Charge the electric screwdriver from a laptop charger

An electric cordless tool is equipped with a factory charger that allows you to replenish the capacity of the battery. If the unit is lost and broken, the owner has the question of how to charge an electric screwdriver without a charger. In addition to purchasing the original unit, there are several techniques for recharging a power source from household or automotive equipment.

How a charger works

In case of failure of battery charger it makes sense to try to restore it first. For the repair it is desirable to have a circuit diagram of the charging device and a multimeter. The circuitry in many of the charging devices is based on the HCF4060BE chip. Its circuitry forms the charge interval time delay. It has a crystal oscillator circuit and a 14-bit binary counter, which makes it easy to implement the timer.

The principle of the charger circuit is easier to understand with a real example. This is what it looks like in an Interskol screwdriver:

This circuit is designed for charging 14.4 volt batteries. It has a LED indicating that it is plugged in, LED2 is on, and the charging process, LED1 is on. As a counter it is used chip U1 HCF4060BE or its analogs: TC4060, CD4060. Rectifier is assembled on the power diodes VD1-VD4 like 1N5408. PNP type transistor Q1 works in switch mode and controlling contacts of relay S3-12A are connected to its pins. The operation of the key is controlled by the controller U1.

When switching on the battery charger, the AC voltage of 220 volt is fed through the fuse to the step-down transformer, which has an output value of 18 volt. Then, passing through a diode bridge, rectified and goes to the smoothing capacitor C1 with a capacity of 330 uF. The voltage across the relay is 24 Volts. When the battery is connected the contact set of the relay is in open position. Microcircuit U1 is fed through the regulator VD6 with a constant signal of 12 volt.

When the “Start” button SK1 is pressed, the stabilized signal through resistor R6 arrives on the 16th output of controller U1. The switch Q1 opens and a current flows through it to the relay outputs. The contacts of the device S3-12A close and the charging process begins. The diode VD8, connected in parallel to the transistor, protects it from a voltage spike, caused by switching off the relay.

Used button SK1 works without fixation. When it is released, all the power is fed through VD7, VD6 and the limiting resistor R6. And also the LED1 is energized through the resistor R1. LED1 illuminates, indicating the charging process has begun. The chip U1 is set to run for one hour, after which the power is removed from the transistor Q1 and, accordingly, from the relay. Its contact group opens and the charging current disappears. LED1 light turns off.

This charger has overheat protection circuitry. This is accomplished with the help of a temperature sensor, the SA1 thermocouple. If during the process the temperature reaches more than 45 degrees Celsius, the thermocouple is triggered, the chip receives a signal and the charge circuit is broken. After the process is complete, the voltage at the battery terminals reaches 16.8 volts.

This way of charging is not considered intelligent, the charger can not determine in what state the battery is. Because of this the duration of operation of the screwdriver from the battery will be reduced due to the development of the memory effect. That is, the capacity of the battery decreases each time it is charged.

What is the optimal amount of time to charge?

Usually the time required to fully charge the battery is indicated in the instruction manual for the screwdriver. It is advisable to follow these instructions. Often the charger has indicators that tell you when the procedure is complete and when it is finished. They will help you understand how long to keep the battery on charge. Remove the battery from the charger after completing the procedure so as not to damage it.

Usually an electric screwdriver battery takes from half an hour to 7 hours to charge. For example, nickel-cadmium batteries with a capacity of 1.2 A/h will take about 7 hours to fully charge with a 250 mA current.

Battery chargers come in two types, conventional and pulsed. The first ones are designed for household screwdrivers and charge a battery in 3-7 hours. The latter are used in professional equipment and can restore the capacity within 1 hour.

What to do if the battery does not charge or does not hold a charge

In this case, there are few options: either the cause of the malfunction is in the charger, or the electric screwdriver “fails” itself. Even the battery pack can wear out over time and need to be replaced. In order to find out the reason, you need to carefully examine the tool itself, as well as its battery pack together with the charger.

Often the reason for poor battery charging is that the contact between the electric screwdriver and its charger becomes loose due to unbent terminals. To fix this problem, it will be enough just to disassemble the battery charger and carefully bend back the terminals.

We should not forget about such a common problem, as oxidation of the metal parts of the battery and the charger itself. Construction dust and dirt can also reduce the charging current from the battery to the charger: Tools are not as good at charging. It is important not to forget to take care of all the components of the tool in order to prevent deterioration of its performance by wiping the metal contacts and cleaning them of dirt.

