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Furnace
11-08-2006, 01:51 PM
My friend at work has a 1983 Skil Cordless Screwdriver that needs a new power supply to charge the batteries. Its output is 5VAC, which seems weird to me and, we can not find him one anywhere and hes not about to throw it in the trash. Would any of you fine gents know of a place that would sell such a thing. Ive looked online for one but the ones Ive found dont have the 115VAC input. Maybe Im not looking in the right place. Any help would be greatly appreciated.

John

japcas
11-08-2006, 02:02 PM
Check out this link to Radio Shack. This might work for you.
http://www.radioshack.com/product/index.jsp?parentPage=family&summary=summary&allCount=48&cp=2032056.2032136.2032153&productId=2261608&accessories=accessories&techSpecs=techSpecs&currentTab=features&fbn=Type%2F12+Volt+or+less&custRatings=custRatings&f=PAD%2FProduct+Type%2F12+Volt+or+less&features=features&fbc=1&support=support&tab=summary

He may have to spice the cord to put on the right type of charging plug but I believe it does have a 5 volt output on it. Hope this helps.

Tin Falcon
11-08-2006, 03:24 PM
Furnace:
IIRC computer power supplies are 5 volt. I have a power supply for a Zip drive in hand that is 5 volt. With all the computer stuff in the world you should be able to find something that will work. I am shure Evan or someone else can add to this.
Tin

Evan
11-08-2006, 03:34 PM
The voltage isn't critical. I have a couple of old 6 volt Skil cordless drills from that time and they use sub-c nicads. I am pretty sure the screwdriver uses two or three of those. They have about a 1200mah capacity so any dc supply with an appropriate resistor to supply around 100 to 150 milliamps of charging current will do fine for overnight charging.

Furnace
11-08-2006, 05:59 PM
Thank you guys for your replies. We have tons of old pc stuff laying around at work to scrounge through, plus some old power supplies too. I love this forum. Thanks.

ligito
11-08-2006, 06:06 PM
Are the Ni-cads still good, after all this time?

matador
11-09-2006, 12:14 AM
AFAIK,as long as the voltage output from the charger is higher than the drill's voltage,the only thing that would matter is the output in amps.
e.g.,it should be no problem using a 6volt charger,but for a drill I would guesstimate at least 1 amp(1000Ma)current output required.
Any electronics place will be able to supply a wall-wart with a 1 amp DC output.
Like ligito,i would look at the battery pack closely.At this age ,it's likely to be on it's last legs.

Evan
11-09-2006, 01:58 AM
It doesn't need 1 amp of output. To recharge nicads of that vintage you should not exceed around 1/10 of the amp hour capacity as the maximum charging current. As I said, 100ma (1/10 amp) to 150ma will be the most you will want to use or need. The supply need not be rated much higher than that.

Some nicads from that time are suprisingly good. Since they could not be fast charged safely the chargers were not designed to do that. If the vent seals are good and they haven't dried out they may well still be ok. I have some that old that still work.

darryl
11-09-2006, 02:04 AM
Many of the cordless tools from that era didn't have fast charge capable cells in them. If you were to need a fast charger, then it would have to be more sophisitcated in order to avoid damaging the cells. But for an overnite charge, Evans suggestion is good. You could use pretty much any ac adapter that gives more than 8 or 9 dc volts output, and use a series resistor to set the current. The adapter that has the 5 vac output does give peak voltage of around 8 or so, so as long as there is a rectifier in the drill, it will work. My skill cordless had the rectifier in the drill, so it would work with either an ac adapter or a dc one. The only real chore is to size the resistor to give the appropriate current output, and to handle the heat dissipation. The higher the voltage output from the adapter, the more power this resistor would have to dissipate.

If you find an adaptor on hand that has more than 9 volts output, then I'm sure we can figure the resistor value for you, as long as you can make some voltage and current measurements.

Furnace
11-09-2006, 07:00 AM
Theres a battery store here that said making a battery pack for it wouldnt be a problem, but the 5VAC 300mA would be hard to find. Im no electronics genius or novice for that matter but AC output from the charger? Is it a typo or am I missing something? Im sure we have an old charger here somewhere, how do I go about figuring what size resistor to use on it?
Thanks guys.

Shaidorsai
11-09-2006, 11:58 PM
A 5 to 6 volt AC 150 - 200 ma wall wart should not be hard to find. Are you sure the output is AC and not DC at 5 Volts. Normally DC is used to charge batteries, and the polarity of the connector is critical. If a DC input is specified, you need to know whether the center pin is positive or negative. You may see a symbol near the power receptecal that indicates the polarity of the center conductor. If it does use AC, there may not be a polarity symbol, and connector polarity will not matter. Any number of electronics surplus houses on the internet will have a suitable Wall Wart, at usually less than $5.

