CCA to Amp Hours???

Is it possible in anyway at all to work out how many amp hours a car battery is by its CCA rating? I just want to know so i can safely tell how long i can run my fridge for until my dual batteries are fitted.

Its just the standard battery that come with my BT50 and is rated at 622cca and is the size of a N70z i would say. Is there a simple way of roughly working out its capacity, or should i just run fridge of it and measure the volts every hour? Will that work? I only have a basic multimeter no other auto sparkie tools.

Me and electrics dont really go well, as i have been told too many different things over the years by so called ''Pros''. I start to understand basic principles but then am told, no thats wrong and given a different opinion, to which i am told is also wrong. Seems to be a never ending cycle.

I'll let you guys steer me in the right direction i think.

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Reply By: Maîneÿ . . .- Tuesday, Apr 27, 2010 at 05:38

Tuesday, Apr 27, 2010 at 05:38

There's no *definitive* way of doing that with CCA's.
Just use your multimeter every few hours.

Go for a drive every now an then to recharge the battery if that's a realistic plan.

Maybe invest in a low voltage disconnect switch fitted in the fridge cable if it's not already a feature of the fridge?

Maîneÿ . . .
AnswerID: 414411

Reply By: get outmore - Tuesday, Apr 27, 2010 at 06:35

Tuesday, Apr 27, 2010 at 06:35
alot of variables as to how long that battery will power your fridge and still start your vehicle

mostly ambient temperature, how cold your fridge is set and how big it is as well as if your vehicle is petrol or diesal

guessing the fridge will be set around 0-2deg, you have a diesal and the fridge is around 40l I wouldnt be leaving it for a full day and night before driving it

in hot weather even one night and a full day could flatten it

if you want to stop and prop - it just wont be adequate. if you using the vehicle to poke around a bit during the day you should have no worries
620cca is a fairly decent sized battery
AnswerID: 414413

Reply By: Sand Man (SA) - Tuesday, Apr 27, 2010 at 07:22

Tuesday, Apr 27, 2010 at 07:22
The most important differentiation between Cold Cranking Amps and Amp Hour rating is in the design of the batteries.

Starting batteries are designed to give a high current for a short period of time.
Deep cycle batteries are designed to provide a small discharge of current over a long period of time.

The safest option for you is to purchase a low voltage cutout device (battery protector) to protect your battery from excessive discharge. They cost about $30 and simply connect in-line between the 12 volt outlet and the fridge lead.

You can buy a Protecta brand from Kmart and some motor retail outlets or order a device online from
Derek advertises on this site and provides a prompt dispatch service.

I use a low voltage cutout device even though I have a good size Auxiliary battery.


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AnswerID: 414415

Follow Up By: fisho64 - Tuesday, Apr 27, 2010 at 09:24

Tuesday, Apr 27, 2010 at 09:24
I reckon you got it there Sandman.
CCA is measuring something that a deep cycle is not designed for, nor does it have any direct relationship to total discharge amps..

Maybe an analogy is comparing a tractor and a family car by measuring drawbar pull?
Car isnt designed or built to withstand or perform longterm in that manner
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Reply By: garrycol - Tuesday, Apr 27, 2010 at 08:32

Tuesday, Apr 27, 2010 at 08:32
Divide by 5 to get the theoretical figure BUT starting batteries cannot go down to deep cycle so the usuable figure is to divide by 10 - rough and ready only.

AnswerID: 414423

Follow Up By: Member - Phil G (SA) - Tuesday, Apr 27, 2010 at 10:27

Tuesday, Apr 27, 2010 at 10:27
Thats a good estimate. My N70 Exides are 620cca and 80Ah (whitten on the case).
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Follow Up By: Member - Barry (NT) - Tuesday, Apr 27, 2010 at 20:01

Tuesday, Apr 27, 2010 at 20:01
Agree with above as a guide,,, matches my auz abttery also Phil.

