Analog Angle Blog

Do Cars Have 12-V Batteries?

The son of a friend of mine recently had a problem starting his car, a conventional gasoline-powered vehicle. When the key was turned, it made that slow grinding sound which usually means the battery is weak.

This 300 Instant/600 Peak Amp Jump Starter from Black+Decker is quite handy, and much safer and easier to use than long inter-car jumper cables.

This 300 Instant/600 Peak Amp Jump Starter from Black+Decker is quite handy, and much safer and easier to use than long inter-car jumper cables.

Since he wasn’t in a rush, and since jump-starting from another car was impractical for various reasons, he headed over to a nearby store for one of those convenient boosters: you connect it to an AC line, charge the internal battery, then bring that battery (with integral cables) out to your car and connect it for a jump, see here. These units are actually pretty handy and easier to set up than a car-to-car jump using those long cables (assuming you can position the two cars), with a chance of misconnecting or shorting things if you aren’t careful (a battery-to-ground short makes a very impressive spark!). The $50 retail price is also reasonable, IMO.

Still, I was surprised when he called me from the store with a “simple” question, to make sure he was getting the right charger/booster: “is that battery 6, 12 or 24 V?” he asked. At first I thought this might be a trick or joke question, but he was serious. (Of course, the answer is 12 V.)

It turns out it wasn’t a trick question; he was serious. I assumed incorrectly that “everyone” knows a standard car battery has a 12-V rating (actually, 12.6 V nominal) and that knowledge is part of the common culture.

Looks like I assumed wrong. My gut reaction was “how could you NOT know that?” but then I gave it some more thought and reconsidered, and realized two things. First, this person is no technical dummy. He routinely sets up and gets different networks and systems connected, talking and even happy with each other; that’s often a real challenge when you are configuring systems from different vendors or apps. I’ve seen him do this and it is an impressive feat.

Second, today’s cars are basically hostile to average people poking around in them. There are so many sophisticated electronic modules, networks, functions, and features that you are diving into a jungle when you open the hood. Unless you have a good sense of what you are doing, there’s not much you can do; you are taking a big chance by going in and poking around. It seems like an unintended and unforeseen consequence of the complexity of today’s cars is to discourage the average person from doing anything on them, even basic maintenance. (A cynic might say this is not at all unintended or unforeseeable, of course). Even the humble, basic car battery is now part of a complicated power-management subsystem which monitors voltage, current flow, cycles, and much more.

The situation is analogous to the dilemma we have with electronics and experimenters. On one side, we urge students to explore, build circuits, and do more than just sit at a keyboard playing with apps and thinking that keyboarding is all there is to engineering. But the reality is that is very hard to build real circuits with today’s ICs and passive components due to their tiny pads and dense interconnects. Building, loading, and soldering a prototype is a real challenge and whole “maker” subculture has found ways to do it with magnifying glasses, tweezers, and modified toasters.

Further, if the beginner does have the patience and skill/luck to build that board, it is still very hard to “play with circuit” as part of the project—it’s hard to probe voltages and currents, or change components to try something out. While there are some ways to minimize the problem by using leaded discrete components, through-hoIe ICs (yes, you can still get some in DIPs), and various prototyping boars, you are very restricted in what you can build.

The more I thought about him not knowing that the car has a 12-V battery, the more I concluded that this is not a matter of technical ignorance in itself, but rather an issue of his technical insight being naturally redirected to where he can actually do something. Maybe it’s the new “normal” that we, as engineers, had better get used to?

What’s your view on increased product complexity diminishing the need or ability to underhand basics such as a car battery?

Related links

Powering the autonomous car

New cars making tapping battery power tough

Will cars get on the 48V bus?

13 comments on “Do Cars Have 12-V Batteries?

  1. mccunets
    January 18, 2016

    Good thought-provoking piece, Bill.  A good way to get into electronics with stuff currently available is Ron Quan's recent book, a piece on it here:  

  2. clayga
    January 20, 2016

    It wasn't a stupid question – most (all modern) cars /do/ have 12V batteries, but not so long ago there were a number with 6V systems (the little DAF saloons for one) and of course lorries standardise on 24V…

  3. antedeluvian
    January 20, 2016

    but not so long ago there were a number with 6V systems

    To say nothing of positive earth cars.

  4. LCVieira_BR
    January 20, 2016

    I am a 54 YO E.E. graduated in Brazil (I'm a native Brazilian) and I teach for 20 years now. I confess I feel like you were talking to one of my students… I face this daily-basis, kinda 'reality disconnection' since I started teaching, but lately it's getting worse. When dealing with computer science students, I suspect that their so needed 'level of abstraction' to understand what programs do without the need to understand how they do it leads to reasoning emptiness. I am terribly worried about the consequences they are going to face in the future… “Idiocracy” is, perhaps, the movie with the best worst scenario.

