AC to charge Nicads? wtf?

[joecool85]

Ok, I have an inexpensive black and decker 7.2v drill, and the batteries are dead. I'm going to replace them with some batteries from batteryspace.com

The issue is, the charger's output is rated at 11v AC @ 130ma! That's right, AC output. And there isn't any rectification inside the drill. So....how could this be? Can you charge nicads with AC directly? That doesn't seem to make sense.

 

[Optikon]

Ok, I have an inexpensive black and decker 7.2v drill, and the batteries are dead. I'm going to replace them with some batteries from batteryspace.com

The issue is, the charger's output is rated at 11v AC @ 130ma! That's right, AC output. And there isn't any rectification inside the drill. So....how could this be? Can you charge nicads with AC directly? That doesn't seem to make sense.

You've measured the chargers output and are sure there is not a diode in it's output? Be sure to measure it under load - use a power resistor.

 

[joecool85]

You've measured the chargers output and are sure there is not a diode in it's output? Be sure to measure it under load - use a power resistor.

When I measured the charger directly I got 15.1v AC, no resistor. I don't have anything bigger than a 1/4w resistor. But the key thing here isn't the voltage, it's the fact that it is AC power going into the batteries. THAT I don't get.

 

[Sceadwian]

You're missing something, there has to be at least a single diode, perhaps inline with a wire inside the drill. Feeding AC like that directly into a battery would cause a fire so there has to be some kind of rectification inside the drill. Any way you look at it it's a piss poor way to charge a nicad battery. Your battery probably would have lasted two to four times longer or more if a sensible charging circuit were used. If you get a new battery get a new charger as well. It'll save you more money over time.

 

[joecool85]

You're missing something, there has to be at least a single diode, perhaps inline with a wire inside the drill. Feeding AC like that directly into a battery would cause a fire so there has to be some kind of rectification inside the drill. Any way you look at it it's a piss poor way to charge a nicad battery. Your battery probably would have lasted two to four times longer or more if a sensible charging circuit were used. If you get a new battery get a new charger as well. It'll save you more money over time.

Definetly getting a new charger along with the batteries. I can't see where the diode would be...it goes directly into the battery as far as I can tell. I dunno, I'll check it out more when I open it up again later. btw, assuming it does have a diode, if I ran DC through it would it work ok or would I need to get rid of the diode?

 

[Sceadwian]

Does the charger plug directly into the battery pack? If so it's likley the diode itself is inside the battery pack, possibly with charge control circuitry.

 

[joecool85]

Does the charger plug directly into the battery pack? If so it's likley the diode itself is inside the battery pack, possibly with charge control circuitry.

The charger plugs directly into the drill. It doesn't really have a pack, it has built in batteries. I can't image there is a diode or anything in with the batteries, they are sealed in a clear plastic case and you can see pretty much everything. I took the battery out and hooked the voltmeter onto the wires that go to the battery while the charger was plugged in. It cycled every 1-2 seconds between 6.6v DC and 8.2v AC. Not sure what that means.

I can take pictures/video if anyone wants to see what I mean.

 

[bananasiong]

Maybe the charger only steps down the voltage, and the rectifier and the filter circuit are built-in in the drill?

 

[joecool85]

Maybe the charger only steps down the voltage, and the rectifier and the filter circuit are built-in in the drill?

I don't see where they would be. It goes directly from the jack to the battery. From the battery it goes up to the trigger. But the rectification couldn't in the trigger because it wouldn't be in line. The battery would still be seeing AC voltages.

 

[Nigel Goodwin]

You already know that you can't charge the batteries with AC, so there MUST be a rectifier somewhere!.

 

[joecool85]

You already know that you can't charge the batteries with AC, so there MUST be a rectifier somewhere!.

Could the rectifier be in the jack? That is the only thing directly between the battery and the charger.

Also, if I got a power supply that was DC, would it pass through the rectifier alright or would I need to find the rectifier and remove it?

 

[joecool85]

Alright, maybe this will help:

**broken link removed**

The jack has two wires going to it, one goes to the positive on the battery, one to the negative. I should note that the battery has 3 wires, one at 3.6v (3 cells) and one at 7.2v (all 6 cells) and when I refer to "positive" I mean the full 6 cells' positive.

And here is a video of the voltmeter when hooked to the positive and negative of the drill (no battery in the circuit at this point.)

https://www.youtube.com/watch?v=qBun9-eUkdw

 

[fingaz]

Just a thought. . . Maybe the rectifier in the power supply has gone to silicon heaven. . . if it's shorted, then you could get AC output. That may be what damaged the original battery pack?

Have you opened the power supply to see if it has a rectifier? this may not be possible as some units are factory sealed, and not designed to be user servicable.

 

[joecool85]

Just a thought. . . Maybe the rectifier in the power supply has gone to silicon heaven. . . if it's shorted, then you could get AC output. That may be what damaged the original battery pack?

Have you opened the power supply to see if it has a rectifier? this may not be possible as some units are factory sealed, and not designed to be user servicable.

Ok, to get this to rest. The power supply DOES NOT have a rectifier. It is SUPPOSE to put out AC, it says so on the back. It's a matter of where the diode/rectification circuit is within the drill. I'll have to check it out more. I didn't know if anyone here had run into something like this, but I guess not. It's ok though, I'll post if I have more questions or if it ends up being really interesting what I find out.

 

[Firnagzen]

I have a suggestion. Use your meter, and measure the wires where the battery was attached while the charger is plugged in. See if the voltage going to the batteries is AC or DC. If it's DC, well, then, trace the wires!

 

[joecool85]

I have a suggestion. Use your meter, and measure the wires where the battery was attached while the charger is plugged in. See if the voltage going to the batteries is AC or DC. If it's DC, well, then, trace the wires!

Maybe I wasn't clear. That's what I've already done. When I did that I got the cycling between 6.6v DC and 8.2v AC. If you want a better explaination of it, look at the vid I posted.

 

[Nigel Goodwin]

I suggest you stick a scope on it, a multi-meter (particularly a digital one) isn't much use for what you are doing.

I presume you haven't had the power unit to pieces?, it's probably got a rectifier in there - but no smoothing capacitor, which is why it's labelled AC.

 

[HelenVBeden]

Hello, I am Helen!. And even though this my first post here

Hello, I am Helen!.
And even though this my first post here, I for a long time on this forum www.electro-tech-online.com I'm not sure if this should go here or in the GENERAL DISCUSS section,
so if it's in the wrong place, my apologies (and can someone move it please! EXCUSE uneducated girl). I just loved this topic.
Honestly we should start some movement or society. Anyway I'm also one of those people who enjoy to discuss "https://www.electro-tech-online.com/".
In my opinion it would be a lot better to get some fact into our STUFF. I'll glad read all new information about this topic
Thanks a lot..!!
Helen V. Beden.

 

[joecool85]

I haven't taken it apart, but it registers as AC when I check it with my DMM. And the thing is, it registers as 15.1v AC, when it goes through the drill to the wires that would go to the battery, it's getting a flip flop of 6.6v DC and 8.2v AC. So there is obviously *something* happening inside the drill. The wires going to the switch are in parallel with the charging wires, could that be whats causing all this?

 

[Nigel Goodwin]

Like I said, get a scope on it, you're wasting your time with a meter - particularly a digital one!.