In the event that the battery itself is dead, you can try to “rock” it, as is usually done in the case of nickel-cadmium cells. If this does not help, you will either have to replace the battery pack completely or do a partial replacement of the cells.

Generally speaking, any electric screwdriver is equipped with two identical batteries. If one of them fails, you can assemble one working battery from two if you want, in case the capacity has become less in both. Once you have assembled one working cell from two blocks, you must not forget to equalize the capacity of the cells, “pumping” the battery with several charge-discharge cycles within 3-4 hours.

You can also try to “revitalize” the batteries individually. For this purpose it is necessary to expose the weakest of them to high currents, and then assemble the battery back and charge it already in a normal mode.

This method sometimes works in the case of nickel cadmium batteries. “Spot” charging with high currents should last no more than 3-5 seconds. In this case, it is desirable not to allow strong overheating of the element in order to avoid its destruction.

Converting an electric screwdriver to lithium, part two, charging properly

Last time I wrote about how to rebuild the battery for a cordless tool. I also wrote that I would talk about the features of the charge, and the subject of the review this time will be the DC-DC converter board. If you are interested, please visit.

Originally I planned to limit myself to two parts, converting the battery and charger. But while I was preparing the review, the idea for the third part matured in my head. And in this part I will tell you how you can remake the native transformer charger, if it still works, or if the power transformer is still alive.

The converter board was ordered quite a long time ago in quantity of several pieces (as a reserve), it was ordered specially for this redesign because it has some features, however I won’t go too far, let’s be consistent.

First I will divide the chargers into three main types: 1. The simplest. transformer, diode bridge and a few parts. These chargers are used for ultra-budget tools. 2. Branded. Essentially the same, but already contains simple “brains” that automatically shut off the charge at the end. 3. “Advanced”. switching power supply, charge controller, sometimes charge several batteries at the same time.

Instruments of the first category seldom get under the alteration, since it’s often easier (and cheaper) to buy a new one, and the third category usually has its own difficulties on alteration. Basically it’s possible to remake devices of the third group but not within the limits of this article because there are many types of such chargers and each needs individual approach.

This time I will be remaking the charger from the second group, a branded, although simple. But this redesign has a lot in common with the first group, so it will be useful for more readers.

In order to charge the battery you need not just connect it to the power adapter, such an experiment usually ends not very well. You need to connect it to a battery charger. And here comes a slight misunderstanding, since quite a few people are used to calling chargers small power supplies from which they charge their smartphones, tablets and laptops. They are not battery chargers but power supplies.

What’s the difference between battery charger and power supply. Power supply is designed to deliver a stabilized voltage within the range of the stated load currents. The charger is usually more complicated, because its output voltage depends on the load current, which in turn is limited. In this case there is a unit in the charger which stops the charge at the end and sometimes it’s protected from connecting the battery in wrong polarity.

The simplest battery charger is just a power supply and a resistor (sometimes a light bulb, which is even better) in series with the battery. This circuit limits the charging current but as you understand there is nothing more it can do.

It is a little more complicated when they have a timer that shuts off the charge after a certain time but this principle quickly “kills” the batteries. For example it is made in one of the inexpensive screwdriver battery chargers (not my photo).

The next class are more “smart” chargers although in fact they are not much better than the previous one. For example here is a photo of Bosch brand charger designed to charge NiCd batteries.

But all these chargers look very simple after looking at modern options for charging lithium batteries.

Of course the last option doesn’t quite fit in our re-design concept since it would be desirable for our charger not only to charge properly but also cost a minimum of money.

charge, electric, screwdriver, battery

The chargers of Chinese screwdrivers certainly look much easier, but then again, to make from scratch such a device is unlikely who wants, although that’s what I plan to do in the third part, though more accurately.

And so, to begin with let’s assume that we have a battery charger which simply does not fit the new type of batteries, but is serviceable. or at least it has a functioning transformer. As I wrote above you can even use just a resistor or a light bulb, but it’s “not our way”.

A notional diagram of a typical inexpensive charger looks like this: Transformer, diode bridge, thyristor and control circuit. But sometimes instead of thyristor you have relay, current is not limited and there could be circuit of thermal control from overheat (though it doesn’t always save you).

But we only need the transformer and the diode bridge from this circuit, although we have to add a capacitor too, so we get a certain initial unchanged part, it’s marked in red and won’t change further.