  5. KD4GT
    January 20, 2016

    My own personal vehicle uses a 24 volt battery plant. Actually, it uses a pair of fairly standard 12 volt batteries in series. Most military vehicles (which is what I happen to drive on a regular basis) use 24 volt primary systems. It may have been selected for economy of production when copper for wire was in short supply – or maybe it was selected because many military vehicles are “lorry sized” machines.

    Regardless of the initial reason, “we've always done it that way” prevails and now we much design to the standard. Lighting, fuses, breakers, relays, automotive HVAC, and any accessories – including radios – for military vehicle use are the 24 volt variety. And, yes, I have a couple of those radios, too!

    With seemingly only heavy equipment and military vehicles using the 24 volt systems, getting the usual 12 volt accessories to operate has necessitated my creation of a subsystem using a switching supply to take 24 to 12 volts. And I use different and non-mating connectors so that never the twain shall meet.

    But at least I know when I am looking for parts or accessories that I must look at the specs carefully to make sure it won't let all of the magic smoke out when I connect it to my Pinzgauer vehicle.

  6. jimfordbroadcom
    January 20, 2016

    Actually not replying but commenting; the Comment function isn't working for me.  I just wanted to point out that Linear Technology had an app note about designing electronics to connect to the car battery via the cigarette lighter.  I don't have a link, but they referred to it as “the power supply from hell”!  Two batteries in series may be experienced for a quicker charge, on the order of 100 V spikes, etc. make it very challenging to keep stuff connected to the cigarette lighter working properly.

    Also, a few years ago there was some momentum to change to a higher voltage such as 42 V, but I've not heard anything lately.  Something about greater efficiencies in alternators and starter motors.  Maybe somebody else with greater knowledge can fill in here?

  7. KD4GT
    January 20, 2016

    I think much of the “efficiency” has more to do with IR drop through conductors than with energy transfer or conversion.

    And anything powered from “the power supply from hell” is suspect! That socket is usually made to the lowest possible standard, very current limited, and certainly subject to all the noise possible. Any sensitive electronic devices wanting to use that thing (or anything directly tied to an automotive electrical system) better have extensive filtering and surge suppression on the input.

    Look at all the tech bulletins from Motorola and the auto manufacturers regarding radio installation. That is one messy environment.

  8. Andy_I
    January 21, 2016

    To add to the confusion…  Many engineers are clueless that a “12V” battery isn't 12.0 volts.

    I have seen electronics designed for “12V” operation, with a maximum voltage specification of 12.5V.  That is, 12.0 +/- 0.5V.  Seriously.  I surely hope their specs were wrong.

    Take a normal American auto and measure the battery voltage when it's running, and it'll probably be something like 13.5 to 14.5V, with spikes going above that.

    People who use lead-acid batteries in static operation (I've got one in my den) usually talk about 13.8V (not 12.6) as the nominal steady-state voltage when connected to the charger that is there just to maintain the battery in a charged state.  Look up most amateur radio transceivers, and that's what they were designed for (e.g. 13.8V +/- 15%).  So even calling it 12.6V is too low and would mean the battery might not be at full charge.

    The inability to measure what you build, is one of the reasons for the popularity of SPICE.  If you can't get a waveform on a scope, the only way to play with your circuit and see what it does, might be on your computer's screen.

  9. Duncandennis
    January 22, 2016

    Actually not replying but commenting; the Comment function isn't working for me. 

  10. OMardadam
    January 25, 2016

    And anything powered from “the power supply from hell” is suspect! That socket is usually made to the lowest possible standard, 

  11. Benwatanabepual
    January 27, 2016

    good post nice information

  12. leornevo11
    December 31, 2016

    Well ' the question was relevant but the answer was not good enough as while most cars are 12V and most Trucks are 24v it is good to specify what is the Amper required to get a smooth ignitions' as my 4×4 cars for examples with engines like 2800-3000 require 100 A while private – saloon car require only 55-65 Amper and in the above case the picture show that the Jump-Starter is very big and up to 300 Amper which means he was lucky and took a strong enough jump-starter ' else could get stuck again with no car ignition ;; so in general simple answer of Yes or No us not good enough in such cases and need to ensure  supplies all required questioins and answers – good luck – leor Nevo = Jeepon-Naim By Nevo 

  13. MelBrandle
    December 4, 2018

    This would help many drivers be independent when on the roads especially at a location that does not witness that much traffic flow. Having one of these units would mean that a weak battery can be salvaged in no time without having to depend on other road users to save them with their own battery. Furthermore, impracticality can also be avoided both for the giving vehicle and the one at the receiving end.

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