Car battery voltage is a parameter by which you can learn a lot. For example, how it is charged or discharged. But this is only the simplest, as judged by the voltage at the terminals. Another correctly measured voltage allows you to assess the state of the vehicle's on-board network, identify malfunctions of the generator or relay-regulator, voltage drops and other problems. How it works is described in detail in the article in question.
Why measure battery voltage
To begin with, let's dwell in more detail on the information that can be obtained using battery voltage. Why measure it at all? Are there any alternative ways to get the same data? How can faults in the on-board network be detected by battery voltage? Etc.
The first thing you can find out from the voltage of a car battery is charge level. That is, how many percent it is charged or, conversely, discharged. For example, if your new battery is running low, a control voltage measurement will allow you to detect this in a timely manner. In addition, by the degree of battery charge, you can evaluate the chances of a successful engine start on a winter morning. And a weakly charged battery should be recharged to avoid sulfation.
The second thing that the voltage at the battery terminals can “tell” about is whether the engine start system is working normally. That is, the starter. For example, if the voltage during its operation sags heavily or disappears altogether, then there are problems. Most likely, the starter "takes over". A slight voltage drop, on the contrary, indicates the health of the starting system, as well as the good condition of the battery itself.
The third is whether the battery is charging normally from the generator. In order for the battery to fully recover during the ride, two things are needed - a voltage in the region of 14.4 V, as well as a sufficient amount of time. If the voltage is less than the specified, then we can drive a car for at least a day and a half, but the battery will never fully recover.
Fourth - how the on-board network "holds" the load. Many are familiar with the situation when, when powerful consumers are turned on, the voltage drops terribly. As a result, neither the consumers themselves fully work, nor the battery from the generator is fully charged. For example, if without load the voltage at the battery terminals is 14.4 V, and with the headlights and the stove drops to 13.5 V, there are problems. Drawdowns in the region of 0.1-0.5 volts are considered the norm.
Fifth - whether the battery is charging correctly from the charger. The vast majority of memory devices are not equipped with voltmeters. But the voltage is a very important parameter of the charge. You can read more about this in the article about why the battery boils.
Let's summarize the above. The voltage at the battery terminals can be estimated:
- charge level;
- serviceability of the engine start system;
- operability of the generator and relay-regulator;
- their ability to carry an increased load;
- the correctness of the battery charge from a stationary charger.
This is not all that can be found out by the voltage of the car's battery. For example, checking the battery with a load plug is also not complete without voltage. The reason why a battery boils is directly related to voltage. To fully check the car engine before buying, it is also useful to navigate the battery voltage. Read about all this later in the materials on the links. And we will continue to delve into the current topic.
How to measure battery voltage correctly
First, consider how not to measure voltage, and why. Incorrect methods include measurements using:
- a standard voltmeter on the dashboard, if it has not been previously calibrated;
- display meters plugged into the cigarette lighter socket;
- cheap Chinese voltmeters brought into the cabin;
- voltmeter on the charger if it has not been calibrated;
- cool multimeter with automatic range detection.
Let's briefly go over each point. Regular voltmeters are not on all cars. If they are, then they usually have two drawbacks. Firstly, they show the voltage without taking into account the voltage drop in the wires that are connected to the battery. Secondly, they measure inaccurately. But some regular voltmeters can be calibrated. The essence of calibration is to achieve a voltage display precisely at the battery terminals.
Let's take a look at how it works. Let's imagine that we have some kind of battery and two obviously accurate voltmeters. We connect one of them with the shortest possible wires directly to the battery terminals. We connect the second with wires of such a length that it will allow us to “reach out” from the engine compartment to the passenger compartment.
If initially both voltmeters were calibrated the same way, then with the connection described, they will show different voltages. Namely, the one on longer wires will display fewer volts. This happens due to a voltage drop. The phenomenon associated with the resistance of conductors.
Now imagine that our voltmeters have the ability to calibrate. This is usually a screw or a tiny variable resistor. If there is such an opportunity, then we can adjust the readings of the second voltmeter to the numbers of the first. As a result, we eliminate the error that occurs due to the voltage drop.
From the foregoing, the main rule for measuring battery voltage follows: it must be measured directly at the battery terminals.
As for the display meters that are inserted into the cigarette lighter socket, they cannot be calibrated without barbaric intervention in the design. Accordingly, they show, as a rule, an underestimated voltage. Its fall occurs because there are wires on the way from the battery to the cigarette lighter socket. This drawdown can be compensated only with a calibration screw.
