Measuring Amps from a 12V cigarette lighter outlet (1 Viewer)

[stevenjj71]

I've read in various threads that the rear 12V cig lighters don't have much power due to small cables/long wire length. What I want to do is actually measure how much current flows from mine. My portable battery can theoretically charge at 8 Amps while I'm driving, but gives me no indication of how much it is actually getting.

How can I determine how much amperage that cig lighter is putting out? If it turns out to be really low I'll either stop using it, or look into installing better wiring.

Is it something I can do with a multimeter - all my googling of 12v and multimeter seems to show measure current at the actual battery.

If it is doable with a multimeter is there a particular feature I need when buying?

Thanks!

 

[spressomon]

You could make up an adapter to connect simple multi-meter cables inline or use a clamping style ammeter. But the battery will need to be discharged enough to flow max/near max current AFAIK...

 

[george_tlc]

The long/small wires will create a voltage drop due to the resistance of those wires (and cig lighter connections and wires to the battery you are charging). The current that flows will change because of this voltage drop AND the state of charge of the battery you are charging AND the alternator output with engine running.

There is no simple single measurement to figure out how well it's charging or not. Anything you put in series (like you meter on amp range) will introduce more resistance and thus voltage drop. And what you want to measure is the voltage applied to the portable battery and the current that is flowing at that voltage. At the minimum measure the voltage across the portable battery with engine running and alternator putting out its nominal 14.4V.

As above, the portable battery will draw different current when it is mostly discharged and decrease as it goes to fully charged. It will initially charge quicker and taper off as it gets to full charge. Voltage losses in wiring/connector etc will just mean it may take longer to charge.

All that really matters is that it doesn't draw more current than the fuse rating protecting the rear cig lighter circuit. Maximum current would flow with the portable battery discharged and alternator putting out 14.4V (at the main battery). A DC clamp meter would show the current flowing to the portable battery (clamp over just ONE wire).

cheers,
george.

 

[e9999]

The current you'll get will be a function of the load you put on the outlet. You have pretty much a fairly constant and predetermined level of voltage supply, so the effective resistance of the load (plus that of the circuit itself) determines how much you get. If you are concerned about a particular device, use that one as a load. And then, yes, put the multimeter in the ammeter mode inline (if you're sure it won't exceed the max capacity of the meter) or use a DC clamp meter of suitable range (easier and a device well worth having around). But just looking at the fuse on that circuit will basically tell you the max it will likely be able to give for any load.

 

[stevenjj71]

Ok, I'm in need of a multimeter anyway if I'm going to get much deeper into my cars electric system.

Sounds like if I can do the DC clamp meter test on various 12V outlets with the battery say 10% full, then I might get some interesting information about how much crappier the rear outlet is (or isn't) than the others.

Does a DC clamp meter include all the multimeter functionality, or is trying to combine them a false economy? Any suggestions for a device that will handle the DC clamp meter ask as well as whatever I'm likely to need a multimeter for?

 

[george_tlc]

^ for occasional use, a combo DC clamp meter with multimeter functions will be just fine.

Pretty decent and inexpensive is the uni-t UT210D (google for it to find your bargain source).

Extech makes decent clamp meters, but a bit more $.

cheers,
george.

 

[e9999]

Steven, yes, your typical DC clamp multimeter has the usual multimeter functions as well (except for the inline current measurement on those I've used). A typical DC clamp current measurement is likely not as accurate as an inline measurement, but it sure is way easier in many cases. For more accuracy, you'll want to zero the meter each and every time with either no current or with the clamp very close to where you'll end up putting it around the cable so you cancel as much as possible of the stray fields.

You'll have to decide on the best range of the device for you. If you plan on doing only relatively low currents, like your charger, down to very low ones like parasitic drains etc, a low range (as close to the max you envision) clamp ammeter would be more accurate, but then you may not be able to measure higher currents, like engine starting. And conversely, if you have a meter capable of, say, 1000A, it won't do you much good at 10 mA.

Be sure that you buy a clamp device that states very clearly that it does *DC* current. There are ads out there for cheaper AC-only current clamp meters that are vague about that, and those won't work with DC systems. The AC and DC clamp current measurements work on different principles.

 

[stevenjj71]

Ok, so purchased the UT210D.

After some initial user issues - I clamped one of the wires BETWEEN the pincers - and showed it was only pulling 2 amps - rather than putting the clamp AROUND one of the wires (thanks Youtube).

After correcting that I got the following results:

Battery charge level between 6%-12% during testing:

1) using front cig lighter, nothing else plugged in - 8 amps (basically as much as the battery can draw per specs)
2) using front 12v outlet, nothing else plugged in - 8 amps
3) using center console 12v outlet, nothing else plugged in- 8 amps
4) using rear 12v outlet, nothing else plugged in- 8 amps ...hmm..was expecting this to be lower
5) using rear 12v outlet, iPhone plugged into front 12v outler and iPad plugged into center console 12v outlet - battery still shows as pulling 8 amp

I expected 4) to be less than 8 amps, and 5) to be significantly less (as it was sharing with two other 12v's in use!!) Is it likely I'm overlooking something in this test or do these results seem believable?

