And I thought I was being clever!? Measuring DC amps via fuse socket help!

[Ben Dono]

Well... I must be missing something.
A long story short, I'm looking at getting a start/deep cycle marine battery for my little ute.
It only gets about 5000km on it a year and can sit for a few weeks at a time. I'm noticing the battery is degrading a little early as it's not getting used enough. The other side of the coin is that it would be good to run the radio while I'm working without the worry of draining the battery so I cannot start it.

So I thought I was being clever by measuring the actual Amps that the radio draws and match a battery size. In my mind, the simplest path to go would be to pull the fuse for the radio and meter it from there... Except...no go! The meter closes the circuit and the radio runs but I'm getting a reading of 0.00DC amps.

Any ideas why it's not working?

I tested the amp function on the multimeter on both DC and AC as well as both and it's fine.
I'm using the 10A fused side set to DC.

I will test it again via the battery as I know this will work. I just thought I was being clever doing it via the fuse panel. That way I can adjust the volume, key and radio all from the cab.
Any ideas why it did not work via the fuse socket?
It's got me stumped!

[BobL]

Maybe the radio uses less than 10mA?
Do you have a more sensitive scale on the meter?
Does the meter flicker at all when you attach it with the radio on?

[mike48]

If you have independently tested the DVM that it measures and displays amps OK down to 0.1 amp, then maybe the radio is getting power via another fuse, or via another DC path, or via a fault path.
Has the vehicle been modified or had any electrical work ie security system etc added?
Maybe the marking of the fuse usage on the fuse box is ambiguous and relates to another use, or is just incorrect?
Are you reading the location of the fuse correctly, as some vehicles are really woeful in the marking.

The mystery is only until you find the rational cause of the observed situation, and there will be one.

bon chance
typo

[BobL]

Can you be more sepcific about the vehicle. make/model/year

[Optimark]

Coming at your problem from another angle, have you thought of adding a small PV solar cell that sits on the dash board?

I say this as someone who has quite a few vehicles, one of which did 2,990 klm for it’s first 12 months of ownership, with the second 12 months being worse due to not being used for 4 months whilst we were in Europe.

I’ve been running small PV solar cells for close to 20 years in this manner, saving batteries galore, compared to previous non charged vehicles. I first thought about using a PV solar panel whilst at a boat yard helping a mate with his yacht. Having a sandwich for lunch one day, I spied something atop the rear of a yacht nearby. I couldn’t figure out what it was, so we asked the owner who was also having a sandwich in the shade of his yacht.

Solar panel was his reply, keeps the radio battery charged. We then asked if it worked well, he replied, “not that well”. After a suitable period of silent munching on sandwiches, he then piped up with, “the battery is 12 years old, I suppose it can’t be too bad”!

I remembered those words made me do some serious thinking with regard to our Nissan Urvan campervan that was always running out of battery as it was usually used about every 6 weeks or so, then parked for another 6 or so weeks. Mostly the batteries in that lasted about 30 months, before they became dodgy enough with camping and what have you, that replacement was a safer bet than asking the missus to walk to the nearest town if I couldn’t start the Nissan after sitting in a bush camp for a few days.

I at first purchased a single PV cell mounted onto a bit of plastic and sealed in some kind of epoxy and with a single lead running out with a diode to stop discharge whenever the sun wasn’t shining. I still have and use that cell, it has been permanently attached to my motorcycle for the last 14 years whenever it is parked at home.

We also have a 12V solar system in our camper running 240Amp of batteries kept charged by 220W of panels, powering our refrigerator, lights, fans and the small computer I am currently using to write this. Experience with this system has made me realise the best way to keep any battery useable, is not the type or construction, but the manner in which it is kept charged.

Monocrystalline units will give you more power for a given square area over others, these are usually what is used in 12V mobile situations as space is at a premium compared to a house rooftop.

Something like this, may work very well for you. Solar Panel 5W 12V Mono Square


Some food for thought.

Mick.

[Ben Dono]

It has to be something simple that I'm missing then! I will have another go at it this arvo.

It's the right fuse as I can close and open the circuit (power on and off the radio) with the Mm probes in the fuse socket.
It should be pulling anywhere between 1 and 8amps. Probably more like 2A.
It's a fluke MM that reads down to .000
I can set it to mA but It should not be anywhere near that range and the MM fuse for mA is only 400. I don't feel like blowing the fuse on a hunch!
I will test it directly from the battery and scratch my head until I figure out why it won't read from the fuse socket.
... It's an odd one

The radio is an aftermarket one but it's on the factory wiring harness.

[BobL]

. . . The radio is an aftermarket one but it's on the factory wiring harness.

I meant of the vehicle.

[malb]

If the vehicle/radio are modern enough to include an electronic clock and digital tuner it will normally have at least two power supply positives, separately fused.
One will be a non switched one to maintain the clock and tuning memories etc when the ignition is off. This circuit should only draw maybe 20-30mA.
The second will operate though the ignition switch ACC and ON positions to supply the power for the radio operation, and allow the general operating circuitry to be isolated from the battery once the ignition is in the OFF position and the key is removed, ie vehicle locked up and left. This circuit can draw 2-30A, depending on the model, the speakers fitted, and the volume control settings.
There may be a third positive supply for lighting etc connecting with the circuit for the other dash lights, to illuminate the radio when the vehicle lights are on.

[Ben Dono]

It's just a simple 98 hilux single cab....damn simple thing to work on as well. No fancy electronics. It did not even come with a little clock.

I had another go at it last night as it was bugging me. The imobaliser is on a live circuit but sends a tone through the stereo (on a different circuit) when you turn the ignition key off.

It must be the reason for non results... Still don't know why.

I metered it from the battery last night. The little radio only draws about 1.2A with loud music but fluctuates as high as 1.5A with heavy Bass.
I'm going to call it 1.4A.
Hahah their are a lot of people out there laughing at the massive specs!... it's loud enough for me. I'm 36yrs old with a life time around machinery and my hearing is still great...I want to keep it that way!

Optimark, your right about the solar cells. I did think about it a while ago but I like my ute pretty low tec. Less can go wrong with it!
I will put in a few small and thin ones on the front of the visors and just flip them down when I park it on the farm. The little led volt meters are only around $20. I will wire one in while I'm at it.

I wonder what are the losses you would get from having the solar cells behind the glass. I might test that as well.

So on to the battery size... It's a big guess but if I cut the RC into a third I should get a safety net to start the little truck.
A good quality standard size battery has an RC of about 60mins at 25A which brings it to 10.8v.
So I have 20mins at 25A and should be able to start the truck.. It always kicks over easily!

At 1.4 amps... I reckon I will get about 5hrs of safe radio from the battery in good condition.
That ain't bad..

Anyone want to comment on the maths? It's was a long bow to draw!

[Hercus71]

I saw you got to measure the current draw when in use. I would bet that you were (initally) measuring the clock/station memory input wire so no current draw.
As to the low useage/radio required problem I would suggest a battery keeper/floater type charger. In my experience those devices are tolerant to the useage you describe without disconnecting and will bring the battery back to fully charged when you are not there.
I have 4 of those devices (different makes) and swear by them.
No matter which car I want to start I know the battery will be ready.

[mike48]

So Ben, re the original post, did you ever get to the bottom of this?