Voltage Sensitive Relays - which one to pick?

This  split charge relay was fitted to my car at the local towbar fitters & I fitted the same to my previous tow car  ..... no problems so far ☺

Assuming you already tow a caravan and your vehicle socket is wired correctly why do you need a VSR as it would duplicate the pin 10 output which should only be live once the engine is running.

MollysMummy

I'd looked at the Maypole relay but it seems to be intended to connect pin 9 once pin 10 becomes live which is unnecessarily complicated for my needs where I just need to be able to charge a battery on the trailer. Hence the appeal of a 'straight through' connection from pin 10 to the trailer battery once it detects 13+ volts on the system. 

Vulcan

i don't have a caravan but I do have a correctly wired 13 pin socket. However my Volvo (like all Volvos apparently) has pin 10 live with ignition on and not just once the engine was running. Hence the need for a VSR to prevent the pin 10 wire connected to the battery on the trailer being live as soon as the ignition is switched on. Otherwise the starter could draw current from the small trailer battery via the 2.5mm2 wire. 

So you don't need a VSR, just a blocking diode to stop the reverse flow of current.  The usual use of a VSR is to protect an engine start battery, and to make sure that the second battery doesn't start charging until the engine battery is charged after starting.

I have one on the boat, and once the engine starts and the ign light is out, there's a period of about 10 seconds after which you can hear the alternator come on more load as the charge is diverted to the 330AH domestic bank...

Most wiring descriptions I have seen use the term "ignition switch controlled" or similar which seems to be the same volvo do. It would not surprise me if power was automatically cut to pin 10 while the starter is operated, otherwise surely all caravan battery connections would suffer the problem you are trying to avoid.

It would be interesting to put a test meter on pin 10, and watch while the ignition is turned on and then started. Does it drop to zero when cranking?

Mike

Seems odd that it is not voltage controlled on the Volvo however this should do the trick.

https://www.ringautomotive.com/en/product/RCT465

Mike

I'd agree with you and I intend to double check the voltage at pin 10 to see if in fact it drops to zero when cranking. It seems logical that it would - the radio cuts out when starting - but when I checked it before the voltage seemed to be unaffected. A more careful check is required before investing in a VSR. 

Vulcan

The relay you linked to looks to be similar to the one MollysMummy linked to and seems to be unnecessarily complicated for what I need (or may need), i.e. a straight through connection from pin 10 to the trailer battery once the engine is running. But thank you for taking the trouble to find it.

Clearly if I find that, despite Volvo's description as 'ignition controlled power supply', pin 10 actually drops to zero while cranking then I don't need a relay at all. More research needed!

I think you will find that most VSR's are wired like this, quite simple for your application. It will only allow power to Term 2 or 6 when the voltage rises above a preset level.

Pin 10 to +12 Term

Pin 11 to OV Term

                  6 Term to trailer batt+ve

Thanks for the explanation, Vulcan. That one would indeed do the job I'm sure. The ones I'm considering just have a simple input and output connection triggered by the rise in voltage - see

https://www.samlex.com/en/products/battery-chargers/battery-separator

https://www.durite.co.uk/itm/Voltage-Sensitive-Relay/12V-Voltage-Sensitive-Relay-140A/072733 

These, and similar ones by other manufacturers, seems to be much more expensive than the Ring or Maypole ones with more terminals - I wonder why? 

Perhaps it's their much higher amperage rating which I don't really need just for battery charging?