Voltage Sensitive Relays - which one to pick?
Not strictly a caravan question but a related topic nonetheless. I want to be able to charge a battery mounted on my car trailer and it would seem that a VSR on the trailer, connected to pin 10 (ignition controlled power supply) and then to the trailer battery, would do the trick. Some possibilities are the Samlex BS100 and the seemingly similar Durite 727-33.
Has anyone any experience of these, or other suitable VSRs?
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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 ☺
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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.
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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...
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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
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Seems odd that it is not voltage controlled on the Volvo however this should do the trick.
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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!
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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
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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?
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And the fact that they are approx. 10 times more expensive. Towsure also stock one similar to the Ring product which would do the job.
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Thanks again, Vulcan. The one you've linked to at Towsure seems to be simply a variation on the Ring one and indeed the similarly specified Maypole one. At least the Towsure one doesn't use the old 12S pin designations!
Since I don't need 100 amp capacity in the relay I may just try one like one of the ones linked to but with just a single 12v feed and see how it works. I double checked pin 10 on my towbar electrics and it doesn't drop out when cranking but naturally enough the voltage drops to around 10 volts while starting but very quickly comes up to over 14 volts. So any of these relays should work ok.
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