Voltmeter for vehicle battery to monitor inside of cab

[NE666]

But even a voltmeter will not tell you if there is enough charge in the battery.

I'm going to pull this out from page one, as it seemingly got lost in the noise.

Battery voltage *alone* is not a reliable indicator of condition, nor state of charge. Lead acid chemistry is 'complex', especially as batteries age and/or when they are damaged. I've two 45Ah lead acid AGM batteries in my garage, less than a year old. They have never been 'abused' but they have been frequently cycled. Both show ~13v off load (measured at the terminals), and ~12.7v when initially connected to a 4A DC electronic load. When measured using said load, one returns a capacity of ~4Ah, the other 10Ah i.e. they are 'done'. Both were 'fully' charged prior to testing.

Equally, about 13.5 to 14v or so (some regulators have temperature compensation) when your engine is running tells you only that the alternator and voltage regulator are doing their job. It says nothing as to whether the battery is actually accepting charge, and even less about how much it can retain.

If you're really concerned about charge and discharge rates, I think you'd be better off looking at something like this: https://www.victronenergy.com/battery-monitors/smart-battery-shunt

[SteveThackery]

Yes, I do consider the act of voting - particularly without a deeper understanding of what is being legitimized - to be a sign of very stupid people.

Everyone knows that democracy is the worst system of government, except for all the others, which are even worster. <sic>

So, @radiolistener, if you think people are stupid to partake in democracy, what form of government do you advocate?

[electroniclearner820327]

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's post below echoes my thoughts on the matter very well. If it is correct that there can be such variance as stated below then that can mean the difference between a battery that can start and one where you would be left stranded thus causing a false sense of confidence.

You mention better than "go/no go" well that doesn't seem much better if at all. The whole point is to not get a false positive/negative as with the battery light and with such variance it seems it would be a very similar situation.

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.

[SteveThackery]

Battery voltage *alone* is not a reliable indicator of condition, nor state of charge.

<SIGH>

Yes, yes, everybody knows this!  The OP is not trying to ascertain the battery condition. He is trying to see whether it is being charged or not. A simple voltage measurement is entirely adequate for that purpose.

[electroniclearner820327]

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 is thoughtful of you to do that test however you must also accept that is not going to do much to prove any kind of rule. My vehicle is much different from yours and I imagine much less reliable being almost 20 years older and already given the many electrical issues stated! So your case does not automatically transpose to mine.

[Sorama]

Battery voltage *alone* is not a reliable indicator of condition, nor state of charge.

<SIGH>

Yes, yes, everybody knows this!  The OP is not trying to ascertain the battery condition. He is trying to see whether it is being charged or not. A simple voltage measurement is entirely adequate for that purpose.

indeed.

its funny to see how some get lost in a non-discussion.
it how's what their (lack of) practical experience is worth.
And even more funnier, the TS fell into that hole also.
Like he did in his first topic.
People only see what they want to see.

As in politics 

[electroniclearner820327]

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

Nope, it is a minority of 2 if that is the case. As I write above, radiolistener is my spokesperson and saying almost exactly what I am thinking but in a much more informed way. 

[SteveThackery]

As I write above, radiolistener is my spokesperson and saying almost exactly what I am thinking but in a much more informed way. 

What, about the voltmeter, or about democracy?

[MrAl]

I just get a cheap battery powered DMM and the lghier plug. No mod and after a while if there is no problem you don't need it. With a 10 Megahohms input impdedance whatever resistance in the wiring harness is insignificant.

Hi,

I use the lighter plug too but it's better to wire direct to the battery terminals with an included inline fuse.  There is no better way to monitor the battery voltage itself if you want the utmost accuracy.

The drop in voltage however is not related to the voltmeter current draw, it's due to the current draw from other items in the car such as the headlights, interior lights, etc.  When the headlights turn on they could cause a drop in the harness voltage which would then mean the voltmeter could see less voltage also.

