I am in the UK and have a 1.3 litre Mazda which is over 10 years old.
I have been reading a few web pages which made reference to the reader's car which "may have a dashmoard voltmeter".
Is a voltmeter actually all that useful to see tjings like what the condition the battery is in and how the battery copes and how it is charging?
Can I permanently connect my cheap handheld digital voltmeter to the car battery. I would then mount the meter permanently on the dashboard.
Is there a real danger that the meter will get damaged by various current or voltage surges? Is it good enough protection to have a quick blow fuse in line with the test lead going to the positive terminal?
Auto voltmeter - also known as the "somebody stole your battery" gauge. Because other than a missing battery, it basically tells you, too late, what you already know - weak lights, poor start, bad battery, etc.
An in-dash ammeter, on the other hand, tells you before you are stranded if you are charging or discharging the battery and how much is going in or out, and the amps are if steady or fluctuating , all of which can be used as an indicator for the condition of the alternator and battery, the connections, and switch-off draining.
If I had only one instrument to connect to the car to watch the battery it would be a voltmeter. A voltmeter will tell you much more about the battery and the charging system than an ammeter.
A properly running alternator should raise the battery voltage to around 14 to 14.2 volts. Some cars may run as high as 14.4 volts. Any higher and you can suspect a problem with the regulator. If the voltage fails to come up this high you can suspect a problem in the alternator, shorted diode etc.
A voltmeter can also tell you if the alternator is capable of putting out enough current. As you turn more things on, lights, air-conditioning etc. the alternator should be capable of maintaining the voltage at the 14 volt range. If the voltage drops well below that range you can look for problems.
After the car has been shut down for a period of time you can read the battery voltage to tell you the approximate charge on the battery by the resting voltage.
You can also watch how far the voltage drops when turning on headlights etc. with the engine not running. If the voltage drops way off you may have a battery that is developing a high resistance, battery is dying.
You can also watch how far the voltage drops when starting the car. This is another indication of battery condition.
None of these things can be done with an ammeter. An ammeter will only tell you if the battery is being charged or discharged at a particular time. If the battery has a high resistance or is fully charged you can not tell the difference with an ammeter.
About the only thing an ammeter is good for is to let you know that the battery is not being discharged while the car is running.
The voltmeter should be connected directly to the battery terminals with its own leads for the above indications to be meaningful. Put a small fuse in the hot lead right at the battery to protect the wires.
or more usually, the connections on the dash. (like my three cars).
Just kind turning on the fan and watching the lights dim, but the voltmeter is not nearly as accurate.
As you turn more things on, lights,
granted, the voltmeter tells you that the computer says to raise the field - but it does not say if anything is being generated and/or sent to the battery. - kind of like my jeep did two weeks back, where the voltmeter said all was ok - and the battery had just enough to turn it over twice. And next time it didn't start with 12 volts shown, so after it was running after a jump (voltmeter had said zero) - voltmeter went to 14.4 and then went down to 13.5 after driving. But the ammeter said it was sending 10 amps into a battery. At the shop, the disconnected battery had all of 11 volts and wouldn't take a charge. Voltmeter was reading alternator output voltage, not energy going into the battery. The battery had the capacity of a AAA, and tyhe voltmeter said 12 volts.
If the voltage drops well below that
Just like a low battery does - turn the key and get a click, but the voltmeter is not as accurate
Of course, turning the key after that test also tells you you need a tow and a battery. An indication which helps a lot when you are driving - that is also indicated by the car not starting.
And nor does one want to - and unlike voltmeters that tell you voltage level rather than energy transfer, ammeters only check the battery input and output running and motor off, and the charging circuit, the starting circuit, alternator output, and cable/connection/belt condition running. You know, the energy the vehicle actually uses.
Sorry, yes you can. Same symptoms as with the voltmeter.
