Help needed w/battery charger circuit

The problem is finding a battery charger for a cordless drill. I inherited a Skil 12-volt drill and battery, but no charger. The only hope of finding one seems to be getting a used one, which I haven't been able to find.

I opened my friend's charger and drew the circuit, which I've attached a link to. (PDF, and a big one, sorry; somehow a little bitty 35 KB JPG made the file bloat up over a megabyte.)

Can someone comment on this? I've got all the values except for the one resistor in series with the LED, which I can probably figure out (or maybe just omit altogether, as it seems only to be an indicator lamp).

What's the function of the capacitor? (I believe it's a non-polarized electrolytic in the original unit.)

Would this work if I constructed it?

The other problem is that the battery has 3 terminals instead of just 2. Two are marked "+" and "-", but there's an unmarked middle terminal. But the charger only had connections to the two terminals. What's the 3rd terminal for? (Picture of bottom in the PDF.)

Oh, yeah, the PDF is here:

some rechargeable batteries have a third contact which goes to a thermistor. this is used with a "fast charger". the battery is charged at a high current. when it gets warm enough it is considered to be mostly charged and switches the charger either to off or to trickle charge.

some chargers (i.e. Makita) you push a button on the charger to start the charge. when its done it shuts down.

with this type of contacts it should be very easy to used an appropriately sized variable supply and alligator clips to recharge. always observe correct polarity.

David Nebenzahl ha escrito:

That charger seems horrendous to me. If for any reason you touch any of the exposed battery terminals of the charger, without the battery in place, while the charger connected to the AC, you will get instantly shocked by 120 VAC. I can=B4t believe that such a charger, without any interlock switches can be manufactured, and approved to be sold.

I would design a charger using an AC transformer to reduce the 120VAC to 24VAC. Then I would rectify that voltage, and use a drop resistor to reduce the voltage to 12V. It would be interesting to know the aH capacity of the battery.

lsmartino spake thus:

Look, save your horror and outrage, OK? The thing is mass-produced by a well-known reputable company, is UL, SA and who knows what else approved up the wazoo, so apparently the regulators think it's safe. The whole thing is safely encased in plastic, so there's hardly a chance someone's gonna get their fingers zapped by line voltage.

Or maybe you prefer the version of the world where everything is covered with soft foam rubber bumpers, with flashing lights, sirens and klaxons going off every time you attempt to plug something in?

This appears similar to my black and decker, but mine has a 12v wall wart.

the circuit makes little sense to me as Imax through the 22K resistor would only be 5 mA the bulk of the current would have to go through the series RC circuit 2 ohm + whatever Xc is (at operating frequency). finding the capacitor might be a trick in itself :) the 120 V rating (of C) is suspicious because when charging a depleted battery nearly all the 120 V will be dropped across it. that's not much of a safety factor.

it was probably done this way to partly avoid the heating you get with series resistors.

lets assume for now its a 1 AH battery. to charge it in 10 hours would require 100 mA (or a wee bit more) all you need is to come up with a charger that will pump 100 mA into a 12V load that wont fail or burn up or set you shop on fire. there is probably 50 or do ways to do it but I'd not go the transformerless rout.

a cheap way might be to get a car battery trickle charger and put flashlight lamp bulbs in series to limit the current or calculate the needed voltage drop by measuring Vno load minus 12 and sizing the appropriate resistor .

another approach might be to use a "wall wart" supply. i'm a little hesitant to recommend that as a buddy of mine clams that one failed and burnt his house down.

i'm in the same situation as you with an old flashlight that someone gave me. it's not worth finding a charger for it. i just pull the battery pac and tape gator clips to the contacts and hook to a bench supply with an ammeter in series. set it to 50 mA and let it charge for a day.

OK, but you would be unable to replicate that circuit safely, because you wont be building a safe enclusure for it, and probably you will be using alligator clips to connect the charger to the battery.

So, using a circuit like that with alligator clips *is* a safety concern.

And still, is an awfull design for a charger.

Easy to make one. I'd go for a constant current 1/10th capacity charge rate. This will charge the battery overnight (approx 14 hours) and it will have a very long service life - much longer than a fast charged one. A constant current charger may be made with a suitable DC supply, and one power transistor with few extra components.

Site's not available at the moment so can't comment.

