Volts vs Amps

I just had an argument with my car dealership concerning a starting problem with my car. They were saying that as long as I had 12.8 volts the car will start. I said that having 12.8 volts isn't enough that I had to have the amps behind it or it wouldn't start. They said that the amps didn't make a difference.

I hadn't had any problems with the car since I got it until today. This morning I noticed that that it was rather slow starting, I drove about 20 miles to work using the radio and lights. At lunch time the car wouldn't start. After getting a jump start at quiting time I drove the car about 25 - 30 miles to an auto store and had them check the system. The store employee said that the voltage looked fine but thought the amps were a low comparing idle output to 2000 rpm output. Said the alternator diodes checked good and seemed to think that either the battery wasn't getting enough amps or the alternator wasn't sending enough amps (don't know about this theory), however the battery appeared to be good. When leaving the auto store I couldn't start the car again and got another jump start and drove about 5 miles to my dealer. Of course they jump in and it start fine everytime for them and I didn't have a problem the rest of the evening.

Here's my question and my take on it. What is starting my car, Amps or Volts? It is my contention that if my voltage is 12.8 volts but my amps are low the car wont start. Which is what I think we were seeing. If voltage is low (a more serious problem) then naturally the amps will be low too and thus it wont start. The voltage can drop but as long as it has the amps the car will start. Are there any automotive and/or electrical engineers out there that can support this theory?

Just moving this thread to a more suitable section where it is more likely to be read.

There are two things the battery has to do - supply enough electricity to run the starter, and while the starter turns the crank, zap the plugs. So it's not doing one of those two things. Assuming that everything else runs fine, of course.

If you don't have enough amperage, then the voltage will drop to an unacceptable level when you try to start the car. You need both. A dead battery will probably have about 11 volts across the terminals. When you try to crank the engine it will drop to almost nothing.

Do you have 14.2v while running, if not it's not charging.

From what you discribe, sounds like a battery problem.

It doesn't necessarily have to have "14.2v" while running. It depends on your battery.

If you look at the structure of a battery, it is actually seperated into ~6 sections of battery acid, which hold the charge. Between these sections are thick lead plates. As the battery ages though, chemical RXs occur(water vapour seep into the acid, lead plates deterioate, etc.). Usually only one section is affected at a time. But since they're linked in series, total voltage will end up dropping(high school physics: Vt=V1+V2+V3 !). Normally, voltage should be over 12, around 13. But when it starts dieing, voltage will slowly drop, at around 11V(depending on car), the car will stop starting.

This is just from MY understanding. If there is incorrect information here, then fine, correct it. But IMO, this is what I feel is most correct.

well you forgot that the alternator is usually regulated to a higher voltage than the nominal battery voltage (or else it wouldn't be able to charge the battery), and typically the alt. output is at about 14V. The voltage across the battery will be approximately equal to the alternator output voltage when the car is running, and there will be a current running "backwards" through the battery as it charges.

voltage basically if the strength of a current flowin through a wire, whereas amps is the speed at which this voltage flows...

and contrary to what some people have said you can have theoretically 10000V and 1Amp, which is often the case in power station where electricty is 'stepped up' which means voltage increased and amps reduced, or 'stepped down' which is the opposite

well, you're not likely to have 1kV and 1A coming from the 12V battery in your car... Your ignition coil, on the other hand, probably puts out double to triple that voltage, but only a tiny fraction of that current.

I have to disagree with you about voltage and amperage:
voltage is a measure of potential, not current
amperage is a measure of current

To use a simple analogy, if you look at your electric system as a waterfall, potential (voltage) is the height of the fall, and current (amperage) is the amount of water flowing over.

so sorry ivy,

i guess my memory of high school physics isnt that good after all:D

You need both 12V without enough amps wont crank. You can't take 8 1.5v AA or a 12v screwgun battery and start your car because you need amps. most batteries have like 500cold cranking amps. the formula is amps X volts = watts or power
it takes about 1-2hp to crank a car. at 745 watts per HP +15-20% motor loss it takes around 80-160 amps at 12v to crank.

If you are trying to sneek a little more HP from your engine. Don't put in a massive sound system in and drop to a 30 or 40 amp alt. this can add up to 5hp over a 70-120amp model

I've done alot of work with electrical systems of all kinds, and let me tell u that aquadave knows his shit. If u dont have amprage u cant start a car not even with 1000 volts. If you dont believe me go get a 1000 volt teslacoil with micor amprage and touch it you wont die u'll just get a little shock. So if u dont have amprage u dont have shit.

nobody said he was wrong...

Go buy a battery, plain and simple. I'm runing a Optima Red Top in my car. 800cca 1000ca with a 120min researve capacity. And is under warranty for 72 months. It's a little spendy, but nothing to bad.