Automatic charger for lead-acid batteries. Charger for lead-acid batteries
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As you know, sealed lead-acid batteries can be constantly connected to the charger, that is, they can be in recharging mode. To know when the battery is fully charged, the charger must have some kind of indicator. Below we describe one of the options for a charger equipped with a charge indicator.
Description of the charger for lead-acid batteries
The voltage to the charger circuit is supplied to terminals X1 and X2 from an external constant voltage source (12...20 volts). The charging current is supplied to the charging current on indicator (LED HL1), transistor VT1 and charging voltage. The stabilized charging voltage is connected to terminals X3 and X4, which are connected to the lead-acid battery.
The charging current indicator includes a current sensor (resistor R1), the charging current flowing through it creates a voltage drop across it. Due to the voltage drop, transistor VT1 opens, into the collector of which an indicator is connected - LED HL1.
The magnitude of the voltage drop at which transistor VT1 opens is set by a resistive divider across resistances R3 and R4. If the charging current is less than the set current level (the current limit is set by trimming resistor R4), the HL1 LED does not light up. As the charging current increases, the LED glow also gradually increases.
The LM317 adjustable output voltage stabilizer is used as a charging voltage stabilizer. According to the voltage level and charging current used, the LM317 regulator should be set to good heat dissipation.
Trimmer resistor R5 regulates the output voltage at terminals X3 and X4. For batteries with a nominal voltage of 6 V, the output charging voltage should be 6.8...6.9 V; for batteries with a nominal voltage of 12 V, this output voltage will already be 13.6...13.8 V.
It should be noted that the input voltage from an external constant voltage source must be approximately 5 volts higher than the voltage at the charger output (voltage drop across R6 and LM317).
When you need to charge a medium and small lead-acid battery (not a car battery), most often you take a regular power supply or a simple transformer with a rectifier, and then connect the battery to it for 10 hours, selecting a current of 0.1C. This is, of course, a collective farm. In more or less decent devices, where the filling is “at the level”, a memory circuit with all tracking and automatic charge control systems is required. This is what this charger circuit based on the BQ24450 chip from Texas Instruments is designed for. This microcircuit takes on all the functions of charging the battery and maintaining the stability of the process, regardless of the conditions and condition of the battery. And the wide range of charging currents and voltages makes it suitable for emergency lighting batteries, RC cars, motorcycles, boats or any other vehicle with a 6 - 12V battery - just connect this charger to the battery and that's it.
Characteristics of the BQ24450 chip
- 10-40V DC input
- Load (charge) current 0.025-1 A
- With external transistor - up to 15 A
- Adjust voltage and current during charging
- Temperature-compensated voltage reference
The BQ24450 chip contains all the necessary elements for optimal control of the charging of lead-acid batteries. It controls the charging current as well as the charging voltage to charge the battery safely and efficiently, increasing the battery's effective capacity and service life. Built-in precision temperature-compensated voltage reference for tracking lead-acid cell performance maintains optimal charging voltage over an extended temperature range without the use of any external components.
The low current consumption of the microcircuit allows precise control of the process due to low self-heating. There are comparators that monitor charging voltage and current. These comparators are powered from an internal source, which has a positive effect on the stability of the charging cycle.
Over time, they lose their charge and must be periodically restored. Aspects of this process will be discussed within the article.
What is called charging
This is the name for the process that is the reverse of discharge. When charging sealed lead-acid batteries, they store energy while being powered by an external current source. The end result is an accumulation of charge, which is equal to capacitance. What do chargers for sealed lead acid batteries look like? They are an energy converter and two terminals, each of which is connected to a sealed maintenance-free lead-acid battery, when connected to the network, it will begin the process of restoring and converting electrical energy (supplied from the network) into chemical energy. So that in the future, as soon as the need arises, he can carry out the reverse process and provide power supply to various devices and devices.
Charging is simple and safe
To do this, you need to use the current-voltage method. What is it? Initially, the battery is charged with constant current. When the required indicators are achieved, constant voltage support begins. To find out the initial charging current, it is usually enough to carefully inspect the case - this parameter is indicated there. Typically this value is up to 0.3 To make it more clear, imagine that we have a device with a parameter of 100 A/h. Then the charge current should not exceed 30A. But this is a safe maximum; many manufacturers use the ten percent rule in their chargers. This allows you to charge batteries without the least fear of doing something wrong and damaging it. How long does it take to charge? If the initial current is 20% of capacity, then the battery reserve will be restored to 90% in about 5-6 hours. The remaining 10% will take about a day. These are the features of its functioning that a charger for sealed lead-acid batteries has. Is there any way to speed up this process? Yes, and we will now look at how.
