A device for measuring battery capacity. Basic methods. Checking the battery: what battery parameters need to be checked and how to do it? Battery meter
If you don’t know how to test a battery with a multimeter yourself, then we suggest you familiarize yourself with the testing methods.
First, let's look at the control discharge method; it is carried out in several steps:
- First of all, the car’s battery will need to be fully charged and then discharged. During execution, it is important to maintain the current or power at the same level.
- After you can achieve the desired voltage parameter, the discharge procedure will need to be stopped, and the time must also be recorded. Ultimately, the results of the control discharge will need to be compared with the main technical parameters of the battery. If you have also tested the device previously, then the results need to be compared.
- If the battery takes a long time to discharge under load, this will indicate that its capacity is high.
This testing method is not suitable for everyone, since to perform it the car owner must not only approach the task correctly, but also have free time. So for testing you can use a simple tester. With such diagnostics, the load size must be such that it can take 50% of the current that is needed for the normal functioning of the battery. This testing method is relevant when the battery is fully charged (video published by the Avto-Blogger.ru channel).
To discharge the device, you can use a regular incandescent light bulb, to which two wires are connected, and which is subsequently connected to the battery. If during the diagnostics you see that the light produced by the lamp has become too dim, this indicates that the battery is discharged. If the testing ultimately shows that the voltage reading is 12.4 volts, this indicates that your battery has good capacity.
Checking battery voltage?
Since the voltage at the vehicle battery terminals is either 12 or 24 volts, the car owner does not have to comply with all safety measures. To obtain the most accurate test results, you should first measure the AC and DC current. The measurement process itself is performed using the leads that are located on the end side of the device. The tester is connected to the battery terminals, and then the tester should be set to DC measurement mode. 12.4-12.6 volts is the most optimal voltage parameter at the car battery terminals.
In fact, the service life of the device can be increased through proper use. When the engine is turned off, you should not use energy consumers for a long time - optics, audio system, stove, etc. Otherwise, your battery will quickly drain. It is recommended to turn off all energy consumers a few minutes before turning off the power unit, this will allow the charge to be restored a little.
It is no secret that over time, the capacity of rechargeable batteries becomes smaller, and they can no longer supply the device with the amount of current that they could supply before. In this regard, many users often have a question about how to measure the battery capacity, or more precisely, how to find out the indicator of its residual potential, with which you can understand whether the battery will need to be replaced in the near future.
If you start from the concept that the capacity indicator is the amount of energy or current given by the battery over a certain period of time, it simply will not work. If we are talking about how to find out the real capacity of a battery in the form of AA batteries, you will have to first measure the current, and then use some simple calculations so that the indicator is as accurate as possible. As for any Android-based mobile phone, using a small USB tester.
A simple test of battery capacity with a USB tester and subsequent clarification
The USB tester for measuring battery capacity has very rich functionality - it can be used to measure the battery capacity of a tablet, smartphone, etc. Based on what this device shows, you can get an idea about the wear of the batteries: is it worth changing the battery, or do you need to purchase a new device.
With just one control button, you can measure various indicators, including the battery capacity of a particular device. The button switches tester memory cells and operating modes. If there is a sufficient voltage level in the connected device, the tester turns on.
The meter usually displays the battery capacity indicator in the lower left corner. The measurement accuracy of the tester is not 100% , and therefore it is recommended to use a simple mathematical formula based on the following example.
Let's say you have a certain device (phone, power bank or tablet), which, being completely discharged, charged in a period of time equal to 3 hours. If, out of curiosity, you measured the current using a tester, and its value was, for example, 1.15 A, the actual battery capacity of your device is calculated by multiplying these two numbers together. 1.15 amperes is 1150 milliamps, multiply this number by 3 and get 3450 mAh. This is how real capacitance is measured. If the current “capacity” on your device indicated by the manufacturer is several times greater than the actual one, this is just a standard advertising ploy that should not be trusted.
How to determine battery capacity using a multimeter
It is impossible to determine the battery capacity itself with a multimeter. To be more precise, this device will help in determining the actual capacity indicators.
In order to find out the capacity of an 18650 battery, as well as other batteries, so-called “smart chargers” are used. But their cost is quite high. It’s not worth buying such chargers simply to determine the capacity of a couple of batteries. This indicator will easily determine the usual calculation method with the preliminary use of a multimeter. However, when making calculations, certain subtleties must be observed.
Checking the battery capacity indicator with a multimeter is not just measuring its real indicator, the calculations of which are made using basic mathematical calculations. It is necessary to measure the current level supplied by the battery (any battery) and calculate the exact amount of time during which the battery could continuously and efficiently produce electrochemical energy. It is important to remember that all measurements will not be 100% accurate. But they are the ones that best reflect the true essence of the matter.
They have their own scale of ranks. It shows how much U depends on the charge rate. This is necessary to know: the current flow through the resistance depends on the voltage level. To ensure that this dependence does not affect the measurements, an additional device should be assembled - linear current stabilizer (2.7-3 volts).
Using a linear stabilizer
Using this stabilizer, set the current indicator, calculating it from a 2.7 volt U battery. Then, using the U stabilizer, connect any resistor device (it can be made independently or purchased from an online store). Measure the current that passes through the circuit and set a stopwatch. Next, we periodically check and monitor the voltage at the battery terminals. When it reaches the number 2.7 volts, the stopwatch will need to be quickly turned off and the time obtained recorded.
So, how to measure the capacity of a 18650 battery and other chemical current sources? Real indicator mwe deduce by doing multiplying the current flowing through the circuit through resistance by the same time (in hours) that was originally spent . This is the most accurate measurement of capacitance. In the absence of technical capabilities, it will be more difficult to design a voltage stabilizer, carry out calculations and measurements. Try to find a way out of the situation using a variable resistor.
Using a Variable Resistor
In order to conduct a high-quality capacity test, you can use a smaller battery. For example, 14500, the actual capacity of which is 300 mAh. Let's take a variable resistor of 100 Ohms. Important point: if a direct current resistor is used, the process will be complicated by the fact that it will be necessary to frequently record the results of its readings and carry out calculations of the spent capacitance for certain sections of the scale .
It is possible to average the indicators as much as possible, focusing the calculation on the “arithmetic average” number of the current. In order to understand how to measure the battery capacity, it is recommended to use a variable resistor with a dosed decrease in the resistance value throughout the entire time the battery is being discharged. It is important that the current level is approximately the same throughout the entire process.
Now switch the multimeter to the voltmeter position (measure U) and measure U at the terminals of your battery. Let's say it has an incomplete charge level, say 4 volts. Next, discharge it by applying a current of 450-500 milliamps, lowering the resistance level from time to time and controlling the voltage. When it drops to 2.7 volts, turn off the stopwatch. To completely discharge the battery using 500 mA current , it takes about half an hour, more precisely, 25 minutes. Now let's multiply this current by the amount of time measured in hours. So, the real capacity indicator is 200 mAh.
Thus, it becomes clear how to find out the battery capacity using the most accurate method - not just by measurements, but by mathematical calculations that can most accurately reflect the actual state of the battery and help the user navigate what its potential is in reality.
How and why is battery capacity measured?
Charge Q, as the amount of electricity, is measured in coulombs (C), the electrical capacity of capacitors C is in farads, microfarads (μF), but for some reason it is measured not in farads, but in ampere-hours (milliamp-hours).
What would that mean? One ampere is a coulomb in one second; we know from a physics course that if an electric charge equal to 1 coulomb passes through a conductor in 1 second, then a current of 1 ampere flows through the conductor.
So what is an ampere hour then? Ampere-hour (Ah) is the battery capacity at which, based on a reduced current of 1 ampere, the battery will be discharged in 1 hour to the minimum permissible voltage.
1 ampere hour is 3600 coulombs. Suppose we want to obtain a bank of capacitors that is equivalent in discharge characteristics, albeit over a short section, to a 12-volt battery with a capacity of 55 ampere-hours. 55 amperes for an hour is 55 * 3600 coulombs.
Let us assume a voltage change from 13 to 11 volts, then since Q = C(U1-U2), then C = 55 * 3600/2 = 99000 F. Almost 100 kilofarads is the equivalent electrical capacity of a car battery, if its discharge characteristics were the same as at the capacitor.
There is a video on the Internet where six supercapacitors of 3000 F, 2.7 V each, connected in series, replace the starter battery of a car. It turns out 500 F at about 16 V.
Let's estimate what current and for how long such an assembly can produce. Let the operating range be taken again from 13 to 11 volts. For how long can you count on a current of 200 A (with a margin)? I = C(U1-U2)/t, then t = C(U1-U2)/I = 500*2/200 = 5 seconds. Enough to start the engine.