 

[e9999]

^ You may want to clarify what you are doing and where.

 

[stevenjj71]

There is a provided cable that goes from a 12v outlet Anderson input on my Dometic PLB-40 battery. I'm measuring while that cable is plugged in in the scenarios described above.

The cable by the Anderson end splits into two wires briefly...I made that split a little longer so I could put one of the wires thru the clamp. So the measurement occurs about an inch or two from the PLB-40.

 

[george_tlc]

8A isn't that much current and clearly toyota sized the cables going to the cig lighter properly so that the voltage drop at 8A isn't significant.

And... straight from the dometic website for their product: " The integrated DC-DC charger "

So, they take care of input voltage (14.4 or 14 or 13...) and ensure no more than 8A being drawn.

As inspector clouseau would say, "problem sol-ved"

cheers,
george.

 

[e9999]

Steven, it's not like you have to share a fixed number of amps between various outlets if the battery is doing its job properly. And it can do way more than 8A. And your OP reading up there that the rear outlets "don't have much power" is not really correct . The voltage at the end of the wire may decrease a bit if the wire is too small for the load, but that should not be an issue here.

 

[stevenjj71]

Thanks for the correction on the wire statement.

I am still a little confused on one item though - I thought the 3 outlets on one circuit were limited to 120w between all three (based on a sticker attached to them).

is that the equivalent of a 10 Amp limit? So I have two high speed charging apple devices - both quick charge capable with quick charge capable cigarette lighter plugs - plus an 8 Amp battery pack all drawing current should one of them be falling short on current?

i.e. 2ish+2ish+8=12ish Amps.

Unfortunately I can't use the clamp on the apple cables, so I guess in theory they could only be pulling an amp each.

I'm pretty sure there is another misunderstanding on my part somewhere in this post, please point it out to me

 

[george_tlc]

^ electricity doesn't work that way. It's not like the 120W limit means nominal 10A (12V) is all that can flow. That is just a recommendation so as to not overload the circuit and potentially blow the fuse that toyota would have put there to protect the wiring from the cig lighter back to the battery area (that would have fusible links to protect from massive overloads).

All that happens if you draw 4A versus 8A versus 10A etc is that the voltage drop will change. Wires have resistance, based on their gauge and the length.

V = I * R, so the more current (I) that flows through the wires (with resistance R) the greater V will become, which in this case is voltage drop. Since you dometic has a DC : DC charge controller built in, it really isn't bothered by fluctuations in the input voltage (from you cig lighter outlet).

cheers,
george.

 

[e9999]

S, it is a very good idea to respect limits specified for a circuit. To ensure that the wires don't get too hot, that the voltage doesn't decrease too much if that is important, or that the fuse doesn't blow. But you can disregard that recommendation and can get more power than specced, if you are so inclined, at least for a short while... (Don't do that!)
There are power supplies with electronics that will ensure that you can't exceed a set current and keep providing that, but a typical 12V Lead acid battery isn't one of them. A fuse on a 12V LA battery circuit will limit the current -in a sense- by burning up or tripping, but when that happens the current is interrupted, not just maxed out and kept there. If you take a LA battery without a fuse and short it, it will keep on giving -there will be fireworks! Interestingly, OTOH, most Lithium battery packs do have overcurrent protection buit-in thanks to a battery management system circuit. That's because Lithium cells can be fairly easily damaged and are potentially dangerous. I imagine your Dometic one likely does have one of those BMS.
I would suggest that you read a bit about vehicles 12V systems, it's really pretty simple stuff, but can also be unsafe if not done right. Trucks have burned down because of bad wiring. It's good that you are asking questions and measuring stuff!

 

[stevenjj71]

Thanks, I'm doing by best to avoid making some disastrous mistake

So it sounds like the specified limit is a warning about what not to exceed, not actually what is possible. I was thinking it was a physical (electrical?) limit I would not be able to pass.

I'll avoid using all three outlets at once in combination with the Dometic 8Amp battery draw (apart from for my test/measuring) - I've determined it is possible to power everything at once, but sounds like it is probably not a wise idea!

I'm slowly collecting the bits to have a solar panel on the roof (bit of a nuisance since ALL the roofrack components no my vehicle are missing) hooked up to the Dometic as my next project. I don't drive enough day-to-day to keep it charged for full time fridge use from the primary battery - but since I park in my uncovered driveway I should get enough power that I can have the fridge turned on and ice cold all the time!

 

[e9999]

In a well-designed system, the fuse is chosen to match the circuit capability (basically wire size) so you should be safe assuming the fuse works as intended. But it's worth checking that the fuse is the correct rating if you are not the first owner. Sounds like your fuse should be a 10A fuse. Do not replace it with a higher rating one, bad idea.

Keep in mind your lithium battery has a finite number of charges available before its performance decreases. If you keep the fridge going 24/7 the battery will be charged every day. So that will affect it eventually. And that Dometic battery is not cheap. The good news, though, is that LiFeP is very good with that, you probably have a thousand or more full charges possible, so a few years' worth. But adding a manual cutoff switch in your solar system or fridge circuit and using it may be a good idea.