Thread: Batteries, CCA and Amps

CCA is how many amps your battery can dump for 30 seconds or a minute (depending on which standard its tested to) while maintaining over 10.5v.


Very very aproximately...

devide CCA by 120 (for the 30 second standard) or 60 (for the minute standard) and that gives you the maximum amps you can draw and have the battery provide power for 60 minutes.

What you really want to look at is the RC (reserve capacity) of the battery, thats how long the battery can put out 20 amps for in minutes and maintain a voltage of over 10.5 or 9.5v (again depending on which standard is used)


you will notice that if you happen to have a battery that clearly explains which standard it was tested to and has both RC and CCA on it that the math on the RC gives a better total AH than the CCA as in

RC(amps*time) > CCA (Amps*time)

its a funky deal with the way that wet cell batteries work and isnt worth stressing about other than to remeber that the less power you draw from the battery the longer it lasts relatively speaking.

Hi Dave and not quite right. The industry standard for rating the RC of a battery is set by the amount of time the battery can be discharged at 25 amp ( not 20 ) and the voltage does not drop below 10.5v

The RC is theoretically the amount of time you can operate the average vehicle using a given battery if the alternator fails.

I have no idea how they arrived at the “AVERAGE” vehicle?

Plus, while you can use the RC rating to get a VERY rough idea of a battery’s potential AH, you still have to divid the RC by 2 to get somewhere “near” the battery’s Ah and again, this will vary greatly from one type and make of battery to another.

ok so that standard is that

SAE, BIC, DIN, IEC, EN, BSR or JIS?

http://www.acdelco.com.au/PDFs/ACDelcoToday_Dec08.pdf

Hi Dave and I’ve seen that brochure before.

I must admit to having a good chuckle when I see it. While there is some handy info in it, it is also obvious that it was written by a journalise and not by a genuine battery technician.

For a starter, the brochure goes on about “How batteries loose their power” listing the different devices and how much they might draw.

All good and well but for all the devices to be drawing power, they have to be connected to the battery, again, so far so good.

They then have a chart showing the charge percentage level, specific gravity and the matching voltages for a 12v and 6v battery.

Problem is that they are talking about the current drain caused by devices connected to a battery but the chart is for OPEN CIRCUIT voltage readings.

While a battery has a load on it, no matter how small it is, OPEN CIRCUIT voltage levels are totally irrelevant. Further more, an OPEN CIRCUIT voltage is only accurate after the battery has been sitting in a No-Load/No-Charge state for at least 24 hours.

And while trivial, but just to add insult to injury, every battery shown on that brochure is a sealed type battery, so how do they expect you to get a HYDROMETER reading?

This chart below is for loaded battery voltage readings, but note, to get a reasonably accurate reading with this chart as a reference, the load should be no more than a couple of amps at most.

the main interesting point of the brochure is that it lists the minimum voltages for the standards... (and those are under load voltages not open circuit)

and batteries are like engines..

which engine do you want the one that makes 150KW or 200HP? (yes thats not a perfect conversion but should serve the point)

Using the fancy battery tester at work I can pass or fail the same n70z by testing 700cca at the SAE or ISO standard.

my point?

The standards aint so standard.

Originally Posted by Blknight.aus

The standards aint so standard.

Hi Dave and you’re absolutely correct there and I posted up something I hate seeing others do, I called it the industry “standard” when, in this case, it is not a STANDARD.

What I should have posted is the the 25 amp RC rating is the most commonly used in the industry but this does NOT make it a standard.

Sorry about that, but back to subject and the RC rating is just not suitable for trying to determine the Ah capability of a battery because there can still be as much as a 50% margin of difference between batteries types and makes, so it’s not really suitable to get an idea of Ah.

yep its getting worse too, Ive seen examples of batteries using different parts of the different standards to get the numbers to look impressive.

and yes there is a simple way of finding out what a battery can provide in terms of RC Ah and CCA (after you pick which one of the standards you want to test to) you plug it in to a multi thousand dollar battery tester and leave it sit for about 20 minutes and it will tell you what your battery is capable of.

you can replicate the testing standard (for RC fully charge the battery then put a 25 amt load in it and see how long it takes do drop below 10.5v)

but theres no seriously reliable way of converting any of the specs to any of the others.

Originally Posted by drivesafe

I thought I would take advantage of the information here and carry out a little experiment to see what voltage was needed to start my 300 Tdi.

The battery is on its last legs and will need to be replaced very soon. If I don't use it for over a week, I am likely to have to top it up with the battery charger. Hardly surprising as the battery is the same one that was in the vehicle when I bought it four and a half years ago.

Anyway after not using the vehicle for five days, I measured the voltage today at 11.5 volts, which according to the table is only about 20% charge. I was a bit surprised that the voltage didn't change perceptibly when I turned on the parking lights. Is that what you would expect? Or don't the parking lights produce enough load to change the voltage?

There was just enough power to allow the glow plugs to operate normally and start the engine, but there wasn't much to spare. It started, but only just.

I realise that running a battery down to 20% is not advisable, but I had hoped that this little experiment would give me some useful information. I thought that if I ever fitted a battery monitor or some sort of volt meter, it would be helpful to know what voltage was needed to start the engine.

Was I wasting my time? Does the figure I came up with of 11.5 being well below what is safe tally with accepted ideas?