12V ACCU Automotive vs 12V DC – What is the difference?
This “rhetorical” at a first glance question has a deeper meaning…
Today you will learn:
…Why fails some exactly 12V DC rated lighting devices, e.g.: LED Strips, when are used on 12V DC Car Accumulator installation?
We need to start much far away… What is Voltage? How to measure it?
Can I Touch It? Is it Dangerous?
Unfortunately, Yes! It is Dangerous!
Fortunately, The Safe Upper Voltage limit as a standard is 36V DC! So Any Voltages under that value are accepted as safe to be touched direct with human skin – it resistance varies from 10k – for wet and sweated skin – over to 10MOhms when is “rude” and dry, Typically 100K – to a few Mohms.
Let’s accept the Lower Resistance value – 10K:
The Dangerous Current Upper Limits: for flowing Alternate Current – AC is 10 mA versus 50mA made by a Direct Current – DC.
So, The AC is 5 times Dangerous than DC. For currents flowing through the human’s body, over these two Limits, is coming a real Danger of the Electric Shock.
The Muscles are contracting, and the human can escape from himself lock…
So, if we accept the sweat and wet human skin as 10KOhms Resistance – if we apply it as load over the Voltage Source with 12VDC output, the flowing current, between source and load will be: I = U/R from The OHM’s Law – or 12 V / 10000 Ohms is equal to 0.0012A or 1.2mA – the current is small to be dangerous for skin, but not so acceptable for the hearth. Fortunately, when we use the typical value of human skin’s resistance -100KOhms is fill-in the last equation, the current slows to ten times less 1.2mA – or 120 microamps., which is more small and unproblematic.
Now, we already know, the 12V DC is low enough and safe voltage to be touched without some special wears or protective gloves.
Automotive parts and Car electronics has rated and are certified for work in whole Lead-Acid Accumulator’s allowable voltage range.
The lower limit of complete discharged Lead-Acid Accumulator – with an absolute low limit of 10.8Volts – Over a Load, causes the flowing of a few ampers discharge current – not the Engine Starter DC Motor with Load Current of 300Amps and Over!
The Upper Limit has also an interesting Value. At the moment we will stay it as a “secret”. It will be quoted later, in the text below.
Now, we reach to the essence of the question:
12V ACCU Automotive versus 12V DC – What is the difference?
Мany people will say: what a stupidity: Of course, the 12 volts equals to the 12 volts.
But, this is only true at one point of the load line of the lead-acid accumulator.
PIC. 1 Example 12V Lead Acid Accumulator Load Line
Each charge and discharge cycle forces the operating point to move along the load line, which is generally not straight but the exponent.
So the thin difference between them is: The 12V DC Automotive Rated Devices have guaranteed working Range of 10.8V for complete discharged Accu to 14.4V for fully charged Accu.
That’s why when any hobbyists use the only 12V DC rated, e.g. LED Tape for fine tuning of the interior lights, he will soon realize, the tape starts light unevenly.
What is happening? The 12V Device has a fault on 12V Car Accumulator installation. Please, read carefully in the text below, for truly understanding the explanation.
Many Light Devices, as LED Strips are rated for exactly 12V with no upper limit additional tolerance. That means they must be powered with no more than 12VDC. The Voltage value can be a little bit lower, but not higher.
Example: The LED Strip manufacturers know, that Many AC/DC Inverters on the market has very good line regulation, and the ability for a custom user’s adjusting the value of stabilized DC voltage output.
The LED Strips have a very high Voltage to Current, and Current to Temperature Dependence.
It is a good practice to set the AC/DC Converter to a voltage small below than 12VDC, e.g. 11.85V. Invisible for a human eye difference, as light luminous flow decreasing, It will give you unprecedented more than doubling of the Tape’s Life. Checked!
Now, you already understand, why your Interior Led Strips doesn’t be used in automotive applications, without any protection circuits, DC/DC Converters, guarantees the exactly 12VDC output for whole LEAD-ACID Accumulator working Range: 10.8-14.4V. At non-working engine state, respectively non-charging DC Generator-Alternator, Stand By Accumulator Voltage, depending the charge status is somewhere between 10,8V and 12.6V. On the Upper Limit of 12.6V, the LED strips still have a real possibility to survive, although, it is not too guaranteed – don’t forget, they are exactly 12VDC Rated. On the Lower limit, the luminous intensity will be slowed in a half and less… But, there is one LARGE BUT! The worse case is coming, when the engine really runs. The alternator starts charging the Lead-Acid Accumulator. Dependant on its charge state, the Accumulator Voltage will start rises, and it will go – up to 14.4 Volt. Here is a key to the mystery! The Voltage is very High, The LED currents are over the maximal ratios, and devices will be damaged. The Bonding and the Galium-Arsenide Semiconductor structure cannot survive in these extreme conditions.
As a General the LED Tape for low voltage, as 12V DC is using a simple resistive current limiting schematic. The Tape can be viewed as an array of 5 cm / 2inch segments, connected in parallel to the powering rails or tracks. Every Segment consists, independent of colors with three LEDs connected in serial with a current limiting resistor. Namely, this cheap realization is very powering dependent. The Rated Current is set, near the LEDs Maximal allowable Current. Even Small overvoltage changes are causing large fatal current changes and tape goes useless.
To enrich the theme content, adding a vision to a kind of completeness, we have found a Suitable Automatic Switching DC/DC Converter, working аs Step-Down [ Buck ] when the output Voltage is lower than the input, and vice versa as Step-Up [ Boost ].
Cheap, Reliable, Flexible: Simply Adjust Output to 11.85 – that’s it!
Adjustable DC-DC Converter wide-input, Automatic Buck-Boost Mode: