Voltmeter for vehicle battery to monitor inside of cab

[Analog Kid]

Pardon my intrusion here, as I am certainly not any kind of automotive expert.

However, it seems to be after reading this thread that the objections to using something like a voltmeter plugged into the cigarette lighter socket are kind of uncalled for.

This because of what seems to be the OP's motivation here:

I just had the idea to get one of these due to not trusting the intermittent battery light so then I can monitor the actual voltage when driving without having to pop the hood at a layby and check manually when that light is coming on and off.

I have been monitoring it more regularly now after driving and so far have not seen any decline in voltage when I park up so doesn't seem to be an indicator of battery drainage.

Having something like this I feel would give peace of mind as regardless, of the light not having correlated with actual battery malfunction yet, it does make me twitchy each time it decides to come on while driving.

Now, they didn't say what kind of accuracy they were aiming for; they'll have to tell us this themselves.
But it seems like they want something a little better than a "go/no go" indicator.
I don't think the voltage drop caused by whatever wiring there is betwixt battery and cig-lighter socket would be significant here. The relative voltage indications between a battery in a known-good state and otherwise should be sufficient for their purposes.

I'll let them (the OP) speak for themself to clear this up.

[radiolistener]

I don't think the voltage drop caused by whatever wiring there is betwixt battery and cig-lighter socket would be significant here. The relative voltage indications between a battery in a known-good state and otherwise should be sufficient for their purposes.

the concern here lies in the precision required to meaningfully assess battery condition or charging voltage. Even a deviation as small as 0.2-0.5 V can lead to a completely incorrect interpretation. Typically you need < 0.1 V error for a good lead acid battery assessment.

For example, a resting voltage of 12.7 V typically indicates a fully charged and healthy lead-acid battery. In contrast, 12.0 V (as shown on cig-ligher voltmeter on the photo above) suggests the battery is significantly discharged, potentially even failing and requires immediate attention (charging or replacement). This seemingly small difference of 0.7 V represents a major shift in state of charge, easily over 70%.

So, if the voltage measurement method (through a cigarette lighter socket) introduces an error margin that large, it can render the reading unreliable and potentially misleading, making a good battery appear bad, or vice versa. In such cases, the convenience of the method doesn’t justify the risk of misinterpretation.

[Sorama]

Pardon my intrusion here, as I am certainly not any kind of automotive expert.

However, it seems to be after reading this thread that the objections to using something like a voltmeter plugged into the cigarette lighter socket are kind of uncalled for.

This because of what seems to be the OP's motivation here:

I just had the idea to get one of these due to not trusting the intermittent battery light so then I can monitor the actual voltage when driving without having to pop the hood at a layby and check manually when that light is coming on and off.

I have been monitoring it more regularly now after driving and so far have not seen any decline in voltage when I park up so doesn't seem to be an indicator of battery drainage.

Having something like this I feel would give peace of mind as regardless, of the light not having correlated with actual battery malfunction yet, it does make me twitchy each time it decides to come on while driving.

Now, they didn't say what kind of accuracy they were aiming for; they'll have to tell us this themselves.
But it seems like they want something a little better than a "go/no go" indicator.
I don't think the voltage drop caused by whatever wiring there is betwixt battery and cig-lighter socket would be significant here. The relative voltage indications between a battery in a known-good state and otherwise should be sufficient for their purposes.

I'll let them (the OP) speak for themself to clear this up.

Exactly my point.

[Sorama]

I don't think the voltage drop caused by whatever wiring there is betwixt battery and cig-lighter socket would be significant here. The relative voltage indications between a battery in a known-good state and otherwise should be sufficient for their purposes.

the concern here lies in the precision required to meaningfully assess battery condition or charging voltage. Even a deviation as small as 0.2-0.5 V can lead to a completely incorrect interpretation. Typically you need < 0.1 V error for a good lead acid battery assessment.

