A battery charge indicator is a necessary thing in the household of any motorist. The relevance of such a device increases many times over when, for some reason, a car refuses to start on a cold winter morning. In this situation, it’s worth deciding whether to call a friend to come and help you start from your battery, or whether the battery has died for a long time, having discharged below a critical level.
Why monitor your battery's condition?
A car battery consists of six batteries connected in series with a supply voltage of 2.1 - 2.16V. Normally, the battery should produce 13 - 13.5V. Significant discharge of the battery should not be allowed, since this reduces the density and, accordingly, increases the freezing temperature of the electrolyte.
The higher the battery wear, the less time it holds a charge. In the warm season, this is not critical, but in winter, side lights forgotten while turned on can completely “kill” the battery by the time it is returned, turning the contents into a piece of ice.
In the table you can see the freezing temperature of the electrolyte, depending on the degree of charge of the unit.
| Dependence of the freezing temperature of the electrolyte on the state of charge of the battery | ||||
|---|---|---|---|---|
| Electrolyte density, mg/cm. cube | Voltage, V (no load) | Voltage, V (with load 100 A) | Battery charge level, % | Electrolyte freezing temperature, gr. Celsius |
| 1110 | 11,7 | 8,4 | 0,0 | -7 |
| 1130 | 11,8 | 8,7 | 10,0 | -9 |
| 1140 | 11,9 | 8,8 | 20,0 | -11 |
| 1150 | 11,9 | 9,0 | 25,0 | -13 |
| 1160 | 12,0 | 9,1 | 30,0 | -14 |
| 1180 | 12,1 | 9,5 | 45,0 | -18 |
| 1190 | 12,2 | 9,6 | 50,0 | -24 |
| 1210 | 12,3 | 9,9 | 60,0 | -32 |
| 1220 | 12,4 | 10,1 | 70,0 | -37 |
| 1230 | 12,4 | 10,2 | 75,0 | -42 |
| 1240 | 12,5 | 10,3 | 80,0 | -46 |
| 1270 | 12,7 | 10,8 | 100,0 | -60 |
A drop in charge level below 70% is considered critical. All automotive electrical appliances consume current, not voltage. Without load, even a severely discharged battery can show normal voltage. But at a low level, during engine startup, a strong voltage drop will be noted, which is an alarming signal.
It is possible to notice an approaching disaster in a timely manner only if an indicator is installed directly in the cabin. If, while the car is running, it constantly signals about discharge, it’s time to go to the service station.
What indicators exist
Many batteries, especially maintenance-free ones, have a built-in sensor (hygrometer), the operating principle of which is based on measuring the density of the electrolyte.
This sensor monitors the condition of the electrolyte and the relative value of its indicators. It is not very convenient to climb under the hood of a car several times to check the condition of the electrolyte in different operating modes.
Electronic devices are much more convenient for monitoring the condition of the battery.
Types of battery charge indicators
Automotive stores sell many of these devices, differing in design and functionality. Factory devices are conventionally divided into several types.
By connection method:
- to the cigarette lighter socket;
- to the on-board network.
By signal display method:
- analog;
- digital.
The principle of operation is the same, determining the battery charge level and displaying information in a visual form.
Schematic diagram of the indicator How to make a battery charge indicator using LEDs?
There are dozens of different control schemes, but they produce identical results. It is possible to assemble such a device yourself from scrap materials. The choice of circuit and components depends solely on your capabilities, imagination and the assortment of the nearest radio store.
Here is a diagram to understand how the LED battery charge indicator works. This portable model can be assembled “on your knee” in a few minutes.
D809– a 9V zener diode limits the voltage on the LEDs, and the differentiator itself is assembled on three resistors. This LED indicator is triggered by current in the circuit. At a voltage of 14V and above, the current is sufficient to light up all the LEDs; at a voltage of 12-13.5V they light up VD2 And VD3, below 12V - VD1.
A more advanced option with a minimum of parts can be assembled using a budget voltage indicator - chip AN6884 (KA2284).
Circuit of LED battery charge level indicator on voltage comparator
The circuit operates on the principle of a comparator. VD1– a 7.6V zener diode, it serves as a reference voltage source. R1– voltage divider. During the initial setup, it is set to such a position that all LEDs light up at a voltage of 14V. The voltage supplied to inputs 8 and 9 is compared through a comparator, and the result is decoded into 5 levels, lighting the corresponding LEDs.
Battery charging controller
To monitor the condition of the battery while the charger is operating, we make a battery charge controller. The device circuit and components used are as accessible as possible, while at the same time providing complete control over the battery recharging process.
The operating principle of the controller is as follows: as long as the voltage on the battery is below the charging voltage, the green LED lights up. As soon as the voltage is equal, the transistor opens, lighting up the red LED. Changing the resistor in front of the base of the transistor changes the voltage level required to turn on the transistor.
This is a universal monitoring circuit that can be used for both high-power car batteries and miniature lithium batteries.