An LED is current driven so needs the series resistor. You'll find tables for this resistor value from the voltage if you Google. Or it can be worked out by a formula. If you emit the resistor the LED will fail.

Some makers use the same battery with different chargers. The third terminal is usually a temperature sensor, and this connection is omitted with nasty or low output chargers.

Quite a lot more. To charge a 1Ah battery would take 100mA for 14-16 Hrs or

I agree the charger is awful. I prefer a transformer and 'proper' constant current designs, such as those based on voltage regulator ICs arranged in constant current mode. The last one I knocked together for my dual band handie used a LM317 and a trimpot to set the current, very simple but effective. Unlike a series resistor this design keeps the current precise over the entire charging cycle, regardless of the state of charge of the battery.

Dave

You asked for people's views, then you jump on someone who offers a perfectly valid and sensible opinion, what is your problem?

What has that got to do with it? The question of whether the original is safe or not is completely irrelevent, the issue is whether your attempt at copying the circuit will be safe or not. The original had to pass stringent safety tests that a wall-wart powered design would not because of the potential mains shock to the user. Just because you copy the circuit does not in any way imply the safety approvals will magically transfer to your build.

Frankly, I'd stay a million miles away from that design, it's crude and unnecessarily dangerous for a hobbyist to copy.

You're being ridiculous now. I don't mean to sound rude, but I hope it is down to ignorance and naivity rather than bone headed arrogance. Either way you need to get over your attitude of self importance before you injure or kill yourself.

Tip- if you need to ask about the suitability of such a simple circuit and how it works then your electronics knowledge is perhaps not yet good enough to be so arrogant and dismissive of people who are trying to help you, or to build mains circuits such as this one.

Dave

Have seen it now. Avoid like the plague. It is not a suitable circuit for a decent battery life. And I'd call it dangerous - it would not pass modern testing in the UK.

Dave D spake thus:

Maybe you can be excused for *assuming* that anyone who asks questions like these is an idiot who's likely to get zapped by line voltage the first time they experiment with a circuit.

I can assure you that I'm not in that category. I am not the world's foremost electronics expert (hence my questions), but I certainly know all about protecting myself and others from shocks, fire, etc.

So you misplaced concern for safety is totally unnecessary. If I did build a charger like this, it would be strictly for my own enjoyment. I'm not going to be selling them on eBay, or over at the flea market.

Apparently, you've never heard of line-operated radios, either. Maybe you're not old enough to remember when the canonical 5-tube radio was operated directly from line voltage with no isolating transformer.

Or perhaps you're one of these safety fanatics, like the ones who apparently are responsible for the fact that now on TV whenever there's the remotest chance of anything getting warm or bubbling, the technicians are wearing plastic goggles and protective clothing.

Don't get me wrong; I'm all for safety. I don't like the idea of environmentally dangerous stuff like lead, asbestos, benzene and TCE in air, water and soil. It's a good thing that those big old exposed knife switches are a thing of the past. But even safety can be taken too far, you know.

And this technique is now rightly banned. Radios in those days didn't have external connections for headphones and CD players etc.

The fact is the contacts on your charger are likely to be accessible to a finger, etc. So it should conform to the regs for such things.

You might find the capacitor 'dropper' technique on some self contained devices - perhaps a rechargeable electric razor. But not with your application.

Please excuse me, but if you have to ask about how to construct a circuit to charge a battery, then one can assume that your electronic knowledge or understanding is not the best in the world. The fact that you had to open a charger just to see what a charging circuit looks like, and after that you couldn=B4t apreciatte that the design you encountered it=B4s pretty crude *and* unsafe to replicate, was also another pointer to your lack of electronics experience.

Just because a suggestion given by one poster didn=B4t suit your mindset, is no excuse for being rude. If you ask questions, you should be prepared to receive any answer, even if you dislike the answer.

And please, don=B4t tell us about vacuum tubes, line operated radios and things like that, which are very well buried in the past because of their limitations.

I didn't assume that at all. I do however think you unfairly jumped on someone who voiced valid safety concerns.

I think you're missing the point. You implied that copying the design would be safe because it had originally passed approvals, and you were rude to someone who was simply offering the opinion you requested.

Yes, and radios where the mains cable was a resistive dropper! Very nasty.

Not at all.

Certainly.

Dave