For example, a resting voltage of 12.7 V typically indicates a fully charged and healthy lead-acid battery. In contrast, 12.0 V (as shown on cig-ligher voltmeter on the photo above) suggests the battery is significantly discharged, potentially even failing and requires immediate attention (charging or replacement). This seemingly small difference of 0.7 V represents a major shift in state of charge, easily over 70%.

So, if the voltage measurement method (through a cigarette lighter socket) introduces an error margin that large, it can render the reading unreliable and potentially misleading, making a good battery appear bad, or vice versa. In such cases, the convenience of the method doesn’t justify the risk of misinterpretation.

Wrong, again.

[fourfathom]

I really doubt that the OP needs a solution with Coulomb-counting and Peukert compensation, just something better than an idiot light.  On my cars I keep a cheap "cigarette-lighter" combo USB-charger / voltmeter  plugged in.  I occasionally glance at it before I crank over the starter, and perhaps turn on the headlights to check the non-charging battery voltage.  Then I start the car and watch to see if the voltmeter shows the battery voltage jump up appropriately.  This isn't a precision test of battery condition or voltage, but it warns me of impending problems.  More than good enough for my needs.

[radiolistener]

Wrong, again.

Simply stating "wrong" without explaining your reasoning doesn’t help move the discussion forward. If you believe something is incorrect, it would be far more constructive to clarify why - ideally with technical details or a practical example. That way, we can have a more productive and informed conversation.

Otherwise, such comments come across as typical trolling behavior - dismissing any opposing viewpoint with a blanket "wrong, again" without offering any substance in return. It looks you're trying to reject any statement without engaging in real discussion. But that approach doesn't hold up in a technical or reasoned debate, where disagreement should be supported by clear, evidence-based arguments - not just empty claims with nothing behind them.

[BillyO]

2023 Tacoma
Accessories on
Fan full blast

1) Engine off, at battery
2) Engine off, at lighter socket
3) Engine running, at battery
4) Engine running, at lighter socket

Maximum of about 1% voltage drop between battery and lighter socket.

[Sorama]

Wrong, again.

Simply stating "wrong" without explaining your reasoning doesn’t help move the discussion forward. If you believe something is incorrect, it would be far more constructive to clarify why - ideally with technical details or a practical example. That way, we can have a more productive and informed conversation.

Otherwise, such comments come across as typical trolling behavior - dismissing any opposing viewpoint with a blanket "wrong, again" without offering any substance in return. It looks you're trying to reject any statement without engaging in real discussion. But that approach doesn't hold up in a technical or reasoned debate, where disagreement should be supported by clear, evidence-based arguments - not just empty claims with nothing behind them.

Instead of repeating that old song of you, you better start reading and trying to understand what some of us are explaining you.
Stop trolling.

[Sorama]

2023 Tacoma
Accessories on
Fan full blast

1) Engine off, at battery
2) Engine off, at lighter socket
3) Engine running, at battery
4) Engine running, at lighter socket

Maximum of about 1% voltage drop between battery and lighter socket.

It’s as simple as that.
 

[floobydust]

BillyO, the Tacoma, seeing below 12.0V terminal voltage on the battery, I think is a problem- that's saying the battery's State of Charge is low roughly 25-40%. I would say the battery needs a decent manual charge and equalization.
I find it's a common problem with modern vehicles - the parasite drains can leave the battery partially charged. Firmware updates to Body Control Module etc. better that, but sometimes a module can't go to sleep because something is hanging it up. Subaru's are notorious for killing batteries 3yr life due to the high parasite drain 75-100mA.

[Sorama]

BillyO, the Tacoma, seeing below 12.0V terminal voltage on the battery, I think is a problem- that's saying the battery's State of Charge is low roughly 25-40%. I would say the battery needs a decent manual charge and equalization.
I find it's a common problem with modern vehicles - the parasite drains can leave the battery partially charged. Firmware updates to Body Control Module etc. better that, but sometimes a module can't go to sleep because something is hanging it up. Subaru's are notorious for killing batteries 3yr life due to the high parasite drain 75-100mA.

