Alteration Of The Hitachi Screwdriver Charger

If you have (or were lying around with friends) an old Ni-Cd screwdriver and all the batteries are depleted, and even the charger is broken (burned out), order an MT3608 card for 40p on Ali, look for the old charger from your cell phone (in bulk) and old laptop batteries (of which we need Li-Ion 18650 batteries).

Today we will remodel the old screwdriver from nickel-cadmium batteries to lithium-ion, and accordingly modify its charger.

Everything is easily redone. Let’s start with the batteries.

If the screwdriver was at 12V, we will need 4 18650 batteries (16.8V maximum), if at 14.4V. 5 pcs (21V maximum), if at 18V. 6 pcs (25.2V maximum). Safety margin of electric motor and other mechanisms in a screwdriver big, and we increase the power not prevent.

First, 18650 Li-Ion batteries are tested, if there is anything to choose from, they are selected with the same capacity. This can be done expensively and accurately using the BT-C3100 V2.2 or similar. The factory capacity of 18650 batteries from 2000-2200 mAh laptops is written on them, if not, then it can be divided into 2 capacities written on the nameplate of the battery. Run will be better if you give 3 charge / discharge cycles. If the measured capacity is 5-10% lower than written, then it is permissible, but if the capacity is much lower, then the batteries have lost capacity. We also measure the internal resistance of the batteries in the device and it should also be the same.

Without precision instruments, it is enough to charge Li-Ion 18650 batteries up to 4.2V in any suitable charging with a voltage limit, give the same load and measure the voltage across them at the same time. If it fell to the same value, then normal. For example, we load a fully charged 18650 with a load of 3-5 Ohms (current from 1.5 to 0.8 A), and after the same time (for example, three minutes) we measure how much is left from 4.2V under load and without load. If the final voltage under load and without load is the same, the batteries are suitable. This indicates the same load capacity and the same internal resistance.

We throw out the old dead / closed Ni-Cd batteries from the body of the replaceable battery, and instead of them we seal Li-Ion to the voltage we need. For Li-Ion batteries themselves, it is better to leave flat connectors from the laptop, but if you still solder wires to the Li-Ion, cool the solder by blowing, solder quickly with flux or acid to reduce the heating time of the battery surfaces, in order to avoid damage. Take soldering wires from an old computer PSU, or thicker.

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It will be better if the batteries are soldered through the charge balancer board: “4S or 6S balance protection board”, it will not allow the batteries to recharge above 4.2V. Also, multi-capacity batteries will be better charged through such a board, but in the case of different batteries, less capacious ones will degrade much faster, because they will be discharged below the minimum voltage of 2.8V, while there will still be a voltage reserve on more capacious ones. The screwdriver still turns, but weaker batteries are already degrading.

Then we check how the screwdriver turns on Li-Ion batteries, usually this is an increase in power by 20-40% and a decrease in the weight of the replaceable battery.

Now we turn to the rework of charging, especially if it burned out, or if it is not. At different companies they are different, Bosch, Shturm, Hitachi, everything is different. From the charging case, you can get all the stuffing, except for the terminal block. By and large, we only need a terminal block for connecting a removable battery. Of course, in the case everything will be better. I was too much current from a heavy transformer, and it was heavy, so I found him the best application (in a laboratory PSU).

Video: Alteration Of The Hitachi Screwdriver Charger


We solder the charger output for the cell to the MT3608 board on the VIn contacts, plus, minus. We turn on, we twist the resistor to the output voltage we need, it is 16.8, 21 or 25.2V, respectively, what kind of Li-Ion battery you have.

MT3608 is a step-up voltage converter with pulse-width modulation, on conventional boards output capacitor needs to be soldered to the large contact of the VOut output and, accordingly, clean the ground next to it to solder the capacitor. This is the imperfection of the Chinese, the board works poorly from the factory.

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Alteration Of The Hitachi Screwdriver Charger

We do a charge current limitation, for this we need a 5-15 Ohm resistor and the simplest and smallest diode. Solder the VOut plus wire directly to the terminal block on the battery plus. And VOut- through a resistor in the negative wire. From the measuring point of the resistor a diode (anode) we solder (a cathode with a strip) to the FB signal of the microcircuit, this is the 3rd contact of the MT3608, it is small, but it rings on the potentiometer on the other side of the board, where it is easier to solder.

We connect the battery for charging and check the charge current, it will be from 50 mA (15 Ohms) to 200 mA (5 Ohms). Accordingly, the current from the cell charge will be, for example, 50mA (21V / 5V / Efficiency) = 300mA, and for 200mA (21V / 5V / Efficiency) = 1200mA (it may be too large, not every cell charge will pull it). We check the charging, if it is warming up or the voltage from it drops from 5V to 2.5V, then the current should be reduced in order to avoid overheating.

You ask why such a small charging current, because it will take a long time to charge. The first moment, at high charge currents close to 1.0C (C-capacity of Li-Ion battery), the charge time is about an hour, the battery definitely dies after 1-2 years of such atrocities. Secondly, even old Li-Ion batteries tend to recover at low charging currents (unless of course the chemistry has flowed and swollen), and low-current charging will definitely extend the life of the battery. You can see https://www.youtube.com/watch?v=ep8o8DVPz_0 to study the issue.

Minuses: long time full charge (10-20 hours). It is highly undesirable to put Li-Ion batteries below 3V per cell, that is, to do a full discharge (when the screwdriver turns much weaker), Li-Ion batteries lose their capacity much earlier in the cold, even at 0 degrees the screwdriver will work a little (You can put on a glove or scarf or a scarf only on the battery of the screwdriver for short-term work in the cold, or to heat only the battery indoors on the heating battery).

Instead of charging from a cellular, you can take 5V or 12V from a computer power supply unit or a power supply unit from a router / modem.

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Somehow I came across a very weak Chinese battery from a cell. Written 5V, 450mA. Even at 21V 50 mA, the MT3608 overloaded the charging and the output voltage dropped to 2V, charging started to boil. What had to be redone:

First, I made a voltage limitation of the beginning of the Uin conversion for MT3608 (so that the converter does not translate the charger BP into the 2V 2A state, when everything started to get very hot and burn) In the diagram of simple parts, the resistor R2 can be replaced by a trimmer by 1-10-100kOhm (optimally 10k and R1 10k then). This made it possible for the StepUp converter to start only from an increased input voltage, the maximum current for Chinese charging was at a voltage of 4.3 V, if you slightly increase the trimmer, the converter stopped working and the voltage jumped to 5V.

I also wanted to raise the charging current, 21V 80 mA was not enough.

The higher the voltage on the secondary winding of the high-frequency transformer of the power supply converter, the more power can be removed at the same current (and the maximum current depends on the cross section of the wire), but you can go to the saturation or overheating of the transformer, and the power supply circuit can go into protection or burn out.

At the output of the charging PSU there is a feedback optocoupler and a 3-4 Volt zener diode or resistors for 5.2V stabilization. I was lucky and got a charger including a protective zener diode for 7.5V, which I soldered instead of a measuring zener diode, and got a 9V charger at the output. Above 10V, it is better not to overclock the charger for the cell, usually at 11-12 volts there is a failure of stabilization.

As a result, I twisted up the limitation of input voltage consumption by 8.2 volts, received 21V 140mA at the output of the converter, as a result, 13 hours of charge for my 2000mAh 18650 batteries is normal.

Tags: from improvised parts, can be found everywhere, easily accessible, easy to remake, simplicity, easier when there is nothing.

Leave comments, share experiences, advise who has what happened, how best to redo. If you shoot a video on the alteration, post the link here.