Is voltage stabilisation neglected in the design of cars?

[Hattee]

Use of electronics has its place in voltage regulation of DC power in modern automobile. The regulation usually handles the rms voltage pretty well. However, its ability to handle quick cyclical transcient changes depends alot on the battery response to the transcient demand under the condition of pulsating DC output from the alternator and its regulation speed.

 

 

POTENTIAL WEAKNESSES OF AUTOMOBILE VOLTAGE REGULATION

 

1. Quite unlike typical DC power sources which have large capacitor as power reserve to stabilise cyclical transcient voltages, automobile uses storage battery as its power reserve, instead. Large capacitor is usually better in supplying very quick and sharp repeated demand of power than battery, particularly when the battery is already supplementing power to flatten the alternator's rectified pulsating output.

 

2. If the battery is no longer new, operating in elevated temperature or not in fully charged condition, its ability to handle cyclical transcient voltages is often compromised.

 

3. DC loadings simultaneously from powerful fans, external lightings and other high power systems can cripple the stability of battery voltage coping with alternator's pulsating DC, hence, affecting the effective electronic voltage regulation.

 

4. Under fast changing load, the electronic regulation is always slow in responding due to the lagging control of the alternator's field strength. The control circuit needs the time to respond with additional current to increase the excitor field strength facing the inductively opposing field from the loaded power generating coil.

 

5. Pulsating ignition can then cause significant sharp voltage dip, particularly at instances when alternator's pulsating voltage close to 0V. Extremely short interval of 1V-4V dip cannot be detected by voltmeter which may still show a healthy 13VDC-13.5VDC (rms).

 

Note: Typical ignition primary draws rms current of 3amp-6amp. Assuming 20% duty cycle for ignition built-up, each ignition effectively draws 5xroot2 i.e 20amp-40amp peak for 20% of the time. This is an important consideration for ripple current in order to achieve effective voltage stabilisation.

 

 

 

POSSIBLE ADVERSE EFFECT FROM INSTANCES OF QUICK CYCLICAL VOLTAGE DIP

 

1. This transcient dip is adverse to performance of engine electricals, particularly the ignition itself and other voltage sensitive systems.

 

2 Ignition energy will be inadequate or even misfires at instances of significant voltage dip. Perhaps, unstable idling and power-lacking acceleration might be the symptoms of such conditions aggrevated with max cold a/con and full headlights.

 

Any comments?

[Chisiang]

2. If the battery is no longer new, operating in elevated temperature or not in fully charged condition, its ability to handle cyclical transcient voltages is often compromised.

 

I'm not very sure about this line. A few months ago when my battery got completely dried up and kaput, my voltage reading became extremely stable and maintained at a high of 13.8V or even upto 14V!

[User12343]

it all boils down to cost.

 

how much 1 Farads of capacitance cost when compared to a normal 55AH battery?

 

the formula for charge, Q when a voltage V is applied across a capacitor having C Farads is Q = C*V

 

Similarly Q is equal to the amount of current, I passing in time T, is given as Q = IT

 

equating both, u have IT = CV.

 

now assuming a starter motor draws a cranking current of 10A. you have a 1F cap charged up with a 12V battery and disconnected. u get 12 coulombs of charge.

 

now if u try to start the engine which takes 10A, from the math given, the cap will run flat in less than 1 seconds.

 

if suay suay ur car cannot start in 1 seconds (and it is almost to guarantee a dead engine), your cap is useless w/o the batt connected.

 

u can counter by using a 100F cap, but what is the cost? and also capacitors self-discharge pretty fast when left alone, and who is to guarantee that a charged 100F in the car is able to start 1 week later???

[Hattee]

Quite true if we only look at the rms voltage.

[Hattee]

Yup, no capacitor can replace a battery in storing energy.

 

Perhaps, comparison here is whether a correct capacitor plus battery will do a better job than just a battery throughout its 2-3 years life.

[Genie47]

First thing's first. Prevent the battery from accelerated dying in the first place. A desulfator comes to mind.

[Jonbummer]

will a Desulfator be something like this:

http://cgi.ebay.com.sg/ws/eBayISAPI.dll?Vi...em=190019246897

 

juz a qnn, will a VS work well together with Desulfator?

or having a desulafator will render VS rather useless?

 

cheers

[Genie47]

I have no idea. I would rather use the desulfator over the VS because the main VS is the battery itself! So to make the battery last longer would be a better idea over a VS.

 

You can construct one yourself but one of the components is quite hard to get.

 

See this link: http://www.mycarforum.com/forum/gforum.cgi?post=1168679

Edited October 31, 2006 by Genie47

[Dermocyma]

pardon me for asking. i m just curious. the thing u listed in ebay cost about the same as a new battery.

 

why would a desulfator be anyway better than changing a new battery every now and then for ur ride?

[Genie47]

Everytime you change a battery you not only spend money buying a new battery. People need to spend money to get rid of the battery. FYI, you cannot suka suka dump a battery. It's material is toxic and I'm not talking about the plastic casing.

 

So by making it last longer, you are securing a cleaner future for your children.

[Dermocyma]

oic. thanks.

Edited October 31, 2006 by Pingman03

[Jonbummer]

hmmm

interesting...

i can't construct thigns lah

the furthest i go is Lego!!!

 

i would rather get some ready for installation

also wat happen if i get the wiring wrong...

kaaaBOOOM!

 

[Genie47]

Just read the instructions. If you can turn on your TV, you can wire this up.

[Jonbummer]

errrmm

beside the points Genie put forth

for me, the most impt thing is that this device

will potentially save one from a dead car in the

middle of nowhere, at an ungodly hour..

 

thus, with all the good of this device i might get

one myself for Christmas!

 

cheers

[Dermocyma]

i believe it only extend the death of a battery. can the battery last forever on this device genie?

[Jonbummer]

now u make it sounds like Lego...

[Jonbummer]

from the device description:

 

< Most automotive batteries are designed with a 4

year lifespan, and yet, we dispose our batteries

every 1 -2 years. Think of the money you will save.

 

The desulfator is made to last for more than 10 years!

Use it over and over! >

 

hmmm

if this thingy can make the batt last its full potential

of 4 years then it's of value rite?

 

the 2nd statement is '?????'

is he saying the Desulfator itself can last 10 years or

the Desulfator can make the batt last 10 years

 

if it's the latter, then it's really

[Genie47]

No but it will last longer than the usual 1-2yrs. 4-5 is thereabouts for a battery's life but 7yrs is the record I have heard. The guy didn't have a desulfator but he was driving in perfect conditions for extended battery life. Still had to get rid of it because the battery outlived the casing!

Edited October 31, 2006 by Genie47