J Tiers
11-10-2006, 12:35 AM
Disagreeing with some other posters...............

1) the output voltage may indeed be more critical than has been said.

2) Dc and rectified AC sources will usually charge differently with cheapo charger setups.

I am assuming that the "power supply" plugs into the drill to charge it, as many do, drills, cheapo weedwhackers, etc.

the reason for (1) is that the peak rectified voltage may have been chosen to stop the charge at more-or-less the correct point. NiCd batteries have somewhat critical recharge requirements, and many of the cheapo chargers totally ignore that, relying on you to take off the charge 10 hours later... with minimal "protection" against over-charging etc, if any.
Using a higher voltage may increase charge current, and fail to taper off the charge at the correct voltage, damaging batteries or charger components.

The reason for (2) is that when rectified AC, often half-wave-rectified, is used, the average current is reduced by virtue of the time the voltage is lower than peak, and in the case of half-wave rectified chargers, the off time in every other half cycle. The designers rate all the parts for that usage. A DC source may have an average charge current of 1.4 to 2.8 times larger than expected, damaging the battery or the charge components.


You need to remember they shaved half-pennies making that. Each part was chosen to work as a whole, and if they could cheat, they did. I've seen lots of them, and in many cases the "charger" components consist of a diode rectifier, only..... Sometimes there is a series limiting resistor. Sometimes the inherent resistance of the charger "wall wart" transformer windings is used as the current limit resistor......

Obviously you can't count on there being a sophisticated charge controller to correct your "mistakes".

I would find a 5VAC supply of similar size and weight, which should give similar amperage. Unless you can read specs off the old dead one, in which case try to duplicate them.

That gives your best chance of same performance, good bad or indifferent, and least chance of damage.

darryl
11-10-2006, 03:23 AM
The ac output from the charger could well be right. All it takes is one rectifier inside the cordless device, be it a drill or a screwdriver. As I said earlier, my skil driver of that vintage had the rectifier.

J Tiers makes some good points, but this isn't rocket science. You don't want to overcharge the battery, or undercharge it either. Because we're talking about an overnite charge, the charge rate isn't anywhere near an amp, so there's more leeway in regards to leaving it on charge past it's intended time. That's always been the case anyway with slow charging. Very few chargers had brains then, and you always risked damaging the battery by leaving it on too long. However at the slow rate of charge, there wasn't much risk of overheating the cells or boiling the p--- out of them. A fairly simple circuit can be easily made up by you or your friend to make some adapter that you may find work. The resistor thing again, its value to be determined once you find some adapter that you're willing to use.

There is a very simple circuit that uses only two parts to make a current regulator, if that sounds like a good idea to you. That will give you an easy way to time the duration of charge, but you would still have to remember to pull the plug after x number of hours of charging. Of course, you do have to verify that the battery is good to begin with. That is probably the biggest problem you face, it's not in making up a suitable charger from scrounged adapters.

By the way, this battery charging issue brings up one of the benefits of the old style meters. (non digital). You can read the average current in a circuit regardless of how it's delivered- in pulses as with ac, or rectified ac, or with pure dc, or anywhere in between. To be in between, the filter capacitance would vary, with higher capacitance values delivering closer to pure dc. I have read that charging with pulses is actually better for a battery than dc, and it certainly is cheaper. Only one rectifer is required besides the actual transformer itself.

Regardless, it does appear that some assumptions have been made. One (which I'm assuming) is that the charge rate is so low that it takes many hours ( 8 to 12) to bring the battery up. The other is that this is to be a fast charge, say one hour or so. (this I got from the discussion about the adapter being able to deliver an amp or so). Very big difference. You simply don't fast charge without a sophisticated charger, which is certainly beyond what that original 5vac adapter was capable of doing. And you don't fast charge cells which aren't designed for that.

Here's another thing- some modern chargers will allow the battery to drain back into them if the battery is left on the charger and the ac power is off. This has killed many battery packs, as some of you probably know. This cant' happen with that old style slow charge using the cheapest of chargers with the single rectifer. The rectifier prevents any back flow. You can simply yank the cord out of the wall in the morning and go to work without fear that your battery will be half drained when you go to use it. Sometimes cheap and simple is better, and safer.

ptjw7uk
11-10-2006, 06:58 AM
the first nicad charger that I remember didnt even have a transformer the charge was limited by a capacitor,resistor and diode in series. I took one look and never used it agaib. I built a current regulated charger with various levels a current, worked fine but if memory serves was not as good as the half wave original
Peter