Is my trailer hook up draining power?

I connected up my towbar electrics yesterday via a bypass relay.

My battery was dead after I finished fitting it, but I wasn't sure if that was because I had my lights on and off testing them at the plug to figure out which wire did what.

When I put my multimeter across the inline fuse on the cable that is powering the relay (which is connected directly to the battery) it will give a reading of 12v or more even with the car off and not in use.

When I switch the multimeter to amps DC and put it across it reads 0.

I am just a bit confused as to wether it is drawing any power or not, with a Volt reading of 12.6 but an amp reading of 0.

Cheers.

To measure current draw, the meter has to be in line with the feed cable so the current flows through the meter.
Voltage measurements are between the live feed and the neutral.
John

John506 said:

When I put my multimeter across the inline fuse on the cable that is powering the relay (which is connected directly to the battery) it will give a reading of 12v or more even with the car off and not in use.

Click to expand...

That would indicate that the fuse is blown.

When I switch the multimeter to amps DC and put it across it reads 0.

Click to expand...

Ammeters must be connected in-line.

Make sure your meter is capable of handling the current load! Some cheaper ones can only handle 1A.
John

I thought by removing the fuse and inserting the prongs into the fuse holder this makes it inline?

John506 said:

I thought by removing the fuse and inserting the prongs into the fuse holder this makes it inline?

Click to expand...

Yes, that's correct.

Was anything that would draw current switched on?

Only thing that was on was the light in the boot, but it's not on the same feed as the trailer hookup.

Nothing was plugged into the trailer socket so it's got me baffled.

If it's showing 12v at the fuse holder does that mean it's drawing current, even with it saying 0 for amps?

You were only measuring the cable controlling the relay so there would be a path to ground through the coil - 12V.

How much current would that draw in series with the meter?


Or have I got that wrong?

John506 said:

If it's showing 12v at the fuse holder does that mean it's drawing current, even with it saying 0 for amps?

Click to expand...

No, that means there is voltage available to drive current into a load.

Ahh I see, so it's not necessarily drawing anything then, I just measured what is available if it wanted it.

This is the first time I've actually tried to understand Volts, amps and resistance, so sorry if I'm a little slow.

EFLImpudence said:

You were only measuring the cable controlling the relay so there would be a path to ground through the coil - 12V.

Click to expand...

Would that not mean that when the fuse is put back in the relay will be energised, thus then draining the battery.
Not really sure what the relays for to be honest

333rocky333 said:

Would that not mean that when the fuse is put back in the relay will be energised, thus then draining the battery.

Click to expand...

It would, but only the coil - plus, of course, any load on the relay switched circuit(s) but that wouldn't be measured on this circuit.
However, I presume this measured 12V will not be there permanently - i.e. when control is off.
(I'm still not certain if it was the control circuit or switched circuit that was being measured)

Not really sure what the relays for to be honest

Click to expand...

Presumably it switches on the heavy current loads of the trailer and/or some disconnect the caravan battery from the car battery when the alternator is not running.


The battery was "dead" after only the fitting of the towbar socket.
It couldn't have taken that long.

The measurements appear to have been taken at this "dead" battery time but still reading 12V.
Perhaps a cell or two are on their way out.

Not sure what relay is for? The old 2TU Lucas relay box was designed for cars where the brake lights were dimmed at night so there was no current draw until lights were switched on, and all power went through a fuse to trailer so any fault with trailer would not effect the tow cars lights. I think there is a modern type designed so the bulb failure circuit in the car is not damaged by trailer load.

There are also relay systems for charging, there are a number of systems, triggered by oil pressure, charge warning lamps, or ign switch. There is also a version with monitors the voltage, the latter will draw a small amount of current all the time, but it is milliamps and not really a problem.

Because of the modern cars only using the alternator on over run to save power, there is a new system, this has two feeds to trailer one for ign and other live all the time, and an inverter boosts the voltage to charge the caravan battery, this removes the volt drop problem on the wires as well, getting the permanent live and ignition live mixed up could cause the car battery to go flat rather quickly.

The split charging as it's called is designed so when the engine is cranked the two batteries are NOT connected together or very high current could be drawn on the connecting cable, it also means when the engine is not running the batteries are independent so if you over discharge one battery it will not effect the other.

The inverter has completely changed this, it can take the alternator power and fox the alternator to thinking the battery is flat, and use a stage charging system or pulse charging system to bang power back into the battery much quicker than the old idea of a float voltage, they are used a lot on boats, but not so much on caravans. With the old system of relays you could on a 4 hour drive put around 10 Ah into a 110 Ah battery which was better than nothing but not very good, with the new system that 4 hour drive could well put 50 Ah into the battery which is a huge improvement.

The problem is old caravans had gas lights, foot operated water pumps, so nothing needed 12 volt power, it was just handy, but modern caravans need 12 volt power for central heating, fridge, water pumps, and lights, the modern caravan needs either electric hook up or solar power to keep battery charged.

It might help if you provided a sketch of how you have wired this - so far it's all guesswork as to what you've wired to where.