I've been using the lighter plug for some time now so I know how to interpret the readings.  I make sure nothing else is turned on when I take the readings, and be sure to always do it that way.  The readings I get are an indicator of how the electrical system is functioning because I know the history of all the readings I have taken over the years.  If a problem comes up, it will still show up in the readings even though they may not be 100 percent accurate.

So the bottom line we can say is if you use the cigar lighter jack then build up a history, and it might be a good idea for the first readings to compare with readings taken directly at the battery terminals, at the very same time as the readings from the jack are taken.  Once a couple weeks go by you should have a good idea what the readings SHOULD look like with nothing else turned on.

I also do not recommend keeping the meter connected to the battery.  Use a switch if needed.  That's because some panel meters do draw some significant current.  Even a couple milliamps is not a good idea.

[MrAl]

But even a voltmeter will not tell you if there is enough charge in the battery.

I'm going to pull this out from page one, as it seemingly got lost in the noise.

Battery voltage *alone* is not a reliable indicator of condition, nor state of charge. Lead acid chemistry is 'complex', especially as batteries age and/or when they are damaged. I've two 45Ah lead acid AGM batteries in my garage, less than a year old. They have never been 'abused' but they have been frequently cycled. Both show ~13v off load (measured at the terminals), and ~12.7v when initially connected to a 4A DC electronic load. When measured using said load, one returns a capacity of ~4Ah, the other 10Ah i.e. they are 'done'. Both were 'fully' charged prior to testing.

Equally, about 13.5 to 14v or so (some regulators have temperature compensation) when your engine is running tells you only that the alternator and voltage regulator are doing their job. It says nothing as to whether the battery is actually accepting charge, and even less about how much it can retain.

If you're really concerned about charge and discharge rates, I think you'd be better off looking at something like this: https://www.victronenergy.com/battery-monitors/smart-battery-shunt

The measurement of battery voltage is a fairly complicated thing, but there are some ways to simplify this.

For one, a single measurement at any time is probably not a good indicator of anything.  However, a history of readings, especially under known conditions, is very good.
For a car battery, a good indicator is to measure the voltage while charging, but also to measure after the car engine is turned off, then also after the car has been sitting for a day or longer.  If the voltage when the car is turned off is say 12.9v and the voltage after a day is 12.4v, it's probably doing ok, as long as that repeats to some degree day after day for a few weeks.  If the car voltage is 12.9v and then after a day of sitting is 12.0v, that may mean the battery is either getting old or needs a good charge with a good charger.  This is especially true if it goes below 12.0v after a day or maybe a few days.

If you let it sit for one day and see it go to a certain voltage, then watch for that behavior again and again.  The point is to look for a change in behavior while measuring in the same way.

As above, there are two ways the battery voltage can get lower than usual.  Either the battery is getting old, or the battery has been drained too low by sitting for too long.  If it is old then there's no good way to help it except to charge it with a good charger and then keep checking it.  If it is just drained too low, then charge it with a good charger and that could get it back to normal.

If the alternator is a good one, then just driving the car for a longer time could help charge the battery back to normal.  That would be one that can put out maybe 14.8v to start.  If it is an alternator that can only put out 14.0 volts, then a good charger will be needed to help get the battery back to normal.

These notes come after years of testing and monitoring the battery right in the car, and testing and monitoring using (for a long time) a solar charger system.

[electroniclearner820327]

My primary reason for adding this meter is precisely to determine how much the voltage is depleted when the vehicle is parked. We travel a lot, and is not uncommon to be away from home 10 to 15 days.

Similar but opposite case for me. I live in my van full-time but travel as little as I can get away with! Which means about once per week.

I only got a new battery last year and the one I had since I bought the van second hand a couple of years ago would be very reluctant to start after one week stationary. It totally died a few days after getting the dc-dc charger and then would not hold a charge thereafter even on mains which precipitated the replacement.

Just looking at my notes I have saved from previous checks it seems it discharges about 40-50% not running for the week and having checked again before setting off.