Capacity is charge Q a new battery or a fully charged battery. Charge (amount of electricity) is measured in Coulombs: 1 Coulomb = 1 Ampere × 1 second. Capacity is usually measured in units of ampere hour or ma hour . The typical capacity of a AAA battery is 1000 mAh, AA - 2000 mAh. A 1000mAh battery can produce 1000mA for 1 hour or 100mA for 10 hours. Considering the voltage U, then we can estimate the energy stored in the battery E = Q × U
To determine the battery capacity, it is fully charged, then discharged with a given current. I, and measure time T, for which he was discharged. Product of current I for a while T and there is the battery capacity Q = I × T. The capacity of the battery is also measured, but after a complete discharge the battery can be charged again, but the battery can no longer be used. The point is that you measure capacity of batteries of this type. By the way, capacity alkaline batteries is approximately equal to the capacity of modern NiMh batteries of the same size - AA (2000 mAh), AAA (1000 mAh).
Circuit for measuring capacitance
The proposed circuit discharges the battery through a resistor R to the voltage of almost complete discharge of the NiCd or NiMh element - approximately 1 volt. The discharge current is equal to I = U / R. (About selecting the discharge current) To measure the discharge time T watches operating on a voltage of 1.5-2.5V are used. To protect the battery from complete discharge, a PVN012 solid-state relay is used. It turns off the battery when the voltage drops U to the minimum permissible Ue = 1V.
How it works
The battery must be fully charged and connected to the device. The clock must be set to 0 and the button pressed Start . At this moment, the relay closes contacts 4-5 and 5-6. The battery begins to discharge through a resistor R and voltage is supplied to the clock. The voltage across the battery and resistor gradually decreases. When the voltage across the resistor R drops to 1V, the relay opens the contacts. The discharge stops and the clock stops.
As the battery discharges, the control current through relay contacts 1-2 decreases from approximately 8 to 2mA. With a control current of 3mA, the resistance of contacts 4-5 and 5-6 is less than 0.04 Ohm. This is small enough not to be taken into account when calculating the current - if you need a discharge current of 1A, take a resistor R=1.2 Ohm.
After the discharge stops, the voltage on the battery increases to 1.1-1.2V due to the internal resistance of the cell.
Contact losses
When repeating this circuit, take measures to reduce the resistance of the battery contacts and connectors. With a current of 0.5-1A, 0.1V or more can be lost on the contacts, which will worsen the accuracy of the measurement. The same type of loss is caused by the steel spring used in some battery holders. The spring and other steel contacts must be bridged with copper wire. I did one of the options AA and AAA battery capacity meter in the case from a simple charger that had good copper contacts.
Additional questions
Self-discharge
Please note that the capacity freshly charged batteries are higher, since over time some of the charge is lost due to self-discharge. To find out the amount of self-discharge, you need to measure the capacity immediately after charging, and measure again a week (month) after charging. Self-discharge of NiMh batteries can reach 10% per week or more.
How accurately is capacitance measured?
The exact amount of electricity can be determined by integrating over time dQ = 1/R × U(t) × dt.
The experimental discharge graphs show that as the discharge progresses, the voltage decreases from approximately 1.4V to 1.0V. The discharge current U/R also decreases. When used as medium voltage nominal Values of 1.2V result in an accuracy of no worse than 10%. This is true if the battery is used at approximately the same discharge current as when measuring capacity.
Example of discharge graphs 
If during measurement there was a current of 0.5A, and when using 5A, then the battery will discharge several times faster than expected. At a current use of 0.05A, the capacity will be greater than when measured. At a current of 0.005A, the capacity may be less than measured due to self-discharge of the battery over a long period of operation. A significant difference between the measurement current and the operating current introduces an error of more than 10%.
Using steel contacts in the device instead of copper can increase the error by 10% or more, especially with a high discharge current.
Some error in the cutoff voltage value of 1.0V is associated with the dependence of the current-voltage characteristic of the solid-state relay on temperature. Under room conditions, this gives an error of 1-2%.
What should the discharge current be?
It is necessary to choose the current at which this battery is usually used. If the discharge current is too high, the internal resistance will cause the battery voltage to quickly drop below 1 volt and the measured capacitance value will be low. If you select too low a discharge current, the measured capacity will be greater than the battery will actually produce when operating in your device.
Why two diodes?