You know, there's a reason so many expereinced people in alt.engineering.elect jumped on the thread to roundly condemn voltmeters as next to worthless-
(BTW, the reason ammeters were removed was to save weight to get fleet mileage up - an ammeter takes a ballast resistor and heavy cables and connectors. A voltmeter takes an inch of circuitboard foil. And, anything a voltmeter does is also sensed by the diagnostics and flagged, on my vehicles)
I reckon both have uses and the interpretation is only a problem to the inexperienced. Each to their own IMHO :-) I like both but if had to choose would go the voltmeter.
If you were to log voltage over time it would tell you a whole lot more than current.
Computer? ignition simply being ON energises the armature field. I've not seen any computer input needed here?
Wrong! It shows exactly this. A car battery will not rest at 14v. therefore if your measuring 14v @ battery you know without any question what so ever that the battery is being charged.
High internal resistance, you'll again see this by cranking the engine and the battery voltage will fall extremely low, again an ammeter will not be able to show this.
Again what you have said shows a tired and worn battery (cracked/sulphated cells) and can be easily demonstrated with a load (cranking) and a voltmeter.
Your completely reading this wrong. The fact that the voltmeter was working and correctly showed your system voltage showed you immediately where the problem was (battery) Process of elimination. You see the symptoms - battery voltage is ok, charge voltage is ok, yet not enough current to crank = dead battery!
Yes it is!
If you do this test and then you are unable to start, like Gary said the battery is dying. Clearly if you felt this was such a risk you would not make this test unless you had another way to start the car. There are thousands of cars on the road with batteries that have nearly no reserve capacity, you need surprisingly little to be able to start a car, but you only really notice after you have left your lights on for a few mins only to discover you cannot start again!
No offence I think your blinded by your own ignorance. Everything you say there can be shown better with a volt meter. You say that an ammeter shows the "cable/connection/belt condition running" ok so what exactly should the current be when its all correct? I can tell you a voltage that I know would be right but since once the car has started and the battery has regained its lost charge (not long under usual conditions) how an ammeter between the battery and anything else would show nothing useful at all, apart from a varying and often small charge. (which again can be shown much better with a voltmeter showing around 14v)
Lets assume you connect it in series with the alternator. An instant disadvantage is the fact that you now have introduced more resistance to the circuit, so you slightly loose efficiency (fact of physics and is how all ammeter works) You could be running in daylight conditions lights off and little load, you could be running at night in the rain lights on heaters on fans on etc etc and again you see load. Great. So are you actually going to be able to see what current is right or wrong as fast as I can read a voltmeter and go PROBLEM or not?!
Nope not so. A battery with high internal resistance with an ammeter all you would see is a charged battery, that assumption made by the fact that there will be little charge current.
Using other peoples posts to back up your own is pretty pointless.
The main reason they were removed as they are not needed as people wouldn't know what's right or wrong, and alternators have simply removed the need. Where as voltage meters are much more common, because that actually shows you something useful.
Further if current meters are such a good way of knowing the status of charge of a battery or condition of a system then intelligent battery chargers would measure and monitor battery charge by current - which they don't. Virtually every type of charger stops when a certain voltage is met, and has no concern with current other than the fact to limit it to prevent damage to cells.
Yes, but there is a huge difference between testing for a fault in a workshop, and providing something meaningful for the driver to use to monitor the health of his vehicle.
IME, even a red "No charge" light is too much for some drivers to comprehend...:-)
The computer controls the field of the alternaotr - think "voltage regulator"
Sorry -WADR - wrong -- since a voltmeter only directly measures potential, it CANNOT directly measure flow. And energy flow is what fills the battery, not electrical potential. 14 volts on the line means 14 volts at the alternator, not amps flowing into the battery or flowing out of the alternator.
Hmm.. think of it this way.. crank the engine and watch the battery voltage fall extremely low and then I can tell that I have a battery problem - a reason to have a voltmeter?
Hint - _I_ do not need a voltmeter to tell me my battery is bad when I can also hear my starter drop off after a second or two of cranking. "rrr..rr..rrr....." "look, my voltmeter is dropping fast" "rr........rr..............rr......... ......r.....................................". (It's not the voltmeter that says my battery is dead.)
