ahmadalzuod
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- jordan
What is meant by the words written on the car battery 40Ah
car battery
- Thread starter ahmadalzuod
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K8MHZ
Senior Member
- Occupation
- Electrician
That is the rating of the battery, based on the current and time it takes to go from a full charge to fully discharged.
For instance, 40Ah would mean (roughly) you could draw one amp for 40 hours.
Here is a decent article on the subject.
http://overlandresource.com/what-is-an-amp-hour-and-how-to-calculate-battery-capacity
gadfly56
Senior Member
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- New Jersey
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- Professional Engineer, Fire & Life Safety
I wonder if that's actually the sustainable discharge rate? A 40 Ah battery is pretty puny for a car; I have fire alarm panels backed up by twin 12vdc 100Ah batteries, and they look like they'd be about the right size for a motorcycle. Unless the OP left off a zero and meant 400 Ah.
Depends on the make and model of the car but very unlikely to be that.
On a Renault 5 or a basic Ford Fiesta it's a 40Ah battery.
On a fully loaded model with all the boys toys (electric everything) like a Range Rover, it might be 100Ah
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
In the US at least, a battery which is sold for motor vehicle cranking use will rarely have an AH capacity specified, let alone marked on the battery itself.
The MCA, and CCA are measures of internal resistance and so the maximum instantaneous current available with a pretty steep voltage drop.
The Reserve Amps rating is a measure of capacity, but it is determined by a 20 minute discharge test rather than the 20 hour constant current discharge test (C20), which is the most commonly used standard for AH capacity.
Ignoring completely the effects of Peukert's Law (which says that the same battery will different AH total capacity depending on the test load current in a predictable way), you can relate Reserve Amps to an AH value by dividing the Reserve Amps number by 3.
K8MHZ
Senior Member
- Occupation
- Electrician
I have some 100Ah SLA 12 volt batteries that are bigger than car batteries by quite a bit. They weigh 102 lbs. each.
It may be case here too. It isn't marked on mine and the battery is made to a Johnson Controls Battery division (USA) specification.
But the OP lists Jordan as his location.
- Location
- Wisconsin
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- PE (Retired) - Power Systems
The car battery I just got, from Baterries Plus, did not have a CCA value, it was rated in AH. This was a first for me
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
To me that indicates that either it is a deep-discharge battery designed for RE use or it was intended for somewhere other than the North American market.
We supply UPS systems from time to time.
Mostly we use Yuasa batteries. They usually have Ah rating on the individual batteries. It's the 20 hour rate as a rule.
The data sheets we get and have to provide our customers have Ah ratings typically for 20, 10, 5, and one hour.
The curves go from 0.05C (20 hours) to 3.0C at 25degC.
Maybe such information is available for batteries used in automotive applications.
Mine is called 60Ah. The main function is to crank the engine to start it. It's a 2.2l Diesel. With the higher compression ratios of that compared to gasoline, the initial torque is greater. But starting is not more than a one or two second event. After that, the alternator takes over.
I can't think of any circumstances where the rated capacity would be stretched.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
That is why in the US the primary rating is the ColdCrankingAmps or CCA, which measures Amps delivered at a defined voltage (>7.2V) for a set minimum period of time (30 seconds) when the battery is cold (0F), simulating a cold weather start. With sufficient engineering to lower the internal resistance you could in theory deliver that kind of starting power in a 5AH battery.
The AH capacity would determine how many start attempts the battery could make without the engine firing, so with no help from the alternator.
The reason that Reserve Capacity (RC) (which BTW is the number of minutes at 25A, not the number of amps for 20 minutes as I originally stated) is important is that we Yanks tend to live in our cars and have tailgate parties without the engine running. The drain of radio, parking lights, vent fan, etc. is approximated at 25A, and we need to know how long we can keep that up before we have to start the engine.