Diodes are used to protect the solid state relay in case of accidental resistor breakage R. If you are sure that a break is impossible, or you are measuring the capacity of batteries with a voltage of less than 1.4V ( one AA or AAA element), then the diodes can be removed. In this case, the circuit is placed inside the alarm clock, as I did before. A 5 ohm resistor protects the relay when the Start button is pressed. It can also be removed if you turn on the button parallel to pins 4-5, as in the simplified diagram.
How to measure the capacity of a lithium-ion battery?
| Um | Ue | I | R | r |
|---|---|---|---|---|
| 1.2 | 1.0 | 0.2 | 6.0 | 0 |
| 1.2 | 1.0 | 0.5 | 2.4 | 0 |
| 3.3 | 3.0 | 0.5 | 2.2 | 4.4 |
| 8.4 | 7.0 | 0.1 | 12 | 72 |
In this case, a voltage divider is connected to the battery according to the example shown in the diagram. Using a voltage divider, you can measure the capacity of a multi-cell battery or the capacity of a lithium-ion battery.
Required discharge current I at medium voltage Um provides the sum of two resistors: R + r = Um / I.
Resistor R is calculated so that at the final voltage on the battery Ue, voltage across the resistor R became equal to 1V: R = (Um / I) × (1V / Ue).
How to check battery capacity by voltage?
Capacitance cannot be determined by voltage. Each type of battery and accumulator has typical discharge curves. From them you can estimate the ratio of charge to capacity ( charge percentage). I'm using a charger Ansmann, which for such an assessment measures the voltage at a given discharge current. However, in NiMh batteries, not only the capacity, but also the operating voltage decreases with age. In some cases, Ansmann gave an estimate of 30%, while the measurement before full discharge gave 80%.
How to measure battery capacity without this circuit?
Connect a resistor to the charged battery R and a voltmeter. Keep an eye on the clock. Over time T voltage U will decrease to the minimum acceptable. At this point, disconnect the resistor. Capacitance is Q = T × U / R
A device with which you can check the capacity of lithium-ion AA batteries. Quite often, laptop batteries become unusable due to the fact that one or more batteries lose their capacity. As a result, you have to buy a new battery when you can get by with little expense and replace these unusable batteries.
What you will need for the device:
Arduino Uno or any other compatible.
16X2 LCD display using Hitachi HD44780 driver
Solid State Relay OPTO 22
10 MΩ resistor at 0.25 W
18650 battery holder
Resistor 4 Ohm 6W
One button and power supply from 6 to 10V at 600 mA
Theory and operation
The voltage on a fully charged Li-Ion battery with no load is 4.2V. When a load is connected, the voltage quickly drops to 3.9V, and then slowly decreases as the battery operates. A cell is considered discharged when the voltage across it drops below 3V.
In this device, the battery is connected to one of the analog pins of the Arduino. The voltage on the battery without load is measured and the controller waits for the “Start” button to be pressed. If the battery voltage is higher than 3V. , pressing the button will start the test. To do this, a 4 Ohm resistor is connected to the battery through a solid state relay, which will act as a load. The voltage is read by the controller every half second. Using Ohm's law you can find out the current supplied to the load. I=U/R, U-read by analog input of the controller, R=4 Ohm. Since measurements are taken every half second, there are 7200 measurements in every hour. The author simply multiplies 1/7200 hour by the current value, and adds the resulting numbers until the battery discharges below 3V. At this moment the relay switches and the measurement result in mAh is displayed on the display
LCD pinout
PIN Purpose
1 GND
2 +5V
3 GND
4 Digital PIN 2
5 Digital PIN 3
6,7,8,9,10 No connected
11 Digital PIN 5
12 Digital PIN 6
13 Digital PIN 7
14 Digital PIN 8
15 +5V
16 GND
The author did not use a potentiometer to adjust the brightness of the display; instead, he connected pin 3 to ground. The battery holder is connected with the minus to ground, and the plus to analog input 0. A 10 MΩ resistor is connected between the plus of the holder and the analog input, which acts as a pull-up. The solid-state relay is turned on with a minus to ground, and a plus to digital output 1. One of the contact pins of the relay is connected to the plus of the holder; a 4 Ohm resistor is placed between the second pin and ground, which acts as a load when the battery is discharged. Keep in mind that it will get quite hot. The button and switch are connected according to the diagram in the photo.
Since the circuit uses PIN 0 and PIN 1, they must be disabled before loading the program into the controller.
After you connect everything, upload the firmware attached below, you can try to test the battery.
The photo shows the voltage value that the controller calculated.
The voltage on it must be higher than 3V