Or a decent (long) drive. but it won’t change the amount of parasitic drain.

[radiolistener]

I really doubt that the OP needs a solution with Coulomb-counting and Peukert compensation, just something better than an idiot light.  On my cars I keep a cheap "cigarette-lighter" combo USB-charger / voltmeter  plugged in.  I occasionally glance at it before I crank over the starter, and perhaps turn on the headlights to check the non-charging battery voltage.  Then I start the car and watch to see if the voltmeter shows the battery voltage jump up appropriately.  This isn't a precision test of battery condition or voltage, but it warns me of impending problems.  More than good enough for my needs.

You're right, this doesn’t require a precision instrument at all. Even an old one-dollar DT830 DMM would be sufficient for the task. But the key point here is that if you're aiming to assess the actual condition of the battery, not a reading affected by voltage drops or wiring issues, then you need to measure directly at the battery terminals. This does not require a super-precision measuring device, you just need to understand what you want to measure and how best to do it.

If you're using the cigarette lighter socket, it’s important to understand that the voltage you see does not reflect the true battery voltage.

A voltmeter in the cigarette lighter isn’t significantly more reliable than a simple indicator LED. I could demonstrate a scenario where such a voltmeter would display, say, 6 V instead of the actual 12.7 V at the battery terminals, and I’m sure you can imagine how that could be done.

In practice, I’ve encountered multiple cases where such kind of gadgets provide inaccurate or misleading readings, so I don’t even bother looking at them to not spend the time. Even though my car reports battery voltage over the CAN bus, which I can view both on my smartphone via Bluetooth and on the remote control from the security system, I don't rely on those when I need to check the actual battery state. I always go straight for the DMM and measure directly at the terminals under the hood.

That said, for convenience, it's sometimes useful to glance at the remote display without leaving the house. But I always keep in mind that these values can differ from the real voltage, and based on experience, I have a general idea of how much deviation to expect.

[radiolistener]

Maximum of about 1% voltage drop between battery and lighter socket.
It’s as simple as that.
 

Are you aware that this 1% can easily turn into 10%, 50%, or even 100% error just depending on the condition of your vehicle’s wiring? What may look negligible under ideal circumstances can become highly misleading as connectors age, corrosion builds up, or contact resistance increases. So relying on such measurements without considering these variables can lead to false confidence.

That’s why, to avoid chasing false leads and wasting time on unnecessary checks, the best approach is to connect a voltmeter directly to the battery terminals. This gives you a reliable, unambiguous reading - one that’s unaffected by wiring issues or contact resistance, so you can be 100% certain of the actual voltage at the battery itself.

Since even a 0.1 V drop can be critical for any reasonably accurate assessment of battery condition (especially for load-test), I don’t even consider solutions that involve measuring voltage somewhere along the vehicle’s wiring. Only direct measurement at the battery terminals can provide the level of accuracy needed for meaningful diagnostics. There is no other reliable solution.

[Analog Kid]

I think you're pretty much in a minority of one here.
Still waiting to hear back from the OP.

[floobydust]

BillyO, the Tacoma, seeing below 12.0V terminal voltage on the battery, I think is a problem- that's saying the battery's State of Charge is low roughly 25-40%. I would say the battery needs a decent manual charge and equalization.
I find it's a common problem with modern vehicles - the parasite drains can leave the battery partially charged. Firmware updates to Body Control Module etc. better that, but sometimes a module can't go to sleep because something is hanging it up. Subaru's are notorious for killing batteries 3yr life due to the high parasite drain 75-100mA.

Or a decent (long) drive. but it won’t change the amount of parasitic drain.