It is also useful for keeping the ignition going to drive home after you jump start a car that has a dead alternator.
gadfly56
Senior Member
- Location
- New Jersey
- Occupation
- Professional Engineer, Fire & Life Safety
In the US, automotive batteries are rated for "CCA", cold cranking amps. This is the number of amps that can be delivered at 0F for 30 seconds without the cell voltage droping below 7.2 vdc. Typical values might be in the area of 500 while a premium battery might deliver 850 - 1000 CCA.
gadfly56
Senior Member
- Location
- New Jersey
- Occupation
- Professional Engineer, Fire & Life Safety
Whoops! Left my post in the edit bin a little too long. Didn't mean to step on yours, GoldDigger.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
No Problem. My reply sat in the edit window for awhile too, so it was a toss up who would post first.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
No Problem. My reply sat in the edit window for awhile too, so it was a toss up who would post first.
I tend to write longer responses, so I am vulnerable to that a lot. I just hope people have the patience to read them.
For yourself and the Gadfly, a couple or three points in no particular order or relevance.That is why in the US the primary rating is the ColdCrankingAmps or CCA, which measures Amps delivered at a defined voltage (>7.2V) for a set minimum period of time (30 seconds) when the battery is cold (0F), simulating a cold weather start. With sufficient engineering to lower the internal resistance you could in theory deliver that kind of starting power in a 5AH battery.
The AH capacity would determine how many start attempts the battery could make without the engine firing, so with no help from the alternator.
The reason that Reserve Capacity (RC) (which BTW is the number of minutes at 25A, not the number of amps for 20 minutes as I originally stated) is important is that we Yanks tend to live in our cars and have tailgate parties without the engine running. The drain of radio, parking lights, vent fan, etc. is approximated at 25A, and we need to know how long we can keep that up before we have to start the engine.
It is also useful for keeping the ignition going to drive home after you jump start a car that has a dead alternator.
Yes, the CCA makes sense. The temperature aspect also makes sense given that oil viscosity changes with temperature. And probably 0degF is a good reference point. It's about -18C. I've been in Canada, mostly around the great lakes and had to endure -20C and, in UK, it has now and again dropped to -27C, around -17F. That's quite cold. At those sort of temperatures you maybe take other measures. Oil heaters for example.
Having to crank for 30 seconds? For me, that would be indicative of other problems. But I suppose there needs to be a defining measure.
Yanks? A friend and ex-colleague of mine uses the term "septics". It isn't intended as derisory. Just the rhyming slang that gets used in Cockney London and has spread to the south east of the country.The connecting word usually gets left out.
"Your dog's barking" he might say to me. He meant my phone was ringing. How so?
Dog and bone. Bone rhyming with phone......you pass wind and it causes a pen........pen and ink, stink....
And my sweet wife, an American, isn't a Yank.
That Mason-Dixon line................
Off topic I know. Mods, be kind.
That is why in the US the primary rating is the ColdCrankingAmps or CCA, which measures Amps delivered at a defined voltage (>7.2V) for a set minimum period of time (30 seconds) when the battery is cold (0F), simulating a cold weather start. With sufficient engineering to lower the internal resistance you could in theory deliver that kind of starting power in a 5AH battery.
The AH capacity would determine how many start attempts the battery could make without the engine firing, so with no help from the alternator.
The reason that Reserve Capacity (RC) (which BTW is the number of minutes at 25A, not the number of amps for 20 minutes as I originally stated) is important is that we Yanks tend to live in our cars and have tailgate parties without the engine running. The drain of radio, parking lights, vent fan, etc. is approximated at 25A, and we need to know how long we can keep that up before we have to start the engine.
It is also useful for keeping the ignition going to drive home after you jump start a car that has a dead alternator.
Funny you should mention starting power from a small battery and engineering. A guy I know who is challenged in the IQ department tried to jump start a work van with a brand new Ultra tech brand 33 AH SLA battery. The terminals melted and sunk into the battery. He then tried a used 33AH Yuasua brand SLA battery... Success without significant terminal damage. I assume he didn't try put the batteries in service after. You get what you pay for, Yuasua is worth the cost. Calling AAA was apparently too hard!
Did he connect it in parallel with the dead battery?
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
Under some conditions, you might actually get better starting performance by letting the fully charged extra battery transfer some charge to the dead battery so that it could deliver a short starting surge to help the new battery out.
Who was is that described a guy turning the second battery upside down and connecting it terminal to terminal since he did not have jumper cables?
- Occupation
- Licensed Electrician
That was on an episode of "Car Talk".
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