1972 Fiat Spyder - Amps

Since you make it utterly impossible to comment on articles, I’ll comment here: I think you should check your math in the LED device drawing down battery posting. You’re off on your calculation by several orders of magnitude. Let’s be generous and say a LED draws 10ma. Multiply by hours, gives you 240maH draw per day. That’s roughly a quarter of an amp-hour, drain, per day. That doesn’t get you many days before a typical-real-world-used-battery 30AH battery is too low to start the car. A reasonably-clever designer could cut the current draw by a factor of 5 or so, but few dollar-barrel adapters make any effort at efficiency, so realistically, in real life you could be looking at worse than 10mA rather than better.

… heck, and you won’t even let me list any of the vehicles I actually own… Fairly annoying site you have here…

I agree with you about not being able to post on the blog articles. I don’t use Facebook myself and there seems to be no other way to comment. I have stopped reading the blogs because it is just too irritating.
What kind of cars do you have? You can always describe the cars in your post.
I only opened your post because I was surprised a Fiat of that vintage had survived outside of a Museum.

I chose that one as a “what car I have” since it actually was the closest thing on the list to any of my vehicles. Mine’s a '72 850 Spider, but it’s in little bits and pieces scattered around the shop waiting for me to eventually figure out how to build a frame for it, since the unibody rusted in half years ago. Next in line is a 62 Austin Healey Sprite, and my daily drivers are a 85 BMW 524turbodiesel, a 93 Ford (made by Sterling) Cab-Forward Cargo-7000, and a 96 Stewart and Stevenson M1081 air-drop LMTV. Doesn’t seem like CarTalk really wants to talk about anything I own. And, they also apparently can’t do math.

One thing that escapes people not too familiar with the site hosts is that they are written from a humorous perspective and everything should not be taken so literally. To me, I can tell they are exaggerating because the time frames listed are ridiculous. I would think very few people would expect any battery source to last a couple thousand years so right there should be a clue as to how to interpret the intention…

So why torture yourself?

1 Like

Mostly, because they’re being quoted elsewhere regarding the potential lifetime of a car battery supporting a USB adapter with a LED lamp on it, and their answer is dangerous - I was hoping they might see fit to provide an actually correct answer so that people were less likely to have problems due to following their advice.

In reality, a LED lamp being powered continuously from a car battery, assuming the use of cheap linear voltage regulators which are common in cheap USB power adapters, will pull a typical car battery down far enough to damage it in 1 to 2 months, even assuming no other drains on the system. If their answer didn’t matter, I wouldn’t bother with their site, but people listen to them, and the answer they gave is irresponsible.

Wouldn’t that be the fault of the one doing the quoting? If you quote bad (or satirical,) information and try to pass it off as real, that isn’t the fault of the original writer.


Whose site are you upset with ?

pretty sure this is the ‘article’ the OP is referring to:


Eddo , that does appear to be the article in question . Which means that Willray needs to know what humor is .


Are you sure about that? 12 volts at 10 mA is only 0.12 watts. That would be a pretty dim light, even if it was 100% efficient. I presume you are referring to one of Ray’s newspaper columns. I don’t recall that particular column. If you can find a link to that column, suggest to post it here. For an archive of the Dear CT columns, click on the “blogs” link upper left on this page.

I owned a late 60’s/early 70’s Spyder for a couple of days. The owner’s wife made him sell it, but he was so sad afterward I gave it back to him in return for my check.

I think that’s a good compromise. Sometimes the comments detract too much from the point of the article.

I was being generous in the direction of trying to produce the longest possible run-time with “not too ridiculous” approximations, so no, it’s really not 10ma. That being said, it is less than .12 watts. A garden variety LED indicator lamp draws 20mA at around 2.5 volts. A 1 watt LED is freakishly bright.

The kicker, and this is why I’m suspicious someone did their math wrong, is that while 2.5V at .02A says that the LED consumes only .05 watts, and car batteries are worth maybe 500 watt-hours, cheap devices use linear regulators, so rather than drawing .05W (= 4mA) from the battery, they draw the full 20mA (about 1/4 of a watt), and just ditch the rest of the wattage as heat.

Do the math that (wrong) way, and you get an estimate that a battery ought to last 8-10 months, which might as well be 2500 years, so you throw in a bit of humor and give a number that says “don’t worry about it, it’ll last so long you won’t care”.

Unfortunately, that’s not true. The reality is that if you’re going to leave the vehicle unattended for more than a couple weeks, that stupid little power light on your USB dongle really might kill your battery.

The 850 was my first car. Didn’t get much use out of it before it rusted in half, but I’ve kept enough of it in boxes through several moves that I should be able to reconstruct it, or at least something that looks like it, if I ever get around to making a frame to build the shell back onto.