This is why I think it would be useful to have those jumper cables to give it a blast with the leisure battery if I ever happened to leave it drop to an unstartable state but from what I have read it can take 6 weeks for that with a relatively new battery? In those cases though people I think are talking about when their vehicle has been parked in storage without them using it whereas there will be more drain in my case since I am constantly getting in and out of the van I guess due to the auxiliary crap. Better to prevent it happening but those jumpers would be a nice safety net.

It hasn't happened yet but if it did get to that, to prevent in future I would think about getting an easy battery terminal remover or battery isolator switch for the starter to stop the parasitic drain. I would not mind opening with the keys rather than central locking. It is a horrid waste of energy anyway to be draining the battery like that constantly for no good reason so maybe something to think about either way. Just unplug when I have done my weekly drive and connect again when wanting to go.

[NE666]

Battery voltage *alone* is not a reliable indicator of condition, nor state of charge.

<SIGH>

Yes, yes, everybody knows this!  The OP is not trying to ascertain the battery condition. He is trying to see whether it is being charged or not. A simple voltage measurement is entirely adequate for that purpose.

indeed.

its funny to see how some get lost in a non-discussion.

You mean, like the dozen or so pissing contest / ego clash posts that preceded it?

Heaven forbid someone add something actually germane to a discussion in the beginners section.

[electroniclearner820327]

In fact, by attempting to prove that the minority is always wrong, you fall into a trap yourself - appealing to the majority opinion as an authority. Logic is not determined by the number of opinions supporting it but is founded on sound reasoning. The quantity of people endorsing a particular idea is irrelevant, what truly matters is the validity of the logic, which remains constant regardless of how many individuals support or reject it. It seems obvious to me, doesn't it?

While I retract my statement that you are my spokesperson as it has veered into this political discussion, I am not against the subject matter as I happen to have become very interested in political philosophy lately and having very in depth discussions on other more relevant forums.

I would not assent to much of what you are saying or at least it is not being put in a very good way but the general idea the majority doesn't = right then sure. An easy example of this would be how the german people were swept up with Nazism. The party were the majority then, did that make everything they said and did right? Countless other examples of tyrannical governments.

[Simon]

lets keep off the politics please.

[electroniclearner820327]

The OP can see for himself what the typical readings are between charging and not charging. Switch on the ignition and read the meter. Then start the engine and read the meter. The fault is intermittent, so the OP can see for himself what the typical "charging" voltage is.

I have done this several times already and also as I have noted in my various posts about it the battery light being on seems to have no bearing on the state of charge as I have been able to check at the battery terminals directly when it is on and the engine is running and saw 13/4+ volts and the alternator running happily.

This is why I suspect it is a false positive hence why I would like a standalone voltmeter. Even if it is a false positive there is nothing to say it would not genuinely be lighting up at some point in the future and if I got used to ignoring it then it would be a case of the boy who cried wolf.

Now I don't really have a horse in this race about cigarette lighter or not but I do think I would prefer to rule out possible discrepancies compared to a direct connection. Just 'belt and braces' and I am skeptical there being an inconsequential difference and I don't care to do the testing to find out if it is the case or not and since it isn't that hard to setup directly and not that much cost then why not. Also a dedicated circuit leaves the lighter socket free and want this reading to be a constant fixture for the foreseeable.

I don't want it to be 'try for a few times then put it away' as someone else says. It has to stay there for the duration of the weird electrical issues which don't seem to be going anywhere any time soon. 

[radiolistener]

The OP just wants some indication of whether the battery is still being charged when the warning light comes on. A voltmeter plugged into the cigarette lighter socket would be fine for that purpose. We are talking about a minimum of 1V difference between charging and not charging, and most likely 1.5V or more.

When I need to verify if the battery is actually charging, I use a current clamp on the wire at the battery terminal and check the current direction and magnitude. I wouldn’t trust any conclusions based on a cigarette lighter voltmeter. 

