car battery ratinsg what do they mean ?

as title suggests i was just wondering what the ratings mean on car batterys

12v = 12volts obviously

but then there is AH and AMPS now what do they mean in basic terms ??

ie is is mroe ah better? more amps better ??

more = better

Fish dude, this is quite a hard thing to explain depending on how much you know.

Basically:

Voltage is a sort of push (it's joules/coloumb of electrons) and you can forget this and pretend it's 12v constantly from a battery* If this was though of in a context of flowing water down a pipe, this is how quickly it would be pushed down really.


The number of amps is the amount of current coming out - so in this case it's like the amount of water being pushed by the voltage.

Generally a high voltage leads to a high current flowing. Power = current x voltage, so you get a higher power output.

For car batteries you get a rating like 50Ahr

For example, 50AmpHrs means that it can provide 12volts at 1amp for 50 hours or any combination (i.e. 2amps for 25 hours at 12volts). This would mean 12watts are being used (watts are power and power = voltage x current).

It gets more complicated however in that some batteries are intended to provide the power over a long period of time but at a small current where as others are intended to provide a lot of power for a short period of time.

The higher the AmpHr rating of your battery the better but you also need one suited to a car application.

Sorry this is very broken up and not very clear, I just realised how hard it is to explain for me.



* This isn't true because car batteries have an internal resistance (this is why your lights dim when you start up the car, not that the battery can't provide enough power like most people think) - you don't need to know about that though.

Paul's right, but this is a bit more simple:

Amp hours (AH) is the amount of energy that your battery can store. i.e. if you got a 200AH battery and a 400AH battery and left each one to power a pair of headlights, then the 400AH battery would last twice as long as the 200AH.

Amps is the "cranking power" of the battery, i.e. how much current (and therefore power) it can supply. Bigger engines (and diesels) have more powerful starter motors which need more current to start the engine - hence something like a 1.9JTD Stilo will need a battery with a higher amp capacity than a 1.1 litre Punto.

ahh ok thanx you 2 for explaining i just wanted to compare my new battery to my old one. as i just had to replace it and noticed ratings were different.

my old was a fiat one and was 40ah and 200amps
my new one is bosch and is 39ah and 300amps

Well they are very similar but the new one can provide a higher surge current.

JB, better explained

How much was a new battery? The one in the Punto is on its way out, its fine most of the time but when its really cold it struggles to turn the engine over. I've no idea what the bloody things cost though as I've never needed to buy one!

hi there it depends on ya car and were you go to get to one, mine was fine in warm but other morning it realy struggled and it got so low on power while turning over it stopped for a second and my clock reset but now its fine.

as for the battery you have a mk1 so they are cheaper.

halfrauds will charge you £55 for there brand but a motorfactors near me sell bosch 4 year garentee ones for about £30 (near me = norwich)

it defiantly pays to shop around phone a few places up from teh yellow pages.

I got mine for my 55s for £30 from Bullseye. Not sure of the make..
Shame my car doesnt start anyway, kind of a waste.

yeah ben folowing yoru post, hope you can get ya car sorted mate. wish i had mroe knowledge to help ya.