That is why the voltmeter is refered to as a "battery is missing" gauge. It tells you there is a problem nearly always AFTER it is fairly obvious there is a problem.
An ammeter,on the other hand, spikes and drops back after start, telling you the alternator is outputting, the regulator is responding, and the battery is accepting. The level of flow tells you the cable condition. The best part is that unlike a voltmeter, it tells you before the battery is missing.
Or rather than use a voltmeter and runa test, perhaps just use an ammeter while driving?
(BTW, the battery in my example was less than 18 months old, and bad.)
Or bad cable, security system lockout, or faulty starter relay, all of which give the same symptoms -- and all only after I am dead in the water..
I put my DC power-ammeter on the battery cable of the running vehicle and saw under 10 amps feeding the just-jumped 18 month old dead battery - THAT was my definitive on the problem. The ammeter on a running vehicle said bad, the voltmeter on the running vehicle said good. If I had had an ammeter on the dash as we used to have or as I have in my RV instead of a "battery missing" gauge I would have been in for a charging system check well before I needed a jump.
both tell you it is dead - how can one be more accurate? - it is dead, not slightly dead.
No offense taken, but you should know that besides having hobbied for 40 years in racing and auto restoration, and having worked on everything from V-12 Ferarris to flathead Fords, Austins to 300Zs; having worked hands on from 120,000 volts at 60 amps to 1 volt analogs; repaired and certified test equipment from scopes to rf meters; as one who consulted on those early/mid-eighties-vehicle analog computers post 80s; and as a PE, degreed in electrical and mechanical, I do have some hands-on as well as theoretical knowledge of measuring and automotive circuits.
And I should note that we need to keep in mind that an ammeter is just a voltmeter measuring load across a fixed known resistance.
An in-dash voltmeter merely uses unknown loads to get qualitative measurements of energy capacity and implied flow, while an ammeter gets quantitative realtive measurents of amount and direction of energy flow at a known common point, using a fixed resistance.
Everything you
a .01 ohm shunt at 12 volts is far less a loss than the corrosion in the battery cable terminals or lamp sockets. 1/8 watt loss is a red herring.
You could be running in daylight conditions
First - While voltmeter readers need to have the actual value of the measurement to make a decision - ammeter readers only need direction and any quantity to make a decision.
In other words, you are mistaken to believe I need a value to determine the existance of a problem using an auto ammeter- the determinates for the auto ammeter are "in or out", "quantity or no quantity", "large amount or not". The specific value of flow is actually immaterial. (and since the definitive amount of flow varies with environment and accessories anyway, an exact amount is meaningless except to the few more knowledgeable, and then in context.)
second - in your example, difference is -
1) I see no spike several seconds after start on an ammeter, that time just after starting being the time when I and most people check all the gauges - and I check the problem out before I am stuck somewhere - - but with a voltmeter, with a good battery but alternator putting out 14 v and way-low current, it says 14 volts and my once good battery soon becomes a 20 amp-hour-capacity-left no-reserve battery, and I am dead and towed.
2) My ammeter says outflow five seconds after I start the car and it stays there - time to get to garage - same condition - my voltmeter says "OK" and I drain the battery as I drive away on my sunny day, and after twenty miles heading to the woods the needle is down enough to notice I have low voltage- and out twenty miles, I will shut down on fault before I can get back to a garage.
A battery with a high resistance will give you a running-vehicle voltmeter reading of 14 volts (had it happen several times), while an ammeter will give you no flow into the battery - (there is always flow into the battery for 30 seconds after start)
Totally different condition - the parameter in a vehicle is condition of the charging and battery system and its controls in a two direction system- the parameter in a battery charger is to control rate of charge and prevent overcharge in a one direction system.
It seems to me that I can just connect a voltmeter across the battery terminals and (as someone in this thread mention) include an fuse inline with the hot wire near the battery as a safety precaution.
