I like capacitors, but why does it make 13v to 18v?

[SimonTHK]

My favorit component is the capacitor. I like charging and discharging stuff, but also that it is good for stabilisation. I put it where ever I think it can have a purpose (it cant hurt atleast).

I have a normal car batteri charger that provides me with 12,9 volt dc. When I put a 6800uF 25v capacitor in parallel with the car charger, it gives me suddenly 18,8 volt dc. I can easily hear that my electric test motor (12v rc motor) gets many more RPM with the capacitor in parallel.

I have learned to multiply with square root of 2 when I go from vac to vdc with a lose of 0,35v over each diode. But I havnt learned anything about why my 12,9vdc can turn into 18,8vdc

can anyone lead me to an explanation why this is?

thanks allready

 

[Hero999]

Probably because the 12.9V is pulsed DC from a rectifier so measures 12.9V on the meater and adding a capacitor gives 18V which is the peak voltage.

 

[SimonTHK]

I got it now I think. So after my ac/dc converter I get dc waves and doesnt really get the peak voltage. I will first get the peak voltage after I put in a capacitor.

By that I mean. If I had 15vac and made an ac/dc converter I would get waves from 0v to 21v, that being the reason why I measure 12,9vdc. Then I add capacitor to "smoothing" ^^ out the waves and will then be able to read 18,8.

Is there any purpose of letting a car charger charge with these pulses, other than saving the capactor.

 

[Hero999]

No, it's just cheaper to omit the capacitor when building a battery charger.

Ideally there should be a timer or a voltage monitor to stop the charge when the voltage exceeds about 14.5V but it might not have it, if it's a cheap charger.

 

[SimonTHK]

it is a cheap charger

thanks for the help

 

[Grossel]

Reason why you measure 18V is because of . . . - see curve I uploaded.

As you see, the cap will charge up to the peak of the curve. And then discharge slightly. The voltage will keep about 18 volts.

Another thing is that a battery charger often has a higher output voltage then 12V when there is no load. Voltage out should read 16,9 - 1,4 = 15,6 volts at maximum.

 

[crutschow]

Is there any purpose of letting a car charger charge with these pulses, other than saving the capacitor.

The capacitor would do nothing when charging a car battery since the battery looks like a very large capacitor all by itself. The voltage sits at the battery voltage and you see very little of the pulsation voltage. So there's no reason to add a part that has no effective purpose.

 

[Grossel]

The capacitor would do nothing when charging a car battery since the battery looks like a very large capacitor all by itself. The voltage sits at the battery voltage and you see very little of the pulsation voltage. So there's no reason to add a part that has no effective purpose.

Well, it may reduce noise/ripple a little. Adding a inductor in series (before cap) will also kill some ripple

 

[crutschow]

But ripple is not a problem when charging a battery.

 

[carmusic]

it would need a pretty big inductor at 60hz! charge a car battery and check with scope and battery absorb almost all the ripple. the ripple from a charger is very little compare to ripple from a car alternator.

 

[crutschow]

it would need a pretty big inductor at 60hz! charge a car battery and check with scope and battery absorb almost all the ripple. the ripple from a charger is very little compare to ripple from a car alternator.

A car alternator would have less ripple since it's a 3-phase, full-wave rectifier, where the charger operating off the line has only a single-phase, full-wave rectifier.

 

[kinarfi]

The ripple of the charger will damage a battery if it is left on constantly, we boiled a bunch of batteries dry at a mine I worked at because we used car batteries to power the radios we used and left the charger on 24 - 7. The batteries were always being charged except when the equipment was powered down, but the radio still worked. After a battery is charged the ripple keeps trying to raise the voltage and boils the battery dry. We finally cured the problem with 14 volt zeners, or at least slowed the problem. This was about 20 years ago.

 

[crutschow]

Cheap, unregulated chargers will overcharge a battery if left on continuously. It's not the ripple so much as just the fact that it's voltage is not regulated. You need a charger with electronics that reduces the current to a trickle charge when the battery is charged.

 

[Hero999]

Adding a low 14V low droput regulator would've also solved the problem.

 

[kinarfi]

Like I said, this was 20+ years ago and all this neat stuff we have today may not have been thought about., but that's alright, I am not there anymore!!!!! hoorah.
Today I have 3 chargers that trickle upto the correct voltage and is then clamped. I don't that problem now.

 

[Voltz]

My favorit component is the capacitor. I like charging and discharging stuff, but also that it is good for stabilisation. I put it where ever I think it can have a purpose (it cant hurt atleast).

I have a normal car batteri charger that provides me with 12,9 volt dc. When I put a 6800uF 25v capacitor in parallel with the car charger, it gives me suddenly 18,8 volt dc. I can easily hear that my electric test motor (12v rc motor) gets many more RPM with the capacitor in parallel.

I have learned to multiply with square root of 2 when I go from vac to vdc with a lose of 0,35v over each diode. But I havnt learned anything about why my 12,9vdc can turn into 18,8vdc

can anyone lead me to an explanation why this is?

thanks allready

That is ACTUALLY what she said

 

[tvtech]

On a sidenote...I have been into Electronics for ever.

Damn....the most fun was in the early days at High School. Take a charged 100MF 400VDC charged mains cap.

The bully is threatening you. In your face and all. Take the charged Cap and simply drop it down his shirt while his face is in yours.

Suddenly he starts a jive reminiscent of St Elmo's Fire. Classic. Cause it's shocking him everytime the charged cap touches flesh on the way down. As gravity would have it.

Never bothered me again. Brain beats brawn everytime.