The diode bridge is usually on the board and can be used if necessary (if it is in good working order). Т.е. Generally speaking, you can unsolder all the radio elements from the board, leaving only four diodes and terminals to connect the battery, and use the board itself as a base. The cathode of diodes is marked with a line, the point where you connect two pins with the line is plus, and the point where you connect the “unmarked” pins is minus. To the other two connection points a transformer is connected.

charge, electric, screwdriver, battery

Really having opened the charger you can see and such picture (do not pay attention to absence of the transformer): In this case it is necessary to unsolder all.

The variants of power supply

Any electric screwdriver requires much less voltage than a conventional outlet. That’s why you need a special converter for feeding, which outputs the necessary voltage. All power supplies fall into two broad groups: switching and transformer. Let’s look at each of them separately.


The principle of operation of switching systems is that the voltage is first rectified, and then converted to a special pulse signal. It is important to achieve a stable voltage. A transformer winding or resistors can help.

Switch-mode power supplies are quite effective and can be used in different conditions. But they have a high level of protection against short circuits and similar effects. However, pulsed systems are clearly inferior to transformer systems in terms of power. Besides such units are very picky about the input voltage. If it is lower than the set value, the element may simply not work.


common power supplies, which have proven their reliability and efficiency in many applications. Consists of a step-down transformer and a rectifier, through which the undervoltage is passed. Rectifiers can be different, depending on the number of diodes used.

These cells are simple to make, cheap and reliable. So they are often preferred. They provide a stable voltage without interference with high maximum power. But there are a few disadvantages. The main disadvantage is that it is bulky, with a much lower efficiency than pulsed sources. This fact requires selecting a power supply for an electric screwdriver with more power than the tool needs. Since part of the power will be spent on side processes.

How the battery of an electric screwdriver is designed

The battery is the power source for cordless screwdriver models. The device is based on electrolysis. This is the name of the physical and chemical process during which this element stores electricity. Then it outputs a DC voltage of the desired value at its corresponding outputs.

Voltage and capacitance are the main parameters of any battery. Voltage shows what the potential difference between the cathode and anode of the battery. It is measured in volts. Capacity determines the amount of current a battery pack delivers in one hour. This parameter is measured in ampere hours.

There’s a variety of models. And yet the battery packs are similar to each other in their design. The battery necessarily consists of such structural elements:

IMPORTANT! The case is usually a two-piece plastic box. It usually has about 10 batteries inside. In some cases there are more. Elements are connected to each other and form a chain.

The free terminals of the outermost batteries are connected to the contacts which are located on the case. The contacts are designed to power the electric motor of the tool and connect it to the charging equipment. In order to determine the output voltage of a battery, it is necessary to summarize the parameters of the batteries that are connected in a single circuit.

Reconditioning a nickel cadmium battery

If you have an old cordless screwdriver, or a rare model, the tool can be equipped with a NiCad battery.

Such batteries have a life time of 20-25 years, withstand up to 900 charge-discharge cycles and may be stored in a discharged state.

To restore the battery you can take any charger, able to give the necessary parameters of voltage, as well as current, equal to 50% of the capacity. over, such batteries can withstand without problems, and current of 2 times the capacity.

Connect the charger and wait. If the recovery process starts, everything is fine. Wait until it is finished and the screwdriver is ready to go. The voltage should be 1.37 volts times the number of cells in the battery. At this point the charger should be disconnected. But this is not a guarantee that the screwdriver is fully charged.

Nickel cadmium batteries are characterized by a memory effect. If they have been frequently discharged to 50-70% and then connected to a charger, eventually the units will get used to it and will continue to stop working as soon as the charge goes down to this 50-70%.

Therefore, it is necessary to discharge the nickel-cadmium type battery with the same current parameter as used for charging. Discharge to 1 V times the number of cells in the battery. The load can be a screwdriver or a car lamp. Then fully charge it. And this procedure of charging and discharging should be repeated 3-4 times.

If after the manipulation the capacity was lower than the stated, then the screwdriver will not be able to return to full operation.

Now you know how to charge a dead screwdriver battery without the original charger. There are many methods. But not every method is recommended on a regular basis. Remember that the best solution is to use the original SAE.

Do you have a cordless screwdriver?? What kind of battery does it use?? What do you charge it with?? What other method of charging has been used??

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