Trying to use existing 18v li-ion charging cradle

[jgarnold]

Not long ago the power supply for my rechargable drill stopped working and the manufacturer cannot supply a replacement.
The old power supply is rated at (output) 21 volts at 1.4A and the charging cradle has marked on it to use use that model power supply with that voltage.
The only power pack I could find which has an output similar is a multiple voltage power pack rated at 2A and with selectable output voltages of 20 volts and 24 volts.
When I connect with 20 volts the green led on the charging cradle never comes on (it comes on when the battery is fully charged) and in fact the battery never shows 18 volts on my mutimeter (when removed from the cradle of course).
When I connect with 24 volts the green led flashes for a minute or two and then remains fully on BUT the battery is not fully charged.
Could it be that the circuit in the cradle will only work correctly when 21 volts is used (and not 20 or 24 volts)? I don't understand electronics enough to know.
I can purchase a step down power converter voltage regulator to drop the 24 volts to 21 volts but not sure it this is the solution.
Any suggestions?

 

[Audioguru]

You forgot to tell us the chemistry of the battery. If it is 5 Li-Ion cells then it needs 21.0V and a proper lithium-ion charging circuit which might be in the original power supply or in the cradle.

A hobby store will have a charger for 5 Li-Ion cells.

 

[jgarnold]

Thanks for your reply.
I have no idea how many cells as being for a rechargeable drill it is a sealed battery pack.
It states that it is an 18 volt rechargeable drill. I have seen other 18 volt rechargeable tools and the power supplies for those (no they don't fit mine) are labeled as 18 volt. I cannot find (Google search) a power supply stated to be 21 volts here in Australia and that is why I purchased the variable one which has fixed voltages of 20 and 24 volts hoping it would work. Can I assume that if I purchase a step down power converter where I can adjust the power output to 21 volts that it will be successful in work correctly in recharging the 18 volts battery pack?

 

[jgarnold]

After 'googling' various things I find to achieve a 100% full charge each cell needs to be charged at 4.2volts so for 5 cells then 21 volts is required. I am now assuming that I need to supply 21 volts and I also assume that if I use 24 volts then the cradle will probably shut off incorrectly. Am I assuming correctly and therefore purchasing a step down power converter to reduce the 24 volts to 21 volts then I should be able to achieve a full recharge?

 

[Harald Kapp]

Am I assuming correctly and therefore purchasing a step down power converter to reduce the 24 volts to 21 volts then I should be able to achieve a full recharge?

Could it be that the circuit in the cradle will only work correctly when 21 volts is used (and not 20 or 24 volts)?

That may be the case. Nothing can be guaranteed from your description as we do not know how the charging cradle's electronics are constructed.
Before you buy a step-down converter, you can try to lower the input voltage to the charging cradle by adding 5 diodes (any silicon type rated at 1.4 A or higher will do, e.g. 1N5059) in series between the power supply and the cradle. Each diode will drop ~0.6 V which in total makes for a 3 V voltage drop. You are then left with 21 V at the input of the charging cradle.

 

[Audioguru]

When a Li-Ion cell reaches its max allowed voltage of 4.2V it is far from a full charge. The charging circuit keeps charging it until its charging current drops to 1/10th or less of its mAh rating, then the charger shits off.
EDIT: Maybe your charger shits off at 21V.

Nobody makes an accurate 24VDC to 21VDC converter. Usually when 21V is needed then 21V is made.
You can risk a battery explosion and fire by using a transistor and two bias resistors to make a 3V reducer but its voltage will change when the temperature changes.
EDIT: 5 diodes in series also change their voltage when the temperature changes.

 

[Harald Kapp]

5 diodes in series also change their voltage when the temperature changes.

Yes they do. But considering the charger will likely be used in a moderate environment (0 °C ... 30 °C - admittedly pure speculation) the change in voltage due to temperature will be negligible. Probably the temperature of the diodes or a transistor circuit will be determined by the power dissipation anyway, less by the environmental temperature.

Nobody makes an accurate 24VDC to 21VDC converter.

Allow me to disagree. You can buy comparatively cheap step-down converter modules in that voltage range, for example this one. One may discuss their accuracy, but I doubt the charger will require better than +-10 % accuracy on the input voltage.

Usually when 21V is needed then 21V is made.

Right. Here's an example of an adjustable power supply which could be set to 21 V.

 

[jgarnold]

Thanks everyone for your replies.

Firstly the charging cradle has on it that it MUST be used with the power supply model number it came with which has on it (the broekn power supply) OUTPUT 21 volts 1.4A so maybe it MUST have a 21 volt power supply for the circuit to operate correctly?

I LIKE the idea of placing diodes in series to drop the voltage by 3 volts. Problem is that I haven't found anywhere in Australia where I can purchase suitable silicon diodes cheap enough. The step down power converter I was thinking of purchasing which IS adjustable via a 'multi loop potentiometer' to allow the output voltage to be adjusted (the description mentions to turn the screw up to 10 revolutions). it uses an LM2596.
My local electronics store has a 1N5404 or 1N5408 both rated at 3A BUT the forward voltage drop is 1.2 volts so maybe I could try out using 2 (voltage drop of 2.4V giving me 21.6).
I can't source any 1N5059 in Australia in small amounts and my local electronics store doesn't have any silicon diodes of at least 1.4A less than about $2 each and so the ADJUSTABLE step down power converter would be half that cost (it uses an LM2596).

 

[Audioguru]

For Chester hill Australia go to www.farnell.com and click on the Australian flag. It is one of the many Farnell (based in the UK) electronic parts distributors all over the world.

