NiCad Power Tool Battery Charger Help Needed

[billd]

The battery charger for an 18 volt portable drill/driver has burned out and the company is no longer in business to provide a replacement. The circuit charges an 18 volt NiCad battery which is made up of 15 1.2 volt cells each rated at 1200mAH. The original AC power supply, which also burned out, is rated at 120v AC 60Hz input and 18v DC 400mA output.

I attached a photo with a crude diagram along with the actual circuit board. I need help identifying the suspected fried circuit component (the large gray one with all of the black behind it), determining its proper value, and finding a replacement.

Out of circuit, the suspected gray component measures 2.6 ohms (assuming it is a resistor). Honestly, I do not know enough about electronics to understand how the circuit works. Obviously, the LED is supposed to light when a battery is placed in the charger. However, I have never seen the LED go off when a battery is fully charged.

The diode appears to be OK, when tested with an ohm meter, and the small resistor measures closer to 90 ohms than what the 100 ohm bands indicate. It is a simple circuit but I do not know how to make it work properly again.

Thank you for any insights.

 

[Neal]

It's a crappy design, the battery is gone(drawing too much current), so the series resistor smoked. Get a new cordless with charger, they are cheap.
Is this the Wagner drill?

 

[(*steve*)]

That circuit is designed to slowly kill NiCad batteries by overcharging them.

It is an excellent circuit with a number of cunning features to ensure that this gets done effectively.

It will almost certainly take just a little longer than your warranty period in which to do its job, and is designed to maximise revenue.

It's obviously got a little hot during its life, but I'm surprised it no longer works. Nothing looks like it has got too hot. If the battery pack shorted, then the LED may have failed.

Place a 1K resistor across the battery terminals and plug it in (assuming the transformer is still working). The LED should glow -- possibly fairly dimly.

The LED will never go off with a battery under charge. It shows 3 states (2 are indistinguishable)

1) LED OFF (no power, no battery, or open circuit battery)
2) LED ON - Battery charging
3) LED Still ON - Battery over charging

Did you find the "Made in China" sticker?

 

[billd]

Neal,
You are correct about the crappy design. Yes, the battery was so dead that it drew too much current and burned out the charging system. And, yes, new cordless tools are fairly cheap - replacement being the main option I've considered. You are good!

The drill is part of a (cheap) 4-tool set (a gift from my son) made by Nikota. The tools have been very useful around the house for 7 years and still work. They only need working batteries. I managed to find new exactly identical batteries at Harbor Freight so I was wanting to repair or replace the charger.

Since I could not identify the burned out resistor type and value, I was just wanting to learn more about it. In all practically, though, it looks finding a better quality tool set makes sense. I just don't know what brand for around-the-house usage would use a better charging system.

Thanks much.

 

[(*steve*)]

Billd, is that large resistor charred on the underside? There really isn't any all too obvious sign that it has failed. It just looks like it has been fairly hot at some stage, and that is probably normal.

 

[Neal]

Neal,
You are correct about the crappy design. Yes, the battery was so dead that it drew too much current and burned out the charging system. And, yes, new cordless tools are fairly cheap - replacement being the main option I've considered. You are good!

The drill is part of a (cheap) 4-tool set (a gift from my son) made by Nikota. The tools have been very useful around the house for 7 years and still work. They only need working batteries. I managed to find new exactly identical batteries at Harbor Freight so I was wanting to repair or replace the charger.

Since I could not identify the burned out resistor type and value, I was just wanting to learn more about it. In all practically, though, it looks finding a better quality tool set makes sense. I just don't know what brand for around-the-house usage would use a better charging system.

Thanks much.

I got the Ryobi drill/saw combo with charger for about 50 bucks on sale at Home Depot a while ago and then picked up another drill with charger charger on sale for about 30 bucks.
At least their charger will not over chrarge the batteries. And it is nice to have 2 drills at times.

 

[billd]

Steve,

Yes, the circuit design did overheat the batteries during charging but they managed to last 7 years. The 4 piece tool set has worked well otherwise, which is why I wanted to find new batteries and repair/replace the charging system. The set (a gift from my son) was made in China. For around the house, it has been adequate.

