Car battery

[zachtheterrible]

How many amp hours is the average car battery?
Is it something like 100?

I was just curious because I was wondering how long my halogen torch would last before my car battery goes flat and i can't start my truck.

If it's 100, and my torch pulls about 5 amps (actually its more like 4 but we'll use 5 for simplicity) that would mean I have 20 hours or so?

Is that how long the battery will maintain 12v, or once it's been on for 20 hours my battery is flat dead?

Thanks :wink:

 

[windozeuser]

I've seen some car batteries rated for 220 Amp Hours.

You may find these sites helpful

http://www.gizmology.net/batteries.htm
**broken link removed**

 

[Oznog]

No, you likely saw 220 reserve minutes. Reserve minutes is how many minutes it can sustain a specified current (25 amps is standard) without dropping below a nominal voltage (10.5v is standard). A reserve minute is about 0.42 amp-hr.

The biggest deep cycles you'll find in normal sizes are around 100 amp hrs. Starting batteries are rarely rated for capacity. They are physically smaller and designed for cold cranking amps rather than capacity. The sizes vary a lot too.

A starting battery cannot sustain deep discharges without strongly decreasing the battery's life, and it gets worse if the battery is left in a partially discharged state for long periods. Exactly how much is how damaging is not well specified. I'd say 20% is safe enough if you recharge it soon.

A partially discharged battery may be unable to start a vehicle, especially in colder temperatures.

If you ask me, if it's a starting battery of unknown specs in good condition at 70F I would say 20 amp-hrs should be an absolute max to discharge it.

 

[bryan1]

Eh Zac,
Why not install a second battery in your truck and when you turn off the ignition the primary starting battery cuts out leaving the second battery to supply the power for the light. Then make up a cutout switch at say 11 volts so you don't drain the battery too much and when you start the truck the second battery gets charged. One of the first electronic kits I made did exactly this and I put it in my landcruiser ute mainly for powering the beer fridge and it was still working ok 5 years later when I sold the ute. Off memory I still have the construction details stashed away somewhere so if you want them I can firstly find them, scan them and e-mail it over to you.

Cheers Bryan

 

[DigiTan]

I've got a battery question. What causes a car battery to lose its ability to hold a charge? And are there any special recharging techniques that will renew a battery like this?

 

[Megamox]

I think its to do with its electrolyte being drained of the chemicals which produce the ions for the electrodes and keep the potential. Rechargeable batteries have a solution in which the process can be reversed, not all the way but enough to keep using it again.

 

[Klaus]

I think its to do with its electrolyte being drained of the chemicals which produce the ions for the electrodes and keep the potential. Rechargeable batteries have a solution in which the process can be reversed, not all the way but enough to keep using it again.

I'm glad you are just "thinking" that , since it is not correct. All that can be lost from the chemical brew in a battery is water, hence that is what one uses to top up a battery that's low in electrolyte.
What you describe is roughly the process of charging and discharging which indeed is a reversable process that can be used a number of times. The exact number depends on battery type, its use and the recharging regime. It does have a limit for the following reasons:

What makes batteries age and lose capacity varies on the type. For starter batteries, which have the 'plates' arranged as a thin mesh grid, the material wears away and falls to the bottom of the cell. It can then build up sufficiently to short out the cell. Or the plates can sulfate. In both cases there is a reduction of the active surface. You can do a google search on that for a more detailed explanation. There are chemicals and electronic gadgets available that 'could' reverse the sulphation, search on 'desulfators'. I built one and it works to some extend to rejuvenate a deep cycle battery but it never got as good as it was when new. I doubt it would do much good on a aging starter battery with its plates worn away.
Klaus

 

[audioguru]

Hey Zach,
Many times I drove to lunch then returned to work with my lights on and didn't notice. After work the lights were extremely dim and my car wouldn't start. After looking around at work for something to charge the battery with, I tried starting it after the half-hour break and it started!
Like I said, it happened many times, the half-hour break allowed the battery to refresh itself.

 

[zachtheterrible]

So I was right about 20 hours then eh? Cool :lol:

The reason I ask is that me n my friend decided to go shoot his gun at a "shooting range" up in the hills above the city and we used my spotlight to light up the targets. I got worried when after an hour I looked at the voltage indicator and realized that the battery voltage was not at 14 volts where it usually is. I couldn't tell exactly where it was because my gauge is screwy. I noticed later that the voltage goes down a bit when I turn the truck off, I guess that's the alternator putting out some extra juice?

So I didn't have anything much to be worried about.

 

[pike]

It might be rated at 20 hours, from 14v to 0v. But you might want to consider the fact that anthing lower than 10.5 volts would be considered dead in a car.

But maybe i'm wrong.

 

[Nigel Goodwin]

It might be rated at 20 hours, from 14v to 0v. But you might want to consider the fact that anthing lower than 10.5 volts would be considered dead in a car.

But maybe i'm wrong.

Not really, but it would be effectively dead long before that, as you wouldn't be able to start the engine. Presumably, as it's in the USA, it will have an automatic gearbox as well?, which means you can't bump start it either.

 

[stevez]

Zach - I've surfed thru some technical stuff on car batteries and came to the understanding that a rather large percentage of the capacity, in terms of total power, is gone if the battery voltage drops below a certain point - and as I recall that point was something like 11 volts. That would suggest little is left below that.

 

[zachtheterrible]

So just how long would I be safe burning my light? 5 hours? 10?

 

[stevez]

Google on "lead acid battery discharge curve" - the first item I got appeared to be a well written and researched article. I scanned it briefly and it might have enough information to help get to the answer you need. The information is consistent with my understanding that a battery is pretty much done when it gets to 9 or 10 volts. There appear to be quite a few factors that might play in to making a reasonable prediction of battery life but none seem to be too difficult to estimate - so that the resulting answer is at least the right order of magnitude. What I didn't want to do is keep doing the "it all depends" kind of response without offering a little more.

Let us know what you are able to determine - will save us some work at some point.