I found a long drive does not help. You have to overcharge a bit, have higher voltage for the flooded types like that in the Tacoma. I find 14.4V is a reasonable top-up, and I never see that in a car (summer time) for more than a minute or two after starting. Some manufacturers advise going to 16.2V for equalize until charging current drops or after an hour max.
One car had one cell constantly needed more water than the others, took me years to realize that cell was just way off from the others. The car's charging system never fixed it.
I manually charged it and years later it's using the same amount of water as the other cells, doing fine.

Most people I know are having to manually top-up charge their car battery. The problem is auto makers don't realize an old or cold battery needs more charging - charge efficiency drops yet the ECU (battery model) does not provide more, so you are constantly losing, it's starved.

I've seen Volkswagen ECU's require you program the battery type (AGM, Wet etc.) and Ah capacity. So it's getting complicated nowadays.

[radiolistener]

Subaru's are notorious for killing batteries 3yr life due to the high parasite drain 75-100mA.

That's quite a lot - my car draws about 30 mA and I think most of it is due to the security system. It periodically transmits a beacon to maintain communication with the remote control unit and I estimate it accounts for around 20 mA.

But a 3-year battery life is fairly typical, mostly due to insufficient maintenance. It needs to be fully charged periodically, but most people simply don’t do that.

[Sorama]

When the alternator starts running, it’s often charging the battery at 14,4V.

When we sell or replace a car battery, we always charge it well before handing over.
We used to fill batteries also, but that we don’t do anymore.

[BillyO]

BillyO, the Tacoma, seeing below 12.0V terminal voltage on the battery, I think is a problem- that's saying the battery's State of Charge is low roughly 25-40%. I would say the battery needs a decent manual charge and equalization.
I find it's a common problem with modern vehicles - the parasite drains can leave the battery partially charged. Firmware updates to Body Control Module etc. better that, but sometimes a module can't go to sleep because something is hanging it up. Subaru's are notorious for killing batteries 3yr life due to the high parasite drain 75-100mA.

Or a decent (long) drive. but it won’t change the amount of parasitic drain.

No Worries,

That's under a heavy load.  Lights on, accessories on, fan on full blast, cabin lights on, infotainment system on, ignition system on, computers on.  it's a 2023 TRD Sport, so LED lights are few and far between. So probably drawing well over 40A.  That  battery is less than 2 years old and has zero problems starting the engine.

I can put it on a trickle for a few hours though.

Edit:  Just measured it with quiescent load @ 12.32V, so yeah, it could you a few hours on a trickle charger.

BTW, my wife's 2017 Honda Civic is still on it's orignial battery too.

[BillyO]

But a 3-year battery life is fairly typical, mostly due to insufficient maintenance. It needs to be fully charged periodically, but most people simply don’t do that.

My last car was a 2008 Ridgeline bought new in 2007.  I replaced the battery once in 16 years.

[radiolistener]

I think you're pretty much in a minority of one here.

Thanks. I consider it a compliment. Being in the minority means you're thinking independently and in technical matters, the majority is frequently wrong simply because the average expertise of a crowd is, statistically, quite low.

As for measuring battery voltage, in my view, this has nothing to do with popular opinion - just because Ohm’s Law operates independently of what the crowd believes about it. 

[radiolistener]

I replaced the battery once in 16 years.

 
That sounds incredible - what type of battery are you using?

[BillyO]

The first battery is whatever Honda put in it.  Yuasa?  I replaced that with an Interstate.

Like I also said in a recent edit, my wife's Civic is going on 8 years on the original battery.

[Sorama]

Being in a minority generally means being wrong.

Just like the crazy ones think they’re right.
 

[Sorama]

Our tech support at headquarters (Bosch) does admit that the original battery in a car is often/almost always lower quality then after market.

Lots of issues with agm type batteries.

[Analog Kid]

Battery longevity data point:

My current battery is 6-1/2 years old, still going strong, absolutely no problems.
And I live in a place with cold winters.
No special brand, bought it at a discount place.
(car = Nissan Cube)