Since the AC power supply burned out too, I tried using an 18v 6.5a laptop power supply. The LED does still work. For testing, I replaced the burned resistor with a 50 ohm 5 watt wire wound resistor. An old battery drew 250+ma and made the LED glow orange. Once the draw got down to 100ma, the LED was back to normal red. The new battery would not draw hardly any current, even though it has been drained some. However, neither the old or new battery would take a charge.

Since I found new batteries, I either need to fix the charging system, learn how to build a better one, or replace the whole set.

Your insights were helpful.

 

[billd]

Billd, is that large resistor charred on the underside? There really isn't any all too obvious sign that it has failed. It just looks like it has been fairly hot at some stage, and that is probably normal.

There is no sign of charring anywhere on the resistor. Since it measures only 2.6 ohms, I thought it may have been burned out. However, the problem may be that I need to use a replacement 18v 400ma AC supply or there is something wrong with the new battery as well. I just did not know if 2.6 to 3 ohms in that circuit seemed correct.

 

[billd]

I got the Ryobi drill/saw combo with charger for about 50 bucks on sale at Home Depot a while ago and then picked up another drill with charger charger on sale for about 30 bucks.
At least their charger will not over chrarge the batteries. And it is nice to have 2 drills at times.

I will have to keep an eye out for such a bargain on a better brand with a better charging system. Thanks for the tip.

 

[(*steve*)]

There is no sign of charring anywhere on the resistor. Since it measures only 2.6 ohms, I thought it may have been burned out. However, the problem may be that I need to use a replacement 18v 400ma AC supply or there is something wrong with the new battery as well. I just did not know if 2.6 to 3 ohms in that circuit seemed correct.

2.6 ohms may be reasonable. You suggest that 50 ohms was not charging the batteries which means that it was too high in value.

The resistor is there to limit the current, and in normal use the battery will probably never fall below 12V, and will typically be nearer 15 volts even when discharged. If the battery (or more than a couple of cells) went short circuit (something that does happen to nicad cells) then the transformer may have been damaged (it may have an inbuilt thermal fuse too).

Such a charger is not too bad if you ensure the batteries never get left for long periods. If you wait until the pack is flat, then leave them on charge for the time required to charge them -- then remove the pack -- it will suffer less. Nicads tend to fail short circuit is left discharged for too long, especially if they've been overcharged in the past.

 

[billd]

2.6 ohms may be reasonable. You suggest that 50 ohms was not charging the batteries which means that it was too high in value.

The resistor is there to limit the current, and in normal use the battery will probably never fall below 12V, and will typically be nearer 15 volts even when discharged. If the battery (or more than a couple of cells) went short circuit (something that does happen to nicad cells) then the transformer may have been damaged (it may have an inbuilt thermal fuse too).

Such a charger is not too bad if you ensure the batteries never get left for long periods. If you wait until the pack is flat, then leave them on charge for the time required to charge them -- then remove the pack -- it will suffer less. Nicads tend to fail short circuit is left discharged for too long, especially if they've been overcharged in the past.

The old batteries are definitely dead - one measures 7 volts the other zero. The new battery measures 18 volts but does not take a charge when using the laptop 18v 6.5a power supply through the charging circuit.

The charger LED does not light when the new battery or a 1K resistor is connected to the battery charger contacts. The LED does light when the large resistor is removed from the circuit and a load is placed across the charger contacts.

I am concluding that there is no way to tell from the gray resistor's color stripes or other factors what its value is supposed to be. Apparently 50 ohms is too high and current is definitely flowing through the gray resistor at 2.6 ohms, the LED just does not light then. For a simple circuit, it baffles me.

Thank you for your time and the education.

 

[(*steve*)]

The problem is that your 18V power supply is too low a voltage. The power supply needs to be at least the battery voltage plus a diode drop plus the LED forward voltage or the LED will never light (that's about 18V + 0.7V + 1.7V = 20.4V.