 

[jgarnold]

Thanks for that info Unfortunately the link (flag) to the Australian site does not work. I found the web site and it now is https://au.element14.com/

 

[Audioguru]

I get to Farnell flags by keying farnell.com into Google. The places ln the world are called Element14.

 

[Harald Kapp]

maybe I could try out using 2 (voltage drop of 2.4V giving me 21.6).

It's worth a try at low cost.

Firstly the charging cradle has on it that it MUST be used with the power supply model number it came with which has on it (the broekn power supply) OUTPUT 21 volts 1.4A so maybe it MUST have a 21 volt power supply for the circuit to operate correctly?

That labeling is typical. Manufacturers want you to use their components so your money goes to them not to another manufacturer who may be able and willing to supply a matching component at a lower cost. It also makes it easy for the manufacturer to blame someone else if you do not use original components.

Not long ago the power supply for my rechargable drill stopped working and the manufacturer cannot supply a replacement.

You may be able to repair the original power supply. Have you tried to open it and look inside? Often a small repair can go a long way.

 

[jgarnold]

Thanks for your reply.
Yes I realise that manufacturers only want you to buy their products but I had wondered though if the power supply MUST be 21 volts for the charging circuit to operated correctly. As mentioned before supplying it with 20 volts never results in the green led being lit to indicate fully charged and supplying it with 24 volts causes the green led to come on almost immediately suggesting the battery is fully charged but it isn't.
I have opened (with difficulty as it was 'welded' shut) the power supply but did not see anything obviously wrong (eg burnt devices, burnt mark on pcb, swollen devices etc) and my knowledge of electronics is not much apart from visually looking for possible problems.
I am sure it is a switch mode power supply (aren't most built in the large few years for the consumer market?)
I have attached a photo of the power supply. I checked the (large) 47uF electro with my multimeter (set to resistance) and it displayed a reading one way and no reading the other. I also checked the resistance across the large diode (marked with a red arrow) which shows nearly a short circuit (1 ohm) in both directions but that could be picking up other connected devices. I can't read any more than SB5 printed on the side of it.
The small red diode indicates that it is on when it is working. Note the red diode is near the output of the circuit, the other end is the inlet side (prongs on other side for inserting into 240 volt ac power point)

 

[Harald Kapp]

There is a fuse at the lower left side (label F1. the cylinder with the separately isolated lead). Check for continuity.
Does the red LED light up?
Measuring the capacitances with a multimeter in Ohm range doesn't tell you whether they are good or not. Measuring components in circuit doesn't yield good results anyway (other components may mislead your readings as you noticed yourself).
This looks like a fairly simple supply, not overly sophisticated. The 21 V are surely not regulated to very tight tolerances. So a replacement set to 21 V should work. I wonder whether it is really the power supply that is at fault. Could it be the charging cradle instead? Do you have the possibility to test the same cradle against another power supply, or the same power supply against another cradle?

 

[jgarnold]

Thanks Harald
Yes I know that caps can't be tested with a multimeter in ohms range however I have noticed in the past that placing the probes on the leads (of an electrolytic cap) usually shows a reading which quickly changes and then swapping the leads it does the same as an electrolytic stores a charge. When I tested the 47uF it displayed a value which never changed.
I tested the device marked f1 (fuse) but it is a closed circuit so indicates it is not blown? Maybe I will cut the leads on that diode and test (out of circuit). The power supply has a fly output lead with a dc plug on the end which shows no voltage and the red led indicator does not light and when plugged into the cradle the led indicators on the cradle do not come on YET they come on when the cradle is connected to an 18v or 24v supply so I am confident it is the power supply which is at fault.
Anyway I will get some diodes to drop the 24 volts (on the multi voltage power supply) and see if it works better OR buy a step down power converter (with LM2596 and variable pot) which is variable and so I can adjust to to a 21v output.

 

[jgarnold]

Added later.
I cut the lead to that large diode (the one marked with the red arrow) and tested the diode. I presume it is a diode as the pcb is marked D3 next to it and it has a band around one end typical of a diode but I cannot read what is printed on it and what I can read is not familiar. It shows a continuity (less than an ohm) in BOTH directions. I will replace that diode but with what type? One with a forward voltage drop of 0.7v or 1.2v eg 1N5404. Obviously it needs a power rating of 2A or better such as the 1N5404.
Or maybe a 1N5822 Schottky rated at 3A?

 

[Martaine2005]

What device is on the heatsink?.
I can’t see that PSU being very accurate unless it has other circuitry on the other side.

Have you got continuity/diode setting on your multimeter?.

Have you checked the output cable for a short?, these cables often get twisted over time and the insulation breaks right at the strain relief. This short will (should) shut the PSU down.

Martin

 

[jgarnold]

Thanks for your reply.

I do not know what device is mounted on the heatsink (no markings). It has 3 'legs' soldered to the pcb.

There are quite a lot of very small devices on the other side of the pcb.

Using the diode setting on the multimeter it 'beeps' with the probes in both 'directions' on that diode (which is now out of the circuit). Surely this indicates it has failed. If it has failed with closed circuit in both directions maybe it has also caused other components to fail?

The output cable was cut off after the power supply failed and it was still not working before I cut the leg on the diode (see above). I measured the resistance across the output and it shows closed circuit for a fraction of a second then open circuit. Surely if it was shorted out it the multimeter would show closed circuit continuously.

 

[Harald Kapp]

Maybe I will cut the leads on that diode and test (out of circuit).

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

I measured the resistance across the output and it shows closed circuit for a fraction of a second then open circuit.

Sounds like a capacitor being charged.

 

[Martaine2005]

If the diode is shorted both directions out of circuit, it is most definitely defective.

Martin