Fast charging of sealed lead acid batteries
The norm is DC charging at a voltage of 13.8. More than this is not recommended due to possible negative consequences. But if they don’t scare you, then you can increase the voltage to 14.5 V (this is for 12 V batteries). As a result, the battery will be charged at 20% in 6 hours. This method is used exclusively when working in cyclic mode.
Effect of temperature
Everything that was written above applies only to the case when the temperature is 20 degrees Celsius. For other indicators, it is necessary to introduce charging voltage compensation. Lead-acid batteries can be charged in the range from -15 to 40 degrees. The higher the temperature, the lower the voltage should be to avoid overcharging. In the opposite case, this indicator, on the contrary, should be increased to avoid undercharging. Because of this, it is advisable to charge a sealed, maintenance-free lead-acid battery in conditions of 20 degrees Celsius, plus or minus a few. Of course, you can calculate it every time, but this is not always convenient. People often choose their homes as an ideal place for temperature parameters, but then it is necessary to take care of high-quality ventilation of the charging area both during this process and a few hours after its completion.
Consequences of non-compliance with safety precautions
The methods described above are aimed at fast and safe charging. At the same time, the task is set to maximize the service life of a lead-acid battery by minimizing its aging factors. Now let's look at the deviations. What happens if you use a current greater than the maximum permissible? The first thing to note is that sealed lead acid batteries will not be able to fully charge. Also, due to a decrease in the efficiency of the gas recombination mechanism, the electrolyte will lose water. Therefore, even a one-time charge is enough to reduce the operating life.
What happens if you reduce the current to 0.5 percent of the capacity? Sealed lead-acid batteries will charge in this case, but this process will continue for several weeks. In addition, the device will be in a state that is equivalent to discharged. And this leads to sulfation and accelerated aging. Of course, low current charging alone is not enough to cause serious damage, but it is better not to use them. It is also necessary to monitor the final voltage so that the device does not undercharge and reduce its service life.
Why do lead-acid batteries have such a temperature range for charging? The fact is that when they leave them, the gas recombination mechanism stops working, and the electrolyte loses its water.
Was everything done well?
To get a good result, you must comply with the required parameters within the required limits. The main considerations in this matter should be current and voltage (take into account temperature). Then sealed lead-acid batteries will charge successfully and can last for a long time. If there is electrolyte, white deposits or bubbles around, then the restoration of the device’s characteristics was done incorrectly. A tester can be used to determine the condition. Restoration of sealed lead-acid batteries is carried out using special chargers (which may require several days) or additional mechanical actions (somehow adding electrolyte).
Conclusion
As you can see, the process of charging lead-acid batteries cannot be called complicated. If you follow safety precautions, it will not be easy to get something wrong. But finally, I would like to recommend charging them in separate rooms, and if the devices are being restored in a residential building, then it is necessary to take care of high-quality ventilation during the process, as well as for several hours after it. These safety measures are necessary due to the fact that, albeit in microscopic doses, lead can enter the air, and through it into the body, from where it is excreted very slowly and constantly has a toxic effect.
This story began when we decided to go to the forest on the night from Saturday to Sunday - it was my brother’s jam day, and we decided to celebrate it in the fresh air with barbecue and vodka. They began to gather. For lighting, we took a couple of flashlights and a small boombox to set the background music. Of course, we bought batteries for all this, which cost us a pretty penny. With the faces of happy idiots, we burst into the forest and briskly began to collect firewood, soberly (for now) reasoning that it would be nice to break this very firewood before it got dark. And firewood was needed for two fires - for barbecue and for heating - lighting the place of celebration. Well, what I want to tell you... the next day I hardly managed to straighten up, because in order for there to be enough light from the fire, I had to constantly throw firewood in there, which had to be cut in the forest, in which after sunset it became dark, like themselves you know where the batteries in the lanterns had to be saved and the place of drunkenness illuminated with a fire, for which it was necessary to chop wood. I'm repeating myself, right? Well, that night I had a lot of repetitions like this. In connection with this, the next day two questions arose - “did I rest?” Or “where and how to make sure this doesn’t happen again?”