Note that a DC voltage in excess of 18V + 0.7V will slowly charge the battery.

And of course the battery voltage may well rise slightly above 18V under charge, further complicating things.

At the minimum voltage, the battery will not charge much either. It is quite likely that the original transformer was just a transformer supplying (perhaps) 15VAC. This gives a peak voltage of around 21.2V and a peak current of just under 1A when the battery is fully charged (these pulses will be very brief though).

Can you read the colours of the bands on that resistor? I find it unlikely that it would have lowered its resistance after damage (it usually goes the other way).

Do you still have the original transformer? It is possible that it has markings which would allow you to determine the voltage, or that it has a fuse that has blown.

 

[billd]

The problem is that your 18V power supply is too low a voltage. The power supply needs to be at least the battery voltage plus a diode drop plus the LED forward voltage or the LED will never light (that's about 18V + 0.7V + 1.7V = 20.4V.

At the minimum voltage, the battery will not charge much either. It is quite likely that the original transformer was just a transformer supplying (perhaps) 15VAC. This gives a peak voltage of around 21.2V and a peak current of just under 1A when the battery is fully charged (these pulses will be very brief though).

Can you read the colours of the bands on that resistor? I find it unlikely that it would have lowered its resistance after damage (it usually goes the other way).

Do you still have the original transformer? It is possible that it has markings which would allow you to determine the voltage, or that it has a fuse that has blown.

The color bands on the resistor all look black to me. They appear about the same in person as they appear in the photo that I posted.

The original transformer is rated at 18v DC 400mA output. I have not tried to pry it apart to investigate - it looks to be glued all the way around. Somehow, this transformer charged the batteries quite well for nearly 7 years. Only when the batteries were drained completely and required high current did the transformer burn out. The 18v 6.5a laptop power supply that I substituted is junk and may not be adequate for the task (it cannot run the laptop and charge the internal battery at the same time).

I just thought that I would ask, here. You have been quite helpful. Thank you.

 

[(*steve*)]

It's quite likely that the voltage from the "18V DC" supply was significantly higher than 18V. The rating is generally given under load. Almost certainly the current would have been limited by the transformer. It probably had a thermal fuse which eventually went open circuit.

The bands on the resistor were almost certainly not all black, however it may have cooked enough that their original colours are obscured.

I gather this is more a curiosity for you at this point. Do you have a replacement pack and a need to charge it? Or have you found some other way around the issue?

 

[billd]

It's quite likely that the voltage from the "18V DC" supply was significantly higher than 18V.

That could be. It never occurred to me to measure the output when it was new - it all worked then.

The rating is generally given under load. Almost certainly the current would have been limited by the transformer. It probably had a thermal fuse which eventually went open circuit.

The transformer did get quite hot during charging, even when new. After it burned out and I tried charging with the laptop power supply, I measured the batteries drawing well over 5amps. That, of course, super charged the batteries and they lasted for one more use. After that, they were totally dead useless.

The bands on the resistor were almost certainly not all black, however it may have cooked enough that their original colours are obscured.

Good point, I had not thought about that. I cannot distinguish any color differentiation now, though.

I gather this is more a curiosity for you at this point. Do you have a replacement pack and a need to charge it? Or have you found some other way around the issue?

Yes, at this point it has become more of a curiosity, though I had originally hoped to fix the charger. I found exact match replacement batteries but the charger is not sold separately, meaning I would have to also buy a power tool to get a new battery and charger. If the charger is the same or similar (as it appears), I could learn the correct component values and still perhaps fix the charger and use the laptop supply or find a replace transformer on eBay. I am not yet sure what I'll do from here.

Neal had a good point, too. So I am considering just getting a whole new tool set with a better charging system to avoid wasting any more time and money on this cheapo tool set. I could spend a lot of time fixing the charger and spend money on new batteries only to have the tools crap out. Being on a disability income, I often have to make undesirable choices with the limited funds available. Since the tools are still working and useful, I thought fixing the charger and buying new batteries would be the least expensive option at this time. When I consider the overall cost of everything, looking for a sale, as Neal suggested, is making more sense, now.