Now about how to measure the battery voltage with a multimeter. Or rather, what device is better to use for this. Do not strive to use expensive Chinese multimeters. They “think” for a long time, that is, they give out indicators with a delay. The same applies to instruments with automatic range selection. Of course, in most situations, such a delay is not critical, if you know about it. But it is better, nevertheless, to use simpler multimeters. They react to voltage changes almost instantly, which allows you to solve certain problems without misleading yourself.
Information for very novice users. To measure battery voltage with a multimeter, you need to do the following:
- Turn on the DC voltage measurement mode.
- Select measurement range up to 20 volts.
- Connect the red test lead to the positive terminal.
- Black to the "negative" terminal.
- Wait for stable indicators and fix them.
Important! The probes must be connected to the terminals securely and securely. Weak or intermittent contact will distort the readings and you will be misled. It is not so important which probe is connected to which terminal. In the case of a reverse connection, simply ignore the minus sign on the instrument display.
The last item is built-in voltmeters in chargers. Firstly, if it is an arrow on your device, then forget about it altogether. The analog voltmeter on the charger is good only to find out if the device is alive. Inaccurate. Exemplary. Also lies often. Secondly, even if the voltmeter is digital, do not rush to blindly believe it.
The first reason is that it can be inaccurate by itself. The second is the lack of calibration. That is, the voltmeter can show the voltage at the output of the charger circuit, and not at the battery terminals, to which there are also wires (not always of sufficient cross section). Finally, for some chargers, the voltmeter shows the voltage not on the battery, but the voltage of the selected charge mode. That is, roughly speaking, what the device will “strive for” during the charging process. This, and other important nuances, is described in the material on how to choose a charger for a car battery.
Car battery modes
Now it’s worthwhile to figure out in what modes the car battery works, and what voltage it can give out. It's the most important. Many miss the moment, misleading themselves and others. To understand what we are talking about, we list the main 7 modes of battery operation:
- Separate from the car.
- On a car with a dead engine.
- With the engine running.
- With increased load.
- Charging from a stationary charger.
- after charging.
- After downtime.
Now separately for each item.
The battery is separate from the car
What is special about this mode? The most important thing is that absolutely no load is placed on the battery. After all, when the battery is connected to the on-board network, the latter consumes a small current necessary for the operation of some devices in standby mode. In addition, there may be a large leakage current in the machine, which is also a load.
How does load affect car battery voltage? She puts him down. Sometimes quite significant. It works according to the following principle. The greater the load current, the more noticeable the drawdown. Also, the worse the general condition of the battery, the more the voltage drops. Let's take an example. Suppose we measured the voltage at the battery terminals without disconnecting it from the on-board network. The multimeter showed 12.4 V. If there is a leakage current in the car or several devices in standby mode (radio, alarm, navigator, registrar), then after disconnecting the terminal, the voltage on the battery will rise by 0.1-0.2 V.
Why is this important to understand? Firstly, knowledge of this simple principle allows one to indirectly determine the presence of a harmful leakage current. Secondly, this is important if you want to determine by the battery voltage how many percent it is charged or to what extent it is discharged. To do this, just measure the voltage, and find it in this table.
|
Quiescent voltage (V) |
Charge (%) |
|
< 11.90 |
0 |
|
11.95 |
10 |
|
12.00 |
20 |
|
12.05 |
30 |
|
12.15 |
40 |
|
12.20 |
50 |
|
12.30 |
60 |
|
12.40 |
70 |
|
12.50 |
80 |
|
12.60 |
90 |
|
12.70 |
100 |
|
>12.71 |
Incorrect measurement |
Now it should be clear that the difference of 0.2 volts mentioned above quite noticeably distorts the indicators of the state of charge of the battery. That is, at 12.4 V it will be approximately 70%, and after removing the terminal and a voltage of 12.6 V, it will already be 90%. If you removed the terminals, and the voltage almost did not increase, then everything is fine with the leakage current on your car, and no consumers “eat” the battery in standby mode.
But don't be in a hurry! These are not all the factors that affect the correctness of measuring the voltage of a car battery. Move on.
Battery in a car with the engine off
Partially it should already be clear how this mode affects the voltage of the car's battery. Therefore, if your goal is to assess the state of charge of the battery connected to the on-board network, make allowance for the leakage current and devices operating in standby mode. To do this, you must first measure this correction by comparing the voltage before and after removing the terminal.