When the battery is discharged, starting the car is quite problematic. To avoid such an unpleasant “surprise”, it is enough to simply use a voltmeter from time to time. However, not all motorists and do not always do this, because it is much more convenient to have some kind of device that shows how long the battery charge will last.
What are the indicators?
The rechargeable battery (or battery) consists of six interconnected elements, the voltage in each should normally be about 2.15 volts, i.e. the total battery voltage approaches 13.5 volts. If the charge drops below critical values (approximately 9.5 volts), this can lead to deep discharge of the battery and, as a result, its complete failure.
Modern technologies “meet” motorists halfway and make their lives as easy as possible. For example, many cars already have on-board computers that also monitor the battery charge level.
However, while this option is not available to everyone, it is necessary to use other types of indicators of this important indicator. So, you can find separate crystal displays on the dashboard, there are hygrometer indicators, and you can also (if you have the appropriate skills) make a battery charge indicator yourself. Many alarm devices of this type must be connected to the vehicle’s on-board network so that they can monitor the battery charge level.
Built-in charge indicator
The most common type of indicator on maintenance-free batteries is a hydrometer. It consists of an eye, a light guide, a leg and a float (that’s why it is called a float). The leg with the light guide is located inside the battery; a float is attached to the leg, with the help of which the electrolyte level in the battery is determined. There is a peephole on the battery case that shows the three main states of the battery:
- the green float ball shines through the viewing eye, which means that the battery is more than half charged;
- the eye remains black (this is visible through the indicator tube), this is a signal that the float is completely immersed in the electrolytic liquid, therefore, its density is reduced, and the battery needs to be charged;
Additional Information. Some models of hydrometers have a red float, which is visible in the “window” when the charge and density of the electrolyte decreases.
- if only the surface of the liquid inside the battery is visible in the “eye”, it means that it is “thirsty” - the electrolyte level is critical, it is urgently necessary to add distilled water (and this is quite difficult to do, since such batteries are maintenance-free).
Note! Although a built-in battery charge indicator of this type allows you to instantly determine the existing problem (or lack thereof), judging by some user reviews, the readings of such devices are often false, and they themselves quickly break down.
As a rule, this is due to the following reasons:
- data comes from only one battery cell out of six, and the liquid level in them can vary significantly;
- indicator parts made of plastic cannot withstand the temperature conditions of the battery, so the data is received incorrectly;
- float indicators do not in any way determine the temperature of the electrolytic liquid, but the density also depends on it, so an electrolyte at a low temperature will show a normal level of density, while it will also be low.
Factory indicators in the form of panels
In specialized stores you can find many different battery monitoring devices; each car owner can choose the design and functions to suit himself. The indicators also differ in the method of connection: to the cigarette lighter or to the on-board network of the car. However, the main task of all devices is the same - to determine how charged the battery is and to signal about it.
There are indicators that you need to assemble yourself, like a constructor. As an example, DC-12 V. It makes it possible to control the battery charge, as well as the operation of the control relay.
Such a small control device operates in the range from 2.5 to 18 volts, consumes very little electricity - up to 20 milliamps, the dimensions of the indicator window are 4.3 by 2 cm.
If you install a second battery in a car, you can use an indicator from TMS - this is a small panel made of industrial aluminum with LEDs with a built-in voltmeter and a switch between adjacent batteries.
Among the expensive models (and unreasonably expensive, for the price of a new battery), we can highlight the voltage controllers of the American company “Faria Euro Black Style”. The body color is usually black, the diameter of the display window is 5.3 cm, and the screen is illuminated in white. 12 volts are required for power supply.
How to assemble a charge indicator yourself
If a car owner is comfortable with a soldering iron, he can assemble the analyzer with his own hands; you can find many assembly diagrams. Using one, the simplest, you can assemble a charge indicator reminiscent of the DC-12 V described above. It operates on the same principles: it is connected to the on-board network and determines the battery voltage within 6-14 volts.
To assemble the device you will need transistors, resistors, zener diodes, a printed circuit board and one red, blue and green LED each. After assembly, according to the diagram, the board is inserted onto the dashboard, and the ends of the LEDs are placed in a place convenient for viewing. In this case, a fully charged battery will be indicated in green, blue - when the charge is normal (from 11 to 13 volts), and if the battery is close to discharge, the red LED will light up.
It is unpleasant when a car cannot start simply because the battery is discharged at the most inopportune moment. A voltage indicator, purchased in a store or soldered yourself, will help avoid unpleasant “surprises” and warn in advance that the battery requires recharging.
Video
The relatively simple design of a simple and small-sized indicator that is powered directly from the car battery and shows the presence of charge in the battery. The circuit contains three LEDs of different colors.
First LED - red, it lights up only when the voltage at the battery terminals is below 11.4-11.6 Volts, this will mean that the battery is discharged. The red indicator will probably light up frequently for owners of old domestic cars with a dead battery; especially in winter, the electrolyte often freezes, and the battery loses some of its capacity and rated voltage. I connected two LEDs in parallel for better visibility.