If Waeco it has switch to protect battery - read the book as to what you want,,, ie maximum BATTERY protection,,, BUT watch the volts as mentioned above

I do this even with an aux battery to maximise battery life,,,, but in hot weather the need to run the fridge is high,,, start and go for a drive is an option when required

another option is to switch off over night and go for a drive first thing before day gets hot,,, BENEFIT is you are driving while fridge runs 100% duty cycle AND you are still charging battery,,, works for me if needed

hope this helps, Cheers Baz
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Reply By: Battery Value Pty Ltd - Tuesday, Apr 27, 2010 at 09:15

Tuesday, Apr 27, 2010 at 09:15
Hello Muntoo,

from top of my head, starter batteries are also rated in RC which stands for reserve capacity I think.
This is an expression for minutes of drive time you get when the fan belt snaps.
The current during RC is defined to be 25A, so f.e. if your starter battery has a RC rating of 90 (minutes), multiply this by 25A and you get 37.5Ah.

This rating pertains to rare occasions, so don't expect your starter battery to sustain a great number of RC discharges.
The RC figure is also battery age dependent and can only be measured accurately by connecting a known load to the battery and measuring the time it takes for the voltage to drop to 10.5V.

It's also important to keep in mind that the recharging times can be over ten hours, if a starter battery is cycled below 30% of its capacity.
Not observing this recharging time will deplete the capacity with every discharge, leading to increased internal resistance which will make cranking more and more difficult.

Best regards, Peter
AnswerID: 414426

Reply By: Allan B, Sunshine Coast (Membe - Tuesday, Apr 27, 2010 at 09:50

Tuesday, Apr 27, 2010 at 09:50
Muntoo, as has been said, you can approximate to your battery percentage of remaining capacity by observing the battery voltage.

You will need to be careful that you do not discharge the battery so low that there is not enough energy remaining to crank the engine for a restart. Maybe 50% is realistic. This is indicated at about 12.2 volts but that is to be observed with no load on the battery and after it has been resting for about 20 minutes. It also is indicated by about 11.5 volts with a load of several amps such as a fridge and accordingly this is where the Low Voltage Cutouts are typically set. Some fridges have a low-voltage cutout inbuilt, sometimes adjustable. Otherwise an accessory cutout can be utilised.

A technique which can be useful is to run the fridge on a colder setting whilst the engine is running then reset the fridge to a warmer setting when the engine is stopped for any length of time. This will keep the fridge as cold as possible with less load on the battery whilst stationary.


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AnswerID: 414432

Reply By: dbish - Tuesday, Apr 27, 2010 at 10:25

Tuesday, Apr 27, 2010 at 10:25
Hi Muntoo I use a N70ZZ battery 600CCA, 130RC min, for running a 12V diving compressor it draws 25A & i get about 2Hrs running time before the compressor starts to slow down then its time to stop diving. So about 50AHR is what i get out of a 5yr old battery. It is always recharged as soon as possible then left on a 5W solar panell. This works for me as the batterys are cheap about $120. Cheers Daryl
AnswerID: 414438

Reply By: Member -Signman - Tuesday, Apr 27, 2010 at 11:22

Tuesday, Apr 27, 2010 at 11:22
Most lead-acid batteries are MISlabeled with a higher Ah rating than what they really usefully contain.

UNLESS you get a true deep-cycle battery, you're not getting what you expect.

Gel-cells are usually WAY overrated on useful Ah. The rating on the battery probably also includes the BTU's you'd get if you were to set it on fire after you powered your load with it.

For any "regular" lead-acid battery with a CCA rating, just divide that CCA number by 20 and you'll have it's true Ah capacity.

But, STARTING BATTERIES are not deep-cycle worthy. So, you can only have the first 20% of that CCA/20 figure without damaging that battery. As an example, a 600 CCA battery has 600/20 = 30Ah. Then take that 30Ah/5 = 6Ah of usable charge without damaging the battery.

Short answer is, DON'T bother with starting batteries, except for STARTING things.

Just buy deep-cycle batteries and be done with it for your storage bank needs.

In any case....

You don't need a lab to know approximately what you can get from a battery, just a CONSTANT CURRENT load.

Electrical charge is defined in measured quantities of electron charge called COULOMBS.

"Charge" in a battery is defined as Ampere-hours.

ONE "ampere" is defined as ONE COULOMB PER SECOND of "flow".

So, you COULD write this as (Coulombs per second)-hours.

"Ah" is much simpler.

Forget the "voltage" except as a starting and finishing point measurement. The batteries are rated in Ah NOT Watt-hours. Ah only considers the COULOMB content of the battery, not the voltage at which it is released.


1. Start by using 80% of your Ah rating as your REAL USEFUL CHARGE CAPACITY. This means a 100ah battery should be considered an 80Ah battery. Got it? Good.

