Automatic chargers, do-it-yourself

Various electronic devices in their work use portable energy sources, batteries. The most common voltage for their operation is 12 volts. Batteries, giving the accumulated energy to devices, periodically need to be recharged themselves. When restoring their energy, it is most convenient to use an automatic charger (charger), which makes it possible to simplify the operations performed by the user to a minimum.

Battery types and how they work

There are various technologies used in the manufacture rechargeable batteries (Battery). Depending on the processes taking place in the middle of the battery cells, different charge recovery methods are used. Having approximately the same principle of operation, batteries are divided according to the materials of manufacture and the chemical processes that take place in them.

1. Nickel cadmium (NiCd). First appeared in 1899. Their production technology improved until, in 1947, they created an element with the ability to annihilate the gases that appear during the charging process. The main advantages of this type are: the ability to carry out a quick charge, high load capacity, low price, good reliability and frost resistance. It is possible to store the battery at any state of charge. At the same time, the following drawbacks stand out: the presence of a memory effect, toxicity, low energy density, the self-discharge rate reaches 10 percent per month. Currently, they are practically not used in everyday life due to their toxicity.
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2. Nickel metal hydride (NiMh). In 1984, the use of the La-Ni-Co compound made it possible to absorb hydrogen for more than 100 cycles. When compared with Ni-Cd batteries, they emit higher specific energy values ​​and are non-toxic. The service life of NiMh batteries depends on the charging time and the charging method. Batteries of this type are sensitive to overcharging and are characterized by from 500 to 1 thous. discharge-charging cycles. The service life is 3 to 5 years.
3. Lithium Ion (LiIon). They are by far the most promising elements. For the price they are more expensive than other types of batteries, but they have practically no drawbacks. The first battery of this type was released in 1991 by Sony Corporation. In addition to their high energy capacity, they have the lowest self-discharge rate of all other types. The number of charge-discharge cycles exceeds a thousand times. The first generation batteries based on a lithium metal anode had an explosion hazard when overcharged or repeated charge-discharge cycles. Replacing the anode with graphite in the second generation products completely eliminated the problem.
4. Lithium Polymer (LiPol). This type of battery was designed to replace the first generation LiIon. The design is based on the transition of polymers to a semiconducting state when exposed to ions. The main difference from lithium-ion batteries is the use of a solid electrolyte. Modern LiPol batteries can be made in a flexible form, while the thickness of the cells is one or more millimeters. The number of charge cycles is 800 times, there is no memory effect. To eliminate the occurrence of fire or explosion, all batteries are equipped with an electronic circuit that controls the charge and prevents overheating.
5. The lead acid device was developed in 1859. Structurally, the battery is assembled from six batteries with a nominal voltage of 2.2 volts, connected in series with each other. Each element is a set of lead grid plates. The plates are coated with an active material and immersed in an electrolyte. The battery has a self-discharge value six times lower than NiCd and has good tolerance to powerful loads. The disadvantages are heavy weight and a rapid deterioration in performance in the cold. With a deep discharge in excess of eighty percent, the battery life is drastically reduced.
6. Helium batteries. Manufactured using AMG and GEL technology with a bound electrolyte. They are characterized by low self-discharge and withstand about two hundred charge-discharge cycles. When recovering energy, they require 10% of the nominal capacity of the battery. The disadvantage of this type is that the battery does not have to heat up, since the liquid helium is possible.

The principle of operation of batteries is based on chemical reactions that take place when different materials interact with each other. These processes are reversible, the cycles of accumulation and release of energy can be repeated several times. The battery case is made of a sealed type with terminal leads.

All modern batteries are maintenance-free.

Types of battery chargers

The capacity and duration of the period of use of the battery depends on the conditions of its use and the choice of the charging method. A high-quality charger should prevent overcharging of the battery and be protected from overheating. There are two methods for performing charge control:

• by current;
• by voltage.

The first method is used for NiCd and NiMh rechargeable batteries, and the second for lead acid, LiIon and LiPol batteries. Automatic battery chargers use specialized microcircuits in their work that control the entire energy recovery process.

Charger with current control

Such devices are called galvanostatic. The main characteristic of the charger is the amount of current that charges the battery. Charge the battery correctly and extend its performance, it will turn out only when selecting the desired value of the value, as well as the charge rate. The higher the current, the higher the speed, but a high value of the charge speed leads to rapid degradation of the battery. Automatic chargers set current values ​​equal to ten percent of the battery capacity (0.1C).

