Electrical Question

Mrwurm · Dec 17, 2002

While driving my tractor the other day I almost bumped my head on the garage door because I was thinking about the following: (tractor-related, see)

My 1 million candlepower spotlight recharges in 12 hours off house current or 1 hour on 12volt from a vehicle. Lets see,
110 volts at 15 amps/2 (half wave rectification?) = 825 watts per hour.
12 volts at 5 amps = 60 watts per hour

I don't get it, why does it charge quicker in the car? My cell phone works the same way, 12hrs on 110v or 1hr on 12v???

Ok, brain-iacks, go to it!!

hazmat · Dec 17, 2002

I'll hazard a science bassed wild guess:

It's easier (Read CHEAPER) to step 12V down to the 6V (or whatever your phone takes) than to convert 120V AC to 6V DC. Therefore they can build a 12V charger to handle a good amount of amps for less money than the 110V model. After all it is just a resistor & maybe some electronics to shut it off when done.

For charging the battery, if you do it slowly the "quality" or purity of the DC may not matter as much at low current. If you want to fast charge, you probably need "high quality" DC thus the 110V charger gets prohibitively expensive.

I have a 12V power supply (left over from my days of my RC Car hobby) I think It cost $100. It doesn't even charge, just provides clean 12V DC.

Edit here:
This is just a guess, maybe some electrical guys can confirm or deny my story

BB_TX · Dec 17, 2002

Your 12v charger may well be charging at 5 amps, but I am guessing the 110 ac charger has a very small step down transformer and is charging at less than 1/2 amp.

JATO_RaT · Dec 17, 2002

I was going to say basically the same thing. If its the little transformer with plug built in type, your probably in the neighborhood of a 12W transformer. This is somewhere in the .1 amp area on the primary side. Your DC side will have various amp values depending on the output voltage and degree of discharge of the battery. Rat...

Mrwurm · Dec 17, 2002

Good answers ! !

I knew you could do it !

/w3tcompact/icons/grin.gif

Anonymous Poster · Dec 17, 2002

Both answers are right on the money. What you haven't entered into the equasion however is how many charging cycles the two different charging methods will yield before your battery is junk.
If you're using NiCad batterys, repeated rapid charges shorten overall battery life.

BB_TX · Dec 18, 2002

Maybe that's why the new battery I bought for my cell phone is going out already after less than a year./w3tcompact/icons/hmm.gif

JATO_RaT · Dec 18, 2002

Franz, nicads have about 500 to 700 charges in them. At Thomas Distributing you can find out more then you would ever want to know about NiCads, Metal Hydride and Lithium Ion batteries. It also explains the various and numerous ways to charge batteries which can get very technical. For the most part, the old standard charger will become a relic as the newer style batteries become more prevalent. Rat...

Anonymous Poster · Dec 18, 2002

Rat, I currently have 2 types of chargers on my desk, and at least 3 in the shop. I'm considering color coding batterys & chargers to keep the NIMH from getting in the rechargable alkaline and vice versa.
Dang good thing none of these mini batterys can get connected to the forklift charger, or someone might get a rapid recharge idea.

k8ny · Dec 21, 2002

Your AC charger is current limited (limiting the max charge current) thus charging a NiCAD at a specified rate. When you charge from a direct 12v source (unless you use a current limiting circuit) you are charging the NiCAD at the max current it is capable of drawing. So, 12volts @ 300 milliamps (current limited) will charge at a rate of 4 watts/time period. 12 volts @ 4 amps (non current limited) will charge at a rate of 48 watts/time period.
The bottom line: Faster with a non limited device but reduces NiCAD life tremendously. Stick with your regulated supply for longer lasting NiCADS.