2. Divide that 80% number by 20. So it's 80Ah/20 = 4 amperes. You can draw a steady 4 amperes for 20 hours from a 100Ah battery without damaging it.

3. Apply the 4amp load to the battery for 20 hours. Then remove the load and see where the battery voltage pops up to.

If it's below 12v, then the battery's labeled rating is wrong.

Recharge the battery with a charger that can get it's voltage up over 12.6v within a couple of hours. If it takes longer than that to get over 12.6v, the battery is already beginning to sulfate.

AnswerID: 414442

Follow Up By: Allan B, Sunshine Coast, - Tuesday, Apr 27, 2010 at 14:33

Tuesday, Apr 27, 2010 at 14:33
Seeing as how Muntoo said...."Me and electrics dont really go well" that foregoing by Signman must be a real help to him !!! LOL
Coulombs already?!!


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Reply By: Muntoo - Tuesday, Apr 27, 2010 at 18:21

Tuesday, Apr 27, 2010 at 18:21
Ha Ha see i have proven myself correct again.

I have been told a few different things here, some saying 10% of cca =a/h , other saying 20%.

It seems everyone has an opinion but which is correct, because im confused. And its weird because people say they are no good for running fridges but my old man and his mates had never heard of deep cycle batteries till 5 years ago. They ran fridges and lights and all sorts up Kalumburu and Mitchell Plateau for decades before they become tourist destinations, and they never had dual batteries, just the good old cranker. So surely it cant harm them to run a fridge a couple of times. Cause there batteries still lasted 4 years or more.
AnswerID: 414484

Follow Up By: Lex M (Brisbane) - Tuesday, Apr 27, 2010 at 18:37

Tuesday, Apr 27, 2010 at 18:37
"I have been told a few different things here, some saying 10% of cca =a/h , other saying 20%. "

There is no standard answer. It will depend on the construction of the individual battery and will vary from manufacturer to manufacturer.
Run the fridge, measure the voltage as suggested and find out the capacity of your battery.

You can run any fridge of any battery. For how long will depend on the fridge and the battery. How much it will harm the battery depends on the type of battery, how far and how many times you discharge it, how long it stays discharged, how it is recharged, the temperature, the day of the week. Well maybe not the day of the week, but there are so many variables it's difficult to say.
Every time you discharge a battery you "harm" it. Otherwise it would last forever.
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Follow Up By: Ianw - Tuesday, Apr 27, 2010 at 18:59

Tuesday, Apr 27, 2010 at 18:59
A lot of people (including those using this forum) use Fullriver HGL batteries. They are not deep cycle batteries but they seem to give satisfactory service and the people are happy with them. One cannot say outright that non DC batteries are not suitable for camping and fridge application. In AGM batteries there is not a lot of difference between the two. Actual performance is very similar, but the life of the non DCs are a bit less. This is counter balanced by the lower price of the HGLs. Batteries are a consumable item. Use them until they fail, then replace them. If we believe the "experts" we should not discharge below 70% SOC, therefore we need 3 x 100 Ahr batts to be able to use 100 Ahrs. Cycled every day they will last for 3 years, then you are up for $1000 + for 3 more. If you have one 100Ahr AGM and discharge 80 to 100% it will last only one year and it will cost you $300-$350 for another. So in the end it is precisely the same cost as having three. And you have to carry a lot less weight !! Don't let your battery rule your life!!

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Follow Up By: Allan B, Sunshine Coast, - Tuesday, Apr 27, 2010 at 19:05

Tuesday, Apr 27, 2010 at 19:05
Yes Muntoo, as you say, ask a technical question and you will get half-a-dozen answers, all different.
I have an electrical background and can theorise until the cows come home, but I recognise the reality of actual experiences as you describe about your old man.

I can say that for some years I had an Engel fridge in a little Subaru running from the cranking battery with reasonable success. Then I added a second battery in the back and it was a regular lead-acid cranking battery that lasted for several years. In fact both batteries were still performing OK when I got rid of the Subie.


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Follow Up By: Allan B, Sunshine Coast, - Tuesday, Apr 27, 2010 at 19:11

Tuesday, Apr 27, 2010 at 19:11
"Don't let your battery rule your life!! " from Ianw.

Now there's the best advice offered Muntoo!


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