The OP can see for himself what the typical readings are between charging and not charging. Switch on the ignition and read the meter. Then start the engine and read the meter. The fault is intermittent, so the OP can see for himself what the typical "charging" voltage is.

No. What the OP will see on a voltmeter connected to the cigarette lighter doesn’t confirm whether the battery is charging at all. It only shows the voltage level at the vehicle’s electrical net at some point, which reflects a redistribution of potentials after starting the engine. This voltage increase doesn’t guarantee that current is actually flowing into the battery. Only a current measurement at the battery terminal can reliably confirm actual charging.

The OP is not trying to ascertain the battery condition. He is trying to see whether it is being charged or not. A simple voltage measurement is entirely adequate for that purpose.

Not necessarily. A voltmeter in the cigarette lighter cannot reliably indicate whether the battery is actually being charged. See description above.

Likewise, it does not indicate the actual state of charge of the battery. For example, in the photo above, the voltmeter shows 12.0V, and I seriously doubt this is the actual battery voltage. Such a low voltage would typically occur if the car had been left unused for 2-3 months and the battery had self-discharged. However, according to the author comments, the photo was taken after just a couple of days of downtime.

If the vehicle has only been parked for a week and shows this reading, something is clearly wrong - either with the battery itself or with the cigarette lighter voltmeter. I would suspect the problem is with the cigarete lighter voltmeter.


In fact, verifying that the battery is charging correctly is not as simple as it seems, and it certainly can't be done based on voltage readings taken somewhere in the vehicle's electrical system or even at battery terminals. To confirm proper charging, a current clamp on the battery terminal wire is required.

[Sorama]

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

Nope, it is a minority of 2 if that is the case. As I write above, radiolistener is my spokesperson and saying almost exactly what I am thinking but in a much more informed way. 

It is not that difficult to find 2 people that make up their own truth.

[CatalinaWOW]

One suggestion, to get most benefit from your effort, look into one of the inexpensive USB charging ports that includes a voltage monitor.  Almost every needs/can use another charging port and you end up with a permanently installed voltmeter with adequate resolution.  They are quite inexpensive.

[electroniclearner820327]

Yes, I do consider the act of voting - particularly without a deeper understanding of what is being legitimized - to be a sign of very stupid people.

Everyone knows that democracy is the worst system of government, except for all the others, which are even worster. <sic>

So, @radiolistener, if you think people are stupid to partake in democracy, what form of government do you advocate?

I too am interested in this. They have said what they don't like, I am keen to see them put their money where their mouth is and propose what system they would endorse.

[electroniclearner820327]

As I write above, radiolistener is my spokesperson and saying almost exactly what I am thinking but in a much more informed way. 

What, about the voltmeter, or about democracy?

I hadn't gotten that far in the thread yet unfortunately and have disavowed them on the latter point in a post above.

I have spent all morning reading through this thread and I am not sure if I have gotten anything out of it.  It has been entertaining at least.

[Sorama]

But even a voltmeter will not tell you if there is enough charge in the battery.

I'm going to pull this out from page one, as it seemingly got lost in the noise.

Battery voltage *alone* is not a reliable indicator of condition, nor state of charge. Lead acid chemistry is 'complex', especially as batteries age and/or when they are damaged. I've two 45Ah lead acid AGM batteries in my garage, less than a year old. They have never been 'abused' but they have been frequently cycled. Both show ~13v off load (measured at the terminals), and ~12.7v when initially connected to a 4A DC electronic load. When measured using said load, one returns a capacity of ~4Ah, the other 10Ah i.e. they are 'done'. Both were 'fully' charged prior to testing.

Equally, about 13.5 to 14v or so (some regulators have temperature compensation) when your engine is running tells you only that the alternator and voltage regulator are doing their job. It says nothing as to whether the battery is actually accepting charge, and even less about how much it can retain.

If you're really concerned about charge and discharge rates, I think you'd be better off looking at something like this: https://www.victronenergy.com/battery-monitors/smart-battery-shunt

The measurement of battery voltage is a fairly complicated thing, but there are some ways to simplify this.