Another important nuance of any car battery is its inertia. Or rather, in the inertia of the voltage. The fact is that immediately after disconnecting the battery from the on-board network, the voltage should not be measured. We have to wait a bit. As a rule, 5-10 minutes are enough. After their expiration, the voltage will increase and stabilize. This happens every time the battery is unloaded.
In addition to assessing the level of charge in this mode, you can monitor the operation of the starter. And just in this case, you need the fastest possible multimeter. And also an assistant. The essence of the test is to determine the voltage drop at the battery terminals during engine start. If from 12.5 V it sags below 9-10 V, then there is definitely a problem. Many of them are described in the material - why the starter turns badly.
With the engine running, with or without load
Measuring the battery voltage with the engine running allows you to determine several points at once. Firstly, if without additional load in this mode the voltmeter shows less than 14.4 V, there are problems with the generator or relay-regulator. Secondly, if the voltage sags by more than half a volt when the load is on (headlights, stove, music, air conditioning), then there are also problems.
Again, too high voltage while the engine is running is also harmful. If the battery is with liquid electrolyte, then the latter will quickly boil. It's sad to end this operation. No less problems await those who have a GEL or AGM type battery. They do not contain liquid electrolyte and will not boil. But on the other hand, the contact of the electrolyte with the lead plates will be broken. The gel, as a rule, swells and lags behind the electrodes. As a result, the capacity of the battery is irretrievably lost. AGM batteries tolerate increased voltage a little better. However, they are not immortal in this respect.
Charging the battery from a stationary charger
In the process of charging the battery, the voltage at its terminals is a key parameter. Along with current. In order for the battery to be charged normally, the voltage must be brought to 14.4 V, and in this mode the charge continues until the charge current (amps) drops to 0.1-0.2 A.
If the voltage is below the specified 14.4 V, the battery will never charge to 100%. If this mark is exceeded, the battery will soon boil, which is not good. Especially sensitive to this are GEL batteries. Read more about them in the article - how to charge a gel battery. A lot of confusion and myths have accumulated around the charge voltage of the so-called Ca/Ca batteries. This topic is fully disclosed in the material - how to charge a calcium battery.
Battery after charging
Immediately, and within a few hours after charging, it is generally pointless to measure the voltage of a car battery. It will simply be overestimated, and no useful information can be obtained from it. Just because of a misunderstanding of this simple truth, there are often statements on the Internet that the table presented above is not for all types of batteries, and others. And all because the voltage in such cases is measured immediately after the engine is stopped or the charger is turned off.
Doing so makes no sense!
The reason for this is inertia. For the first time after charging from a charger or a generator, the battery always produces an overvoltage. Nothing meaningful can be determined from it. Moreover, it is impossible to estimate from it how many percent the battery was charged. Immediately after charging, the voltage in the region of 13.0-13.5 V will be the norm.
Battery after idle
But this is a completely different matter. After the battery has been idle, the so-called quiescent voltage can already be measured at the terminals. For the battery to reach this state, it takes different times, but not less than 8-12 hours. Without charging. Without load. Only in this case it is possible to measure the voltage in order to find out how many percent it (the battery) is charged or discharged there.
Accordingly, if you have just turned off the engine, let the battery sit at least overnight. Then measure and evaluate. The same if you recharged the battery from a stationary charger. How well you charged it, evaluate the voltage the next day. It's pointless to measure before.
During the operation of a calcium battery, the author experimentally revealed the following. The age of the battery is 2.5 years from the date of purchase. The generator has an outdated relay-regulator that maintains a voltage in the on-board network of 14.0 volts (it says so on it). It is clear that this is not enough to fully restore the battery charge while driving.
Accordingly, periodically you have to put the battery on charge from a stationary device. So. What was noted during such operation. When the battery was charged directly on board, without disconnecting the terminals, the rest voltage was reached after 7-8 hours. That is, it did not decrease any more. When charging from the charger was carried out with the terminals disconnected, and the battery was left to stand outside the on-board network for the same 7-8 hours, the voltage showed around 13.2 V.
From this we can draw the following conclusions. When the battery settles without a machine (leakage current and load from waiting appliances), it takes a little longer than 7-8 hours to reach the resting voltage. At the same time, the leakage current on the author's machine is normal. Something around 0.05 A. Perhaps these observations will be interesting or useful to someone.
Video: car battery test with a multimeter
Brief summary
Correct measurement of the car battery voltage - allows you to diagnose it. By voltage, you can determine the battery charge level, evaluate the operation of the starter, generator and relay-regulator. The main thing is to measure the voltage correctly, taking into account the specifics of the batteries themselves, their inertia, as well as different operating modes.