That’s entirely possible. Please explain to me how advice that is likely to cause damage to vehicles if followed by the uninformed, is humorous.

I don’t know what article you are talking about since I don’t read them and I’m not on Facebook either. Just to add my real world two cents worth though, my wife parked our Olds at the airport for 2-3 days. Can’t remember exactly anymore. Usually in a hurry and the person she was with did not fully close the trunk, so the light was on. The high power Delco battery was so dead after that, that the airport service had no effect on it. The motor club finally got it going but I just had to replace it. So a little bulb for 2-3 days and that was it.

1 Like

This info is good and helpful for me.
I agree with you.

The sleep current of my ECM/PCM electronics is slightly higher than 20mA. I have cars that last up to 4 months with this kind of draw and can still start the engine. One is a Camry and I believe even it has a battery that is 50% more capacity than your example. That 45AH rating is low by today’s standards IMO.

Of course, leaving a lossy parasitic load connected would double that draw and therefore halve the time it could be supported without being started. But two weeks, in my estimation, is way too short of a time frame for a 20mA load to wipe out most batteries that are in reasonable shape. That being said, I’ve never been one to leave any power supply connected unless it is doing its intended job. Much less in a car I’m going to leave unattended for long periods…

Well, let’s take it to the next level then. Let’s assume the entertainment aspect is removed and they only supply a straight laced answer for the uninformed. Imagine the surprise when someone that has a 3 year old battery that is maybe 50% of its capacity new, expects the USB charger to be OK for a week? And it’s not. Then the literal police would be on here anyway bemoaning the incomplete advice given to this poor unsuspecting soul that got stranded at the airport parking lot…the horror!

I suspect you have a fairly healthy battery if you’re doing OK 4 months out. Most people have a battery that’s several years old, and that has lost significant capacity since new. I wouldn’t worry, for most, that 2 weeks would be a big problem, but, add additional drains like an LED, and a month starts to get worrisome, and 2 months probably puts a lot of “what people have in their car” batteries at risk.

The responsible thing to do, would be to caveat the article and say “All humor aside, there are a lot of differences between cars, and a lot of differences between USB adapters and LED lamps, but, in reality leaving a USB adapter plugged in could draw as much power from your battery as all the rest of your car’s parasitic loads. As a result, if you can leave your car sitting for 4 months now and it still starts OK, leaving a USB adapter plugged in could reduce that to 2 months. If you’re worried about your car starting if you leave it alone for 2 weeks, the USB adapter could cost you a week. Overall, we recommend unplugging them when you’re not using them, just to be safe”.

Wiilray , do you not understand that the CarTalk show was about 60 % entertainment and 60 % help . The column is pretty much the same way so relax .

I dunno VOLVO_V70, perhaps I just need more of your erudite instruction, but it seems awfully mean spirited to me, if the only way you can think of creating humor is by misleading people into breaking their toys. Or, maybe it’s not my idea of humor that’s problematic…

I always found the CarTalk show to be useful information, couched in a humorous presentation. I don’t think Click and Clack would hold much truck with your notion that giving people bad advice is funny.

The humorous thing, from my perspective, is that it’s a near certainty that you are defending a math error that the original author the column would not defend, if they noticed it. Your insistence that that is funny, is, sadly, funny.

1 Like

I’m not sure where you think you’re going with this. The logical conclusion of the argument you’ve presented is that under no circumstances should the column try to give any form of useful or accurate information, because it will inevitably be incomplete and therefore problematic for some. I have a suspicion that’s not really what you meant.

My position would be that it’s important for popularly-acknowledged authorities to present information that is, at its core, true, at least under reasonable assumptions and caveats. More-so when the information is regarding a topic where much of the public is poorly informed, and where they have little ability to reason their way to a ballpark answer for themselves. From a communicating to the public standpoint, I think it’s also necessary to couch the presentation in as palatable terms as possible, so yes, narrative and humor are important ingredients. However, the trappings of presentation should not ablate the basic truth of the content.

In the case of the mentioned column, I believe the “truth” they were attempting to convey, was “it’ll last a really freakin long time”. 2500 years is a humorous way to say that. Unfortunately, that “truth” is not true. Either someone did the math based on some incorrect assumptions and came up with a number that seemed long enough that the battery would be dead of natural causes before the LED had any meaningful impact, or, just looked at a car battery and said “that’s a freakin huge battery and a tiny little LED, obviously it’ll last forever”. Arriving at that conclusion, they chose a humorous way to present it.

In reality, LEDs draw more than people realize, and cheap USB adapters/etc are poorly implemented and draw more than necessary to support the LED. I’m pretty sure that if the authors had realized that, they’d have found a humorous way to convey that USB adapters actually can be a problem. No reason to remove the humor, or hyperbole, just a necessity to direct it towards the audience understanding the truth.

Where are you getting your information . . . ?!