay control circuit before the relay closed. I don't know what the peak voltage is likely to be or whether the two alternators combined produce some high peaks. It's been Ok for a long time. I know you should connect the VSR supplying the leisure batteries but the leisure batteries were not getting charged at low revs which is most of the time. I suppose I could take a feed from the leisure battery to a relay to prevent the engine being started with the cut off switch open circuit.

[Tony Brooks]

If you are energising the alternator(s) from the engine battery and if I understand your setup, you may well be correct. It's a short voltage surge that may be produced and that could be very high.

 

Don't discount the VSR contacts burning because they for some reason carried the full charge plus some to the engine battery.

 

Seems an odd setup to me when  a bi-directional VSR and conventionally wired separate alternators would effectively combine both alternators.

[David Mack]

My VSR is connected to the battery side of the two isolators, with a 100A (I think) fuse in each connection. That way the solar panel, connected to the leisure battery (via its own breaker) will charge both batteries, even with both main isolators switched off (which they are when I leave the boat). So as long as neither of the fuses blows my VSR will see the battery voltages all the time, regardless of the isolator positions.

[Tony Brooks]

3 minutes ago, David Mack said:

My VSR is connected to the battery side of the two isolators, with a 100A (I think) fuse in each connection. That way the solar panel, connected to the leisure battery (via its own breaker) will charge both batteries, even with both main isolators switched off (which they are when I leave the boat). So as long as neither of the fuses blows my VSR will see the battery voltages all the time, regardless of the isolator positions.

 

I think with the OP's set up the alternators outputs were open circuit but they were still energised so they may well have produced a voltage surge. It means his alternators are probably connected to the batteries connected as per the BSS regs.

[Mike Adams]

23 minutes ago, Tony Brooks said:

Don't discount the VSR contacts burning because they for some reason carried the full charge plus some to the engine battery.

Good point. I've taken the sealed unit apart and the contacts are not  burned out and the led lights up when the switching voltage is reached but neither of two parallel relays operate. Without a circuit diagram and with surface mount components it's about as far as I can go with it. Just want to avoid it happening again. One of the reasons for connecting it the way I did was to avoid excess current in the relay.

[David Mack]

5 minutes ago, Mike Adams said:

One of the reasons for connecting it the way I did was to avoid excess current in the relay.

Surely connecting one alternator to each battery means the VSR only carries the difference between the supply and the demand of each battery, whereas if you connect both to the same battery then the VSR carries the full demand of the other battery.

[Tony Brooks]

8 minutes ago, David Mack said:

Surely connecting one alternator to each battery means the VSR only carries the difference between the supply and the demand of each battery, whereas if you connect both to the same battery then the VSR carries the full demand of the other battery.

 

Yes, plus any extra current the better charged battery can supply because of the voltage difference but as I understand the OP the alternators are connected to the domestic bank and that one is normally the least well charged, so if anything the better charged engine battery will tend to limit the current through the relay. But two paralleled alternators could well get close to or even exceed that 140a relay current.

20 minutes ago, Mike Adams said:

Good point. I've taken the sealed unit apart and the contacts are not  burned out and the led lights up when the switching voltage is reached but neither of two parallel relays operate. Without a circuit diagram and with surface mount components it's about as far as I can go with it. Just want to avoid it happening again. One of the reasons for connecting it the way I did was to avoid excess current in the relay.

 

That is normally the best way to do it but, as I said, connecting each alternator to its own battery and fitting a bi-directional VSR now you have to buy another would be more standard yet still combine both alternator outputs.

[GUMPY]

I run two alternators one 90a to the domestics and one 70amp to the starter with a vsr connecting the banks with the domestic bank down at 70% the most current I have seen going into the domestics is 125amp and that was only for a short while after starting the engine. There was nothing being shown on the ammeter on the starter battery this means that about 40amps was going through the relay from the starter alternator as I said it was only for a short while.

Much more sensible to wire one alternator per bank and let the vsr do its job.

[Mike Adams]

10 hours ago, Tony Brooks said:

Tony - I was unaware of the  bi-directional VSR.  I now see how to use two of the cheap VSR's in parallel(one sensing from each battery) and connect one alternator to each battery. Due to physical constraints I fitted a 32A alternator with a large pulley that charges at tickover and the standard 140A one that cuts in just above tickover. That, though, would not solve the problem of an open circuit VSR input on the domestic side. I haven't found a bi direction VSR yet so maybe I will power the ignition through a relay from the domestic battery with a bypass switch which will prevent anyone starting the engine with the domestics open circuit.

 

[Tony Brooks]

Mike Adams said:

Tony - I was unaware of the  bi-directional VSR.  I now see how to use two of the cheap VSR's in parallel(one sensing from each battery) and connect one alternator to each battery. Due to physical constraints I fitted a 32A alternator with a large pulley that charges at tickover and the standard 140A one that cuts in just above tickover. That, though, would not solve the problem of an open circuit VSR input on the domestic side. I haven't found a bi direction VSR yet so maybe I will power the ignition through a relay from the domestic battery with a bypass switch which will prevent anyone starting the engine with the domestics open circuit.

 

 

Reply

 

Here you go, about the third hit for VSR on Google, probably loads of others

 

https://www.12voltplanet.co.uk/user/downloads/VSR Installation Instructions.pdf

 

1. I hope that you don't really mean a large pulley on the alternator because that will make it turn slower.

 

2. Why faff about with two relays when one will do the job. I think that you are overcomplicating things.

 

3. If you ensure each alternator is energised from its own battery bank, then an open isolator would prevent that one energising so it can't produce a voltage surge. It will just sit there spinning but doing nothing else. I suspect that you are energising both your alternators from the engine bank, so an open domestic isolator will allow them to energise with no load. If you want to keep things the same, just energise both from the domestic bank. That will require a relay energised by the engine bank, but switching a feed from the domestic bank to both warning lamps. However, it's unusual and will confuse others.

 

Far better to simply revert to the normal way of doing things with each alternator energised from its own bank and charging its own bank with a VSR joining both banks.

Edited August 6, 2021 by Tony Brooks

[GUMPY]

I run a directional relay sensed from the domestics, it kicks in within a minute or so of starting the engine so avoids any inrush currents but gives the benefit of two alternators charging the domestics very quickly.

Look at it as two separate systems that don't depend on each other and are only connected by the vsr

You shouldn't overcomplicate things it will only backfire in the long run.

[Tony Brooks]

Just now, Loddon said:

I run a directional relay sensed from the domestics, it kicks in within a minute or so of starting the engine so avoids any inrush currents but gives the benefit of two alternators charging the domestics very quickly.

Look at it as two separate systems that don't depend on each other and are only connected by the vsr

You shouldn't overcomplicate things it will only backfire in the long run.

 As it seems to have done!