At the very least, the sensor will show that your car is not charging precisely because the battery is low, while the industrial sensor does not always work accurately.
Second LED - yellow (or orange). This LED lights up if the voltage at the battery terminals is about 11.6-13.6 Volts, and you can find out exactly what voltage is on the battery by the glow of the LED. If this indicator shines brightly, by that time the car is not started, which means there is normal voltage at the battery terminals, but if it shines dimly, then the voltage is no more than 12 Volts and the battery needs easy recharging.
The third indicator - green, it usually lights up when the engine is running, when the voltage at the battery terminals is above 13.6 Volts. If the green LED is constantly on while driving, then this is quite normal, because this is how it should be. In my case, I added 2 more green LEDs and connected them in parallel.
The circuit can be assembled on a piece of a breadboard, placed in a small plastic case and hidden, say, in the glove compartment so that the glow of the LEDs does not distract the driver.
The circuit uses three domestically produced transistors, two of them are of reverse conductivity, which, if desired, can be replaced with the more common KT315, the KT3107 transistor can be successfully replaced with KT361. The choice of transistors is not critical, literally any transistors (even medium and high power) of a certain structures, a lot of imported analogues, for example KT3107 can be replaced with BC556/557, KT3102 with S9012/9014/9018, etc.
Not all cars have an indicator that displays the battery charge level. The car owner must independently monitor this indicator, periodically checking it with a voltmeter, having previously disconnected the battery from the car's electrical network. However, a simple electronic device will allow you to get approximate indicators without leaving the salon.
Selection of circuit and components
Finished design
Structurally, a homemade battery charge monitoring indicator consists of an electronic unit, on the body of which there are three LEDs: red, blue and green. The choice of color may be different - it is important that when activating one of them, the information received is correctly interpreted.
Due to the small size of the device, you can use a regular breadboard. The optimal device circuit is pre-selected. You can find several models, but the most common and therefore workable version of the battery charge indicator is shown in the figure.
Diagram of the board and its components
Before installing components, it is necessary to arrange them on the printed circuit board according to the diagram. Only after this can you cut it to the desired size. It is important that the indicator has minimal dimensions. If you plan to install it in a housing, you should take into account its internal dimensions.
This circuit is designed to monitor the operation of a car battery with a network voltage from 6 to 14 V. For other values of this parameter, the characteristics of the components must be changed. Their list is shown in the table.
The simplest version is shown in Figure 1. If the voltage at the B+ terminal is 9 V, only the green LED will light because the base voltage of Q1 is 1.58 V, while the emitter voltage is equal to the voltage drop across the D1 LED in a typical case is 1.8 V, and Q1 is held closed. As the battery charge decreases, the voltage across LED D2 remains essentially the same and the base voltage decreases, and at some point in time Q1 will begin to conduct current. As a result, part of the current will begin to branch into the red LED D1, and this share will increase until all the current flows into the red LED.
| Picture 1. | Basic circuit diagram of a battery voltage monitor. |
For typical elements of a two-color LED, the difference in forward voltages is 0.25 V. It is this value that determines the region of transition from green to red. A complete change in the color of the glow, set by the ratio of the resistances of the divider resistors R1 and R2, occurs in the voltage range
The middle of the transition region from one color to another is determined by the voltage difference between the LED and the base-emitter junction of the transistor and is approximately 1.2 V. Thus, a change in B+ from 7.1 V to 5.8 V will result in a change from green to red.
Voltage differences will depend on specific LED combinations and may not be sufficient to fully switch colors. However, the proposed circuit can still be used by connecting a diode in series with D2.
In Figure 2, resistor R1 is replaced by a zener diode, resulting in a much narrower junction region. The divider no longer affects the circuit, and a complete change in the color of the glow occurs when the B+ voltage changes by only 0.25 V. The voltage of the transition point will be equal to 1.2 V + V Z. (Here V Z is the voltage on the zener diode, in our case equal to approximately 7.2 V).
The disadvantage of such a circuit is that it is tied to a limited voltage scale of the zener diodes. Further complicating the situation is the fact that low-voltage zener diodes have a characteristic curve that is too smooth, which does not allow one to accurately determine what the voltage V Z will be at low currents in the circuit. One solution to this problem would be to use a resistor in series with the zener diode to allow for slight adjustment by slightly increasing the junction voltage.
With the resistor values shown, the circuit consumes a current of about 1 mA. With high-brightness LEDs, this is enough to use the device indoors. But even that little current is significant for a 9-volt battery, so you'll have to choose between drawing extra current and risking leaving the power on when you don't need it. Most likely, after your first unscheduled battery replacement, you will start to feel the benefits of this monitor.
The circuit can be converted so that the transition from green to red occurs when the input voltage increases. To do this, transistor Q1 must be replaced with NPN and the emitter and collector must be swapped. And using a pair of NPN and PNP transistors, you can make a window comparator.
Given the fairly large width of the transition region, the circuit in Figure 1 is best suited for 9V batteries, while the circuit in Figure 2 can be adapted for other voltages.
