Voltage Sensing Relay (VSR) question

[KevinE]

I don't think it matters where you have the vsr. As long as it disconnects the starting battery at <12.7v (or whatever it's set at) it's doing what it is supposed to. Not a bad idea to have fuse near the starter battery though.

The problem is that voltage drop is very real & the linear distance between the battery & the VSR is actually crucial.

Take any 12v deep cycle battery & connect it to a fridge with 5 metres of 6 B&S cabling. Run it as is & it will run until the battery discharges to a voltage that the fridge can't run off any more.

Now recharge the battery & put a VSR into that set up, right up on the battery; it will run until the VSR shuts off at about 11.8v or whatever setting you have your VSR on. What is crucial here is the actual time it ran for.

Now move the VSR incrementally away from the battery & towards the end user of the energy (the fridge)

Each time you move the VSR away from the battery, the time that the usable energy is available to run the fridge will drop dramatically. Add the variables such as older batteries, or hot humid weather & the issue gets greater.

Thus ends my involvement in any future 12v threads or discussions lol.

 

[OldManBeard]

Objectively; I ran the same cabling from under the bonnet into the tub on both scenarios.

1. Having the VSR in the tub, caused the VSR to continually cut out just a few minutes after the ignition was switched off (making the power connection useless)

2. Having the VSR under the bonnet, no such issue. The battery ran down overnight with a fridge connected, as it should. Still plenty of juice in it in the morning.

The VSR simply doesn't see the voltage in the tub that it did at the battery.

I've run three different types of VSR & the results vary from one to the other. But none were brilliant.

All I can say about that is that we have very different notions about what constitutes suitably heavy cabling.

 

[Horatius]

The problem is that voltage drop is very real & the linear distance between the battery & the VSR is actually crucial.

Take any 12v deep cycle battery & connect it to a fridge with 5 metres of 6 B&S cabling. Run it as is & it will run until the battery discharges to a voltage that the fridge can't run off any more.

Now recharge the battery & put a VSR into that set up, right up on the battery; it will run until the VSR shuts off at about 11.8v or whatever setting you have your VSR on. What is crucial here is the actual time it ran for.

Now move the VSR incrementally away from the battery & towards the end user of the energy (the fridge)

Each time you move the VSR away from the battery, the time that the usable energy is available to run the fridge will drop dramatically. Add the variables such as older batteries, or hot humid weather & the issue gets greater.

Thus ends my involvement in any future 12v threads or discussions lol.

Fair enough Kevin, but I think we have different ideas here about what the VSR is for lol. In my scenario it's aka a "battery isolator" and can go anywhere between the + of the start and + of 2nd battery, to disconnect the start battery so that it doesn't run flat when the engine is stopped from using appliances or whatever. That's all it does. It has no effect on the output of the second battery (not a bad idea to have a cheap low voltage relay on 2nd battery output either though so it doesn't run completely flat, they don't last long doing that). So it doesn't matter where it goes, the voltage drop is the same no matter where you put it, as long as it closes when the engine starts and opens when the engine stops.

 

[Horatius]

Objectively; I ran the same cabling from under the bonnet into the tub on both scenarios.

1. Having the VSR in the tub, caused the VSR to continually cut out just a few minutes after the ignition was switched off (making the power connection useless)

So you have the vsr between 2nd start battery and (3rd) deep cycle battery and it's cutting out? Isn't that what it's supposed to do? You know there are still old fashioned types who have an isolator that they manually switch off when they stop, to make sure they isolate everything from start battery?

If you want your 2nd start battery to run appliances, wouldn't you put the vsr between 1st/2nd crank batteries? That way your 2nd/3rd batteries will run fine in parallel (will work like one big battery) and you'll have a start battery that is isolated from them.

For most people it won't matter, as long as the vsr isolates the auxiliary from the start battery so it doesn't go flat, and then connects the auxiliary when the car starts so it gets topped up again.