Trying to use existing 18v li-ion charging cradle

[jgarnold]

If you cut, then putting it back in place will be difficult. Better de-solder one lead. Oh, I see, too late.

I decided to do that as the power supply did not work and not being able to test other components on the pcb I felt it was destined for the bin in any case. A last resort you might say. I will replace the diode as they are very cheap but don't hold much hope. Looks like it's the 24v power supply with 5 schottke diodes in series is the next option.

 

[jgarnold]

Some success. I purchased a couple of 'ordinary' silicone diodes and desoldered the 'dead' diode from the circuit board and replaced it. The red led on the power supply now is lit and the output voltage is 21.2V - it is rated at 21.0v
I connected it to the charging cradle and connected a battery. BOTH the green and red led came on immediately indicating that it is fully charged and it isn't.
A short time later the green led was not lit indicating it is charging.
Could someone please explain why these batteries have FOUR pins (2 for the charge current but what about the other 2)?
Also the 'dead' diode is a SB5150 (I couldn't read that until removed from the board).
I have found that it is a schottke diode. Would it matter that I used an ordinary diode in its place which has a higher forward voltage drop.

 

[Harald Kapp]

the output voltage is 21.2V - it is rated at 21.0v

Sounds fully o.k., forget the decimals.

why these batteries have FOUR pins

Afaik, usually the other pins are sense pins for charge balancing the different cells within a battery pack.

Would it matter that I used an ordinary diode in its place which has a higher forward voltage drop.

1. Dissipates a bit more power within the power supply.
2. The SB5150 is also designed specifically for use in fast switching inverters. An ordinary diode may be too slow. I suggest you replace the ordinary diode by a SB5150 or equivalent.

.

 

[Kiwi]

What is the make and model of the drill?

 

[jgarnold]

What is the make and model of the drill?

Its an OZITO brand mode; LCD5000 which is the marketed name for a hardware chain in Australia and probably not available anywhere else.

I suggest you replace the ordinary diode by a SB5150 or equivalent.

The only equivalents I can find are the 1N5822 (DO27) and the MBR735 (TO-220).
They are rated at 40V 3A and 35V 7A respectively. The SB5150 is rated at 150V 5A.
The power supply has on its label that it is rated at 21V 1.4A.
Would the 1N5822 be ok?

 

[Harald Kapp]

1N5822 looks promising.

 

[jgarnold]

Strangely when I changed the diode to a 1N5822 schottke the red led started flashing slowly (usually remains steady) and then stopped. I checked the diode and it was shorted in both directions just like the original one. I put the ordinary diode back and it works again and it appears to charge the battery ok.
Could there be something else wrong which causes the schottke diodes to short out?
What could go wrong using a normal diode instead of a schottke?

 

[jgarnold]

Could the 470uF 35V electro cap next to the schottke diode be causing them to break down (short out in both directions) - note that ordinary diodes don't break down.
I have learned that this one is commonly used as filters in devices in various power supplies to reduce the voltage ripple as well as input and output smoothing as an LPF.
I know that they can leak etc but don't know if it could cause the schottke to break down. Sorry my knowledge of most devices is limited.

 

[Harald Kapp]

Yes it could. Usually the capacitor is charged and with rising voltage across the capacitor charge current decreases.
If the capacitor has leaked, it may not charge to the full voltage, thus requiring permanent charge current which may stress the diode over its limits. The non-Schottky diode may simply be more resilient to stress than the Schottky. It's a good idea to replace the electrolytic capacitor, too. Please note the correct orientation (polarization) when reinstalling the new capacitor.

 

[jgarnold]

Thanks Harald

 

[jgarnold]

Something strange
If I measure the voltage of the 18v li-ion battery with no load my multimeter displays 5.38 volts DC YES just 5.38V.
When I place a small load across the battery terminals (680 ohm - wire wound resistor rated at 5 watts) the multimeter displays 17.16 volts DC.
Could someone tell me the reason for this?

 

[Harald Kapp]

Not sure but the battery may have an internal BMS (battery management system) that powers the output down when no load is applied.