Circuitry: question about charging batteries.

[ƒÓÒl]

Okay, I got a pretty good grip on electronics, but I ran into a problem that I need some "old memory" backup.

I have a cordless drill. It has old batteries, but I have empty pockets.
The transformer died for the charger when my daughter left the sink running the other day (submerging electronics in water lets the magic smoke out :shrug: )

Anywho, I also have a doorbell transformer that puts out slightly less voltage than the drill charger did (1/2 volt), but many more amps...charger=210ma and the doorbell transformer puts out .9A

Now if memory serves, I won't have any problem as it's the battery (load) that determines current output, not the transformer. So I believe that it would work fine, but that's where I need the boost to my memory. :D
Smoking batteries=lost work=lost money.

Anyway, I also have a bridge rectifier that I can use to make sure the transformer isn't pushing AC instead of the necessary DC into my battery (5A plenty robust) and I also wanted to make sure that that was a good idea as well.
Vivisection (love that word) showed no such circuitry in the expired tranformer, only a side by side transformer.

Thanks to those who browse and pass, but a big wet sloppy kiss to those who can answer! (kidding of course!)

[Flip-Mode]

Interesting question, I know that it will fry your batt if it starts recharging at all but I can't really explain it.
I want to see a definite answer.

[ƒÓÒl]

Oh, and a funny thing I found out about drill batteries (NiCad's).
The batteries are 14.4 volts, the charger charges them to 17 volts, and they quit running the drill alltogether when down to 6.5 volts.
Not exactly the way to keep NiCad's up to snuff for very long, eh?

[ƒÓÒl]

Well, skip the bridge rectifier question, I found one in the "shoe" that fits onto the battery for charging...well same thing anyway, 4 diodes set in opposing directions. There's also a pair of resistors for the charging LED.
Neat little package the size of a postage stamp.

[four4875]

the lower voltage should be fine, it might not charge to as high of a voltage, but it shouldnt kill them or anything. im sure too high of voltage owuldnt be such a good idea, and adding the bridge rectifier would drop the voltage by a volt or so i believe.

[ƒÓÒl]

The question was about the current, not the voltage. But thank-you much for taking a stab at it.

Well, it seams my impatience has gotten me. I went ahead and tried it.
The resisters on that postage size circuit board have given up their smoke protecting the LED. The diodes test okay, but the experiment is over. :(

On the good side (as in, good that I unplugged it after a 40 second charge), the doorbell transformer was able to bring it from 7 to 9.5 volts in under a minute.
A brilliant fast charge, but alas, it would have smoked the battery too if I hadn't ended the experiment quickly.

I guess I just have to get my cheap behind down to the Home Depot and buy the correct charger.

[Skeith]

yeah nicads arent easy to charge unless you build a regulator circuit for em. They will draw as much current as they can overheat and explode.

[Flip-Mode]

Hate to say "I told you so"...

[ƒÓÒl]

It just doesn't make sense though. If the battery were a high amp load, sucking up all the amps you can feed it, why doesn't the charging transformer that's wound with tiny little hair-fine wire burn out?
It would seam like a direct short wouldn't it?

I'm missing something here....

[ƒÓÒl]

I went to Lowe's yesterday and priced two new batteries and a charger...$70 bucks!
They had this nifty 18 volt 1/2" drill for ~$15 more so I went with the new drill. :D

As an added bonus, the new drill comes with their new quick charger that will also charge the 14.4 batteries, so my 13 y.o. son now has his own drill to use (future case modder in the making).

It's kind of funny though when you look at the prices...the batteries and charger make up a majority of the packages' cost, and they throw in the drill itself for $6!!!...I'm no Huckleberry!

[Flip-Mode]

I went to Lowe's yesterday and priced two new batteries and a charger...$70 bucks!
They had this nifty 18 volt 1/2" drill for ~$15 more so I went with the new drill. :D

As an added bonus, the new drill comes with their new quick charger that will also charge the 14.4 batteries, so my 13 y.o. son now has his own drill to use (future case modder in the making).