For one, a single measurement at any time is probably not a good indicator of anything.  However, a history of readings, especially under known conditions, is very good.
For a car battery, a good indicator is to measure the voltage while charging, but also to measure after the car engine is turned off, then also after the car has been sitting for a day or longer.  If the voltage when the car is turned off is say 12.9v and the voltage after a day is 12.4v, it's probably doing ok, as long as that repeats to some degree day after day for a few weeks.  If the car voltage is 12.9v and then after a day of sitting is 12.0v, that may mean the battery is either getting old or needs a good charge with a good charger.  This is especially true if it goes below 12.0v after a day or maybe a few days.

If you let it sit for one day and see it go to a certain voltage, then watch for that behavior again and again.  The point is to look for a change in behavior while measuring in the same way.

As above, there are two ways the battery voltage can get lower than usual.  Either the battery is getting old, or the battery has been drained too low by sitting for too long.  If it is old then there's no good way to help it except to charge it with a good charger and then keep checking it.  If it is just drained too low, then charge it with a good charger and that could get it back to normal.

If the alternator is a good one, then just driving the car for a longer time could help charge the battery back to normal.  That would be one that can put out maybe 14.8v to start.  If it is an alternator that can only put out 14.0 volts, then a good charger will be needed to help get the battery back to normal.

These notes come after years of testing and monitoring the battery right in the car, and testing and monitoring using (for a long time) a solar charger system.

You clearly don’t know how car electronics work and their impact on a battery.
As if a battery with low voltage is by default defective.

[electroniclearner820327]

I also do not recommend keeping the meter connected to the battery.  Use a switch if needed.  That's because some panel meters do draw some significant current.  Even a couple milliamps is not a good idea.

Can you explain what the problem is here?

What would happen if I connect it on the connection between starter battery and leisure batter, with dc-dc charger in between of course? Will the high amps of 40A+ blow the voltmeter? If so then why is it you can connect things direct to a battery and it doesn't blow in the same way?

Also if it is the case can I not regulate the offshoot circuit so that the voltmeter would no receive too much power? I am focusing on this just because, as mentioned, there are existing battery cable runs from the starter battery engine bay into the van habitation space, so it would be much simpler to tap into this somehow, if it can be done practically.

[Sorama]

Battery voltage *alone* is not a reliable indicator of condition, nor state of charge.

<SIGH>

Yes, yes, everybody knows this!  The OP is not trying to ascertain the battery condition. He is trying to see whether it is being charged or not. A simple voltage measurement is entirely adequate for that purpose.

indeed.

its funny to see how some get lost in a non-discussion.

You mean, like the dozen or so pissing contest / ego clash posts that preceded it?

Heaven forbid someone add something actually germane to a discussion in the beginners section.

My remark wasn’t addressed to you.
Is yours to me?

[Sorama]

Can you explain what the problem is here?

What would happen if I connect it on the connection between starter battery and leisure batter, with dc-dc charger in between of course? Will the high amps of 40A+ blow the voltmeter? If so then why is it you can connect things direct to a battery and it doesn't blow in the same way?

You must be joking.,.
You really should start thinking less and doing more what some of us here are telling you.
Once again it seems you are completely off track/reality.

[radiolistener]

I’ve remember several automotive electricians who relied on a voltmeter to check if the battery was charging properly and, as a result, drew incorrect conclusions and missed actual faults. If I recall correctly, there were even heated discussions on one automotive forum about why it happens. Measuring voltage at the cigarette lighter socket is even less reliable for this purpose.

For example, you might see the voltage at the cigarette lighter increase with engine on, yet both the alternator and the battery could be faulty - with the battery degraded as a result of the alternator failure. This is a real issue that occurs quite often in practice.

Some vehicle owners replace their batteries frequently (once a year), unaware that the root cause of early battery failure is often a faulty alternator.