First of all, batteries - it is clear that batteries are needed, but after looking at the prices of modern nickel-cadmium batteries, my toad categorically refused to buy them. Then I remembered about UPSs - you know, those kind of racks to prevent your computer from cutting out at the most inopportune moment, when you finish completing the 100x100 minesweeper, and a good neighbor has already plugged in a home-made welding unit into the socket and, smiling joyfully, turned it on, turning off the power , thus half the house.
So, these banduras use sealed lead batteries - they are also called gel batteries. In terms of cost, they are not comparable to Ni-Cd batteries - the former cost significantly less than the latter. I went to the store and bought myself a fairly average battery with a voltage of 12 volts and a capacity of 7.2 ampere-hours.
Fig.1 Photo of the battery.
Then everything was simple - we take a 10-watt car light bulb, hang it on a long wire on a tree and connect it to the subject - the light is ready. And to connect the radio, we sculpt a simple stabilizer on the KREN8A or its bourgeois analogue LM7809, screw the wires to the terminals in the battery compartment - e voila - we have light and music. I must tell you that a similar scheme has already been tested - it lasts for an entire night of continuous operation and the battery is not completely discharged.
But you understand that everything is never good until the end - there must be somewhere a drop of waste from human metabolism, which must poison the entire idyll. The catch in this case is that these batteries cannot be charged with conventional car battery chargers. Conventional lead-acid batteries are charged with a constant current, while the voltage at the terminals increases all the time and when it reaches a certain value, the electrolyte in the battery boils, which indicates the end of the charge. Let's imagine what will happen when a sealed battery boils. I believe that casualties and destruction are unlikely to be avoided. Therefore, these boxes are charged differently: the charging current is set equal to 0.1C, where C is the battery capacity, and the charging current is limited, since this comrade is “unsatisfied with the gastrointestinal tract” and is ready to gobble up everything that is given to him, the voltage is stabilized and set to within 14-15 volts. During the charging process, the voltage remains practically unchanged, and the current will decrease from the set value to 20-30 mA at the very end of the charge. That is, it was necessary to assemble the charger.
I really didn’t want to mess around, but then the bourgeoisie came to the rescue - ST Microelectronics - they, it turns out, have an almost ready-made solution - the L200C microcircuit. This chip is a voltage stabilizer with a programmable output current limiter. The documentation for this microcircuit is here: www.st.com/stonline/products/literature/ds/1318.pdf The charger circuit in Figure 2 is an almost typical connection circuit
Fig.2
In general, there’s nothing special to describe; I’ll just dwell on a couple of points. First of all, current-setting resistors R2-R6. Their power should be no less than indicated in the diagram, and preferably more. Well, unless, of course, you are a fan of smoke special effects and don’t get sick of the sight of blackened resistors.
Fig 3.1 Device on a breadboard
The microcircuit, of course, must be installed on the radiator, and don’t be greedy either - all this equipment is designed for long-term operation, therefore, the lighter the thermal regime of the elements, the better for them, and therefore for you. Resistor R7 adjusts the output voltage within 14-15 volts. It is better to take our domestic diodes in metal cases, then they do not need to be installed on radiators. The voltage on the secondary winding of the transformer is 15-16 volts. Personally, I didn’t make any board, there aren’t that many details - I assembled everything on a breadboard. What happened can be seen in the photo.
Fig 3.2 Everything is assembled, only without the housing
Everything works as predicted in theory - the current, at first, was large, but by the end of the charge it dropped to insignificant and has been living in this state for several days. By the way, the manufacturer recommends just such a small current for a long time to preserve the battery capacity.
Fig 4.2 Assembled device on the board
You can download the printed circuit board in LAY and Corel formats for plotter cutting on film below
List of radioelements
| Designation | Type | Denomination | Quantity | Note | Shop | My notepad |
|---|---|---|---|---|---|---|
| DA1 | Voltage regulator | L200C | 1 | To notepad | ||
| VD1-VD5 | Diode | D242 | 5 | 1N5400 | To notepad | |
| C1 | Electrolytic capacitor | 4700 µF 25 V | 1 | To notepad | ||
| C2 | Capacitor | 1 µF | 1 | To notepad | ||
| R1 | Resistor | 820 Ohm | 1 | To notepad | ||
| R2 | Resistor | 3 ohm | 1 | 0.25 W | To notepad | |
| R3 | Resistor | 0.33 Ohm | 1 | 2 W | To notepad | |
| R4 | Resistor | 0.75 Ohm | 1 | 1 W | To notepad | |
| R5 | Resistor | 1.5 Ohm | 1 | 0.5 W | To notepad | |
| R6 | Resistor |