To eliminate the effect of self-discharge, after the end of the charge, the charger switches to the recharge mode with low current. Some energy recovery devices are equipped with the ability to quickly charge, while the current rises to 1C of the battery capacity. It is often not recommended to use this mode due to the reduced service life of the energy cells.

Battery charging is completed if the charging current does not change for three hours.

Charger with voltage control

Devices operate in potentiostatic mode. The process itself consists of two stages, at the first the current is controlled, and at the second the voltage. The end of the charge occurs at the value of the current decrease by a set value or after a certain time. Lead acid and lithium ion batteries are charged using different algorithms than nickel cadmium and nickel metal hydride. For the latter, there are three charging rates: slow (0.1C), fast (0.3C) and ultrafast (1C). The battery charge stops when it reaches the set voltage value.

Requirements for chargers

12 Volt batteries are more widely used in cars and uninterruptible power supplies. On trading floors you can find ready-made automatic chargers for 12V batteries. The main requirements for them are as follows:

1. Current regulation. The charging device must be able to regulate the charging current both automatically and manually.
2. Accounting for heating. The charger must control the temperature. The temperature value of the battery changes during the charging process, while it is correct to change the voltage across it. For example, if the temperature rises by 5 degrees, the voltage on the batteries must be reduced by 0.1–0.2 volts. With strong heating, the charging process should stop.
3. Charging in several stages. The staging of the charging process in the charger allows you to extend the life of the battery. The first stage consists in analyzing the state of the battery and, if necessary, discharging it to the threshold value (eliminating the memory effect). The second stage is the charge by increasing the voltage and decreasing the current strength. At the third stage, recharging takes place while maintaining the minimum current and voltage.
4. Working temperature. The charger must ensure trouble-free operation in a wide range of operating temperatures.
5. All the stages that occur should be easily identifiable on the indicators of the device.
6. The charger must be short-circuit and overvoltage protected at the input and output.

All processes in automatic memory are controlled by microprocessors. Thanks to them, the digital device does not require intervention, and itself selects the required voltage and charge current. When using such devices, the possibility of overcharging the battery is completely excluded. Recently, in memory, they began to use not a constant signal, but a pulsed one, providing an effective and gentle mode. Of all the models on the market the following chargers can be distinguished:

• Нyundai НY400.
• Daewoo DW450.
• WelleAwO5-1208.
• Dexa Star SM150.
• Vitals 2415ddca.

Universal DIY device

Requirements for the device, protection of the battery from overcharging when the voltage reaches 13.7 volts. The power of the device itself is carried out from external source with a voltage of 20-25 volts. The charger does not contain scarce radio elements, is easy to set up and has short circuit protection.

An integrated circuit on the LM317 is used as a current regulator, its value is set by the SB1 switch. The second microcircuit is switched on according to the principle of voltage limiting. The required value is set by resistances RP2 and RP1. When the set voltage is reached, the charging process stops. Then, the battery can be connected at any time without fear of overcharging.

Comparator DA4 is used to drive LED indication. A two-color diode is used as an indication. Red color indicates preliminary discharge, green indicates charge.

When installing the battery, its voltage is compared with the second output of the comparator. The transistor operating in the VT1 key mode is open and the current passing through the leads of the LED causes it to glow red. The second and fourth inputs of the comparator receive a voltage from the Zener diode VD5, equal to 6 volts. Transistor VT3 is connected according to the source follower circuit. In a battery requiring charging, it turns off the voltage limiting assembly so that only the current limiter operates.

As soon as the voltage on the battery approaches the set value and is 12.8 volts, a high level will appear on the first terminal of the comparator. The threshold is set by collecting RP3 and RP4. Transistor VT1 closes and converts the second and fourth output of the microcircuit to inversion. The red LED goes out and the green one lights up. VT3 closes and the voltage limiter starts to work.

Stabilization of 12 volt power supply for the control and display unit is carried out using an integral stabilizer DA3 7812. Since the power keys are heated during charging, they must be installed on the radiator. The cooling system is turned on on VT4. If the radiator starts to heat up, the thermocouple sends a signal to the third leg of the comparator, which opens the VT4 transistor and turns on the fan.

Setting up with correct assembly and serviceable parts comes down to setting the required charging parameters. A signal of 20 volts is applied to the input and the presence of 12 volts at the 3rd leg of the comparator is checked. At terminal X2, without connecting the load, the voltage of 12.8 volts is set by the variable resistor RP1. The variable resistor RP3 achieves a state in which the LED glows green. RP5 is used to set the moment when the fan is switched on.