It's kind of funny though when you look at the prices...the batteries and charger make up a majority of the packages' cost, and they throw in the drill itself for $6!!!...I'm no Huckleberry!
Good find!

[RoadWarrior]

Yeah prices of replacement batteries for things are nuts. For instance, hardware store round here puts those bigass flashlights on sale once or twice a year for $5 with a battery. It's those big square section lantern batteries they take, new batteries are like $9-10. That's only dry cells. So every couple of years when my bigass flashlight is dead I go buy another one.

I noticed something though, dollar stores round here have started getting in AA and AAA nicad rechargables for a buck a pair. Also they've got some of the button cell packs for portable phones, and once I saw a 2 cell sub-C pack for something. Now these aren't the high capacity latest LIon or NiMh, but if you've got older NiCad packs with standard cells in, you can crack them open and re-cell them with dollar store cells for a few bucks. As soon as I saw them I picked up an old NiCad charger in a thrift stor for $2, and got $15 worth of AA and AAA for all the crap round the house that takes them. Even if they're so cheap they only do 50 cycles instead of 1000, it's still worth it, since they're as cheap as regular disposable batteries. You can usually figure out what cells a pack has got in, divide total voltage by 1.2V since the cells are 1.2V each, so a 14.4V pack has 12 cells of some type in. Then hold up a couple of different cells to it to figure which one's fit, then crack the pack, to make sure you can do it cleanly and get the old cells out, then buy the new cells, wire them up, and stick them back in, might need to tape up the pack when you're done, but it'll work again. Often it's one cell in a pack that's failed, so if you've got a few dead packs, you can break all the cells apart, test them individually, and make up the best set to get another working pack.

[ƒÓÒl]

I thought about busting open the battery packs, but I use the drill for my work across western Michigan, so I can't afford a failure on my own count :D

I was also looking at those new Toshiba drills, but they're even more outrageous on replacement battery cost...$56 ea.!!

Oh by the way, I think I figured out what I was missing with the attempt at replacing just the transformer on my old charger.
In the transformer itself, the primary coil is the load (adjustable by wire size and length), so output amperage is constant on the secondary coil no matter what the secondary load is. A transformer like that will only put out it's rated amperage, and that explains the smoke now. :bang head
I just had to step back from the problem for a while to figure it out. :thup:

[RoadWarrior]

Yeah, simple PSUs I think, when the voltage differential is high they put out rated current, however, when the voltage differential is low, when the packs getting more charged, the current goes higher, it's unregulated, that's what screws things up with nicads. Therefore you need to have current limitation in the supply to hold it to the spec.

The converse happens in low current situations, the voltage goes high. Beware of the cheap adapters with low current devices, say you want to run a 150mA device off a regular transformer type wall wart of 6V rating. Now, you'd think it would be okay because the voltages match, but if you're running that 150mA device off a 1500mA supply, what can happen is that it doesn't pull enough current to pull the voltage down, and it's actually getting 10V instead of 6. Which might not be something it can cope with.

[Skeith]

Current limitation is key. Unless you want to get a bench psu with ampere readings and watch the battery while it charges for 6hrs to keep it in spec :P. Some nicad chargers use thermistors to disable charging when they get warm (aka finished charging) Ill see if i can find a schematic for a NICAD charger. What you could try doing is cracking open the charger you have and pull out the regulation circuit and attach a similar rating transformer to it. I wont garantee the results as some cheaper chargers use the low current transformers as part of current regulation.



EDIT!!

Here (http://www.web-ee.com/Schematics/Battery%20Charger/charger.htm) is a circuit for a Nicad Charger. It doesnt have a shut of section so you would have to put a timer on it or something. (a little household timer) set it for the charging period and put the batt in. You will probably have to modify the circuit for your battery since its a bank of more than one cell. Unless you want to open it up and charge each cell individualy (maybe make some sort of battery holder to pop them back into place after. You will need to know the charging voltage and charging current of the batterys.

I hate nicads, why can they be as easy to charge as lead acid or gell cells. :bang head