I'm not familiar with current shunts (which you mention elsewhere).
Where can I get a suitable current shunt from for my car and very roughly what do they cost?
Do I simply install the shunt across the battery terminals and then place a voltmeter (sic) across the two terminals of the shunt?
Would I need to calibrate my multimeter to the shunt in order to measure current values correctly?
Most external ones that I have seen are mounted on the firewall. The cases were about two-three inches long. They are more used on vehicles with big alternators. Connection: Remove cable between alternator post and battery and run to post on the shunt case which is bolted on the firewall; Alternator post cabled to the other post on shunt case. Small leads off the two case posts go to dash meter movement.
Dash mounted ammeters (one in my hand) have internal shunts inside the case that fits in the dash, and there are posts on the back of the case to connect heavy wire eyes. That would be the easy way to go for a smaller vehicle. Connection: Alternator post cable goes to dash gauge post, other dash gauge post goes back to battery post. About number six wire, I think.
(Note that some European vehicles used to run the starter lead to the alternator post and then on to the battery, unlike American cars that ran separate leads from the battery to starter and alternator - the former had a lot of problems in the US in cold climates/corrosion since some of the hundred plus starting amps naturally went through the alternator rather than all through the starter cables, smoking alternator parts as cars aged. Note that If you put a regular auto ammeter in line with a starter, you may smoke the ammeter)
Again - an ammeter is connected in series with the alternator-battery cable so it reads the smaller charge-discharge amps of the alternator, and it is not connected so it reads the large starting current.
The ammeter gauge I last bought was about $10 US. Truck repair places probably have internals on hand (and probably also external)
For a while, they made dual gauges - top part an ammeter and the bottom part a voltmeter (or vice versa). Haven't seen those in a while, though
background on the shunts in ammeters-
The shunt is in parallel with a meter movement (a voltmeter, bascially) having an amp scale on its face, and the "voltmeter" reads the voltage drop across the shunt. Since the shunt is an accurate resistance, the amps is interpolated internally for the amps of the "ammeter" face (from ohms law).
The particular shunt and particular ammeter movement are coordinated - electrical supply houses sell shunts, and you are looking an ammeter with an external shunt around ,around .001 ohm/ 60-100 amps.
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NOT across the terminals. That would short out the battery.
Basically, it is connected in line with one battery cable, with the meter movement reading ONLY across the shunt. It is the shunt voltage drop that the meter movement is "calibrated to" so as to give accurate amp readings
Yes - but, again - in-dash ammeters are all set up - internally shunted - just connect them in-line.
The way you are explaining to hook up the ammeter will only give indication of charge output from the alternator. It will not show any discharge from load on the battery.
The ammeter needs to be in the alternator lead but between the point where the supply to the car's electrical system is taken from and the battery. In other words, the lead to the cars electrical system will be on the alternator side of the lead that connects to one side of the ammeter. The other side of the ammeter would go to the battery.
Sometimes this is difficult to do as often the car's power is taken from the battery cable where it attaches to the starter. Sometimes it is taken at a splice / junction in the lead between the alternator and the battery, much easier to do here.
I imagine that vehicles may differ, and perhaps I wasn't clear - in mine, the starter gets a heavy cable off the hot post, and the alternator gets a lighter cable off the hot post. The vehicle non-starter power is then taken off that alternator post which feeds the battery. That puts the ammeter between the alternator and power panel lead, and the battery. If there are three feeds off the hot battery post (starter, alternator, and accessory power panels), you may well need to cut that (third) power panel feed cable at the battery post, add a crimped on eye on the end, and move that power panel end over to the alternator post holding the alternator-to-battery cable. In other words, move the panel-feed end from the battery post to the alternator post ( it's the same point electrically)
Yes, agreed
While I have personally not seen that panel-connection coming off the starter, I would think the moving of the panel-connection cable end over to the alternator post would solve the problem.
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