18V Dedra Screwdriver Battery Charger Overheating & Resistor Modification: Ni-MH 2Ah Charging

Hello

I have a problem charging the Dedra brand screwdriver battery, which charger looks like this:

The charger was terribly hot, I also undressed it and its diagram is given:


Without the battery for charging, the R1 resistor (250? was heating up) , and after connecting the battery, the R2 resistor (4.2 ? was also warming up) . Both warmed so much that after a minute it was impossible to touch them.
I changed the R1 resistor with 250? to 1.2k?, and I changed the R2 resistor with 4.2? to 15?. The diagram now looks like this:


After changing the resistors, the battery (18V, Ni-MH, 2Ah) is charged with a current of 250mA at 20.7V and please look at the modification and prompt if the voltage is correct?

I have 2V more on the original charger.
250mA is a bit too low, it will charge for a long time.
I had resistors when the diode went down, check if it is working properly.

The diode is OK and the charger is slightly warm after 7 hours of charging. Earlier, after 5 minutes it was burning. I will add that this is the second charger, because the transformer in the power supply burned out first and I bought a set, i.e. the power supply plus charging foot. Strangely enough, this bought charging foot had other components and SMD. Unfortunately, it burned so much that it is impossible to reproduce the diagram. So I returned to the first charger and modified it a bit to make it last longer.
The power supply is not a complete set, 24V (exactly 24.3) and 1.5A because 400mA also gave a lot of heat.

I will add to the information that after about 8 hours of charging, the charging current is 150mA and the voltage is 21.6V and I thought the battery was charged.
Now I have connected a 1.2Ah Ni-Cd battery and charging with 370mA current at 18.5V.

After 15 minutes of charging, the charger is quite warmed up and in my eye it has something around 50 ° C, and the charging current has increased to 390mA.

I'm learning. Such a question: diode D1 should not be drawn the other way around?
And in addition, it seems that the battery has 15 Ni-Cd rechargeable batteries, which at maximum charge will have a voltage of 21V.
They can be held on a 1.2Ah / 20 backup, which gives a current of about 6mA. How are you (24.2V - 0.7) / R = 0.06 A is R = 12 Ohm. How is 15 is also ok.
When the battery is flat, you have a voltage of approx. 15V on it. Then charging takes place with a current of about 0.5A.
The LED L1 lights up when the power supply is connected. The L2 diode lights when the current flowing through R gives the appropriate voltage, i.e. when the battery is being charged by some current value. No ordination does not mean no charge. It's just such an indicator. Charging should take about 16 hours. For my knowledge, you have converted this power supply well.
Let the wiser ones say whether I have analyzed it correctly.

WRadek wrote:

diode D1 shouldn't be drawn the other way round?

She should.
But this is probably just an error in the drawing, as if it was the case, it would not have charging.

It looks like I actually made a mistake with this diode and I will check it physically but later, because the screwdriver is already hidden.
The original battery has 15 cells of 1.2V and 1.2Ah, but I'm already finished and I'm using a battery of 15 cells Ni-MH, 2Ah (Eneloop), which occasionally need to be charged every few months.
The L1 diode shines after supplying power, while the L2 diode when the battery is loaded (plugged in) and slightly dims as the battery charges.
The discharged battery (Ni-MH) has a voltage of 18V (the screwdriver will spin a few turns), and a charged 21.6V.
I selected the resistors from what I had, guided above all so that R2 does not heat up dramatically and I hope that now leaving charging on overnight, in the morning I will get up and the charger will be all right.