Thank you so much for your interest and input. It has really helped me work through the issues and options that needed consideration.

 

[(*steve*)]

This article may be of interest to you.

Most is behind a paywall though :-(

 

[billd]

This article may be of interest to you.

Most is behind a paywall though :-(

Nonetheless, it is a device that I had not known about and worth considering. Thank you for posting it.

 

[billd]

I got the Ryobi drill/saw combo with charger for about 50 bucks on sale at Home Depot a while ago and then picked up another drill with charger charger on sale for about 30 bucks.
At least their charger will not over chrarge the batteries. And it is nice to have 2 drills at times.

Ryobi Recall Notice October 15, 2010: About 455,000 Ryobi model HP 1802M cordless power drills, manufactured in China and imported by Ryobi Technologies Inc. of Anderson, S.C., because the switch can overheat and pose a fire and burn risk. The company has received 47 reports of overheated drills, including 12 reports of property damage and two reports of minor burns. The drills were sold by Home Depot from January 2001 to July 2003. For more details, call 1-800-597-9624 or visit www.cpsc.gov.

 

[billd]

The battery charger for an 18 volt [Nikota 01305] portable drill/driver has burned out and the company is no longer in business to provide a replacement.

In my original post, I was seeking help with repairing a power tool battery charger. As *steve* pointed out, it turned out that the charger circuit was fine. The power supply had burned out because of overload from attempting to charge a totally drained battery. Since the batteries were 7 years old and no longer held a charge for long, I put them on the flashlight to totally drain them, hoping that recharging them from zero would help revive them. Apparently "deep cycling" that way was wrong - I guess I should have researched how to do it properly.

The bigger problem was either finding replacement batteries or affordable replacement tools. Since the 4 tools still worked, finding replacement batteries and a new charger were the preferred option. As it turned out, I was able to get 2 equivalent batteries, a new charger, a new drill, and a new flashlight all for $32 US. If they last me another 7 years or so, I will be pleased. In the event that others experience similar problems, below are my discoveries.

The 4-in1 tool kit is a Nikota 01305. Nikota is a Chinese company that has been making tools since the 1970s under various brand names. At some point about 10 years ago, Nikota started selling tools under their own name and I found them at Pep Boys (in the U.S.). The quality was not great but sufficient for around-the-house usage. By the time the batteries died 7 years later, Nikota was nowhere to be found.

As I started researching replacement tools and batteries, I discovered that Harbor Freight Tools sold drills and drills sets under the Drill Master brand with similar battery packs as my Nikota. The batteries turned out to be identical in both physical size and rating (15 1.2 volt 1200 mHA cells) as the Nikota (in the photo, the red one is new, the black is Nikota's). The only obstacle to putting the new cell pack into the old Nikota battery case was the tape band around the cells. That was easily dealt with by using a screw driver to push the tape around the screw posts while inserting the cell pack.

The charging circuit was upgraded from Nikota's (see photos) but there is still no charge rate regulation. Honestly, I do not understand just how the new charging circuit works but included it for those who do. I am hoping that I identified the transistor correctly, since there was no number visible to reference.

The new power supply is rated at 21 volts 500ma output - the Nikota was 18 volt 400ma output (see photo). When I measured the new power supply with no load, it was 29 volts.

On the initial charging of the new drill battery, it drew 360ma. Eventually the draw dropped and leveled off at 210ma. The separate replacement battery started at 330ma then dropped and leveled off at 260ma. The manufacturer suggest an initial charge time of 7 hours then 3 to 5 hours on subsequent charges. After 7 hours, the first battery did not feel warm but the second battery was. When I used to recharge the Nikota batteries, they and the charger would always get hot.

Hopefully these new units will work well for me and others who may be "stuck" with battery-less Nikota tools will find this post helpful in getting more use from them at an acceptable cost.

 

[amsandiego]

Can nicad power tool batteries be recharged ?

A simple procedure will enable your Nicad cordless power tool batteries to again charge in your battery charger and give their full power potential.