Relationship between CCA & State-of-charge

Hi Wok,
To answer part of your question re CCA and state of charge. It has a significant bearing. If for example a battery when fully charged is capable of producing 40 amp hours - that is 40 amps for 1 hour or 1 amp for 40 hours and so on with other combinations of figures, then the same battery half charged cannot produce the same amps for the same time and the CCAs are similarly effected. Best solution is to keep the battery in good condition and even more so keep the engine in good tune. That way it will need nowhere near the max CCAs of the battery. When choosing a battery be sure that it exceeds the needs of your engine and you will have no trouble.
Best wishes

Hi Wok,

What is it you are trying to achieve and in what sort of vehicle? I notice in your reply that you seem to want to remove a parallel battery setup (maybe to install an isolator and aux battery?).

Someone here may have already done what you want to do.

Cheers

Wok,

Have a look here, great information on batteries.

http://www.windsun.com/Batteries/Battery_FAQ.htm#AGM,%20or%20Absorbed%20Glass%20Mat%20Batteries

Cheers,

Jim.

There's really no meaningful relationship between CCA and amp/hour capacity. Each measures a different parameter: in fact amp/hour capacity is nowadays only rarely quoted for starter batteries.

Very briefly CCA is a measure of the number of amps a battery can sustain whilst cranking for a short time (typically 30 seconds) without its voltage falling below 1.2 volts a cell.There are four different versions of CCA - so battery CCA comparisons are tricky.

There is another measurement called 'Reserve Capacity'. This is the number of minutes that a fully-charged battery can be discharged at 25 amps before dropping below 10.5 volts (for a 12-volt battery). This is vaguely related to amp/hour capacity. Multiplying Reserve Capacity by 0.4 is more or less amp/hours.

Beware of any normal-sized battery with high CCA! It's odds on the Reserve Capacity is lacking.

As batteries sulphate, both CCA and Reserve capacity is lost.

We are rarely if ever concerned about how many peak amps a deep cycle battery can produce - but we do need to know how much it can store and release. This is usually quoted in amp/hours but is complicated by the percentage of charge it hands back being related to rate of discharge. (At high dischargeates, a battery heats up and thus energy is lost.)

The internal resistance of batteries (and its effect) was thoroughly understood and described by Peukert (in 1897). If you are interested in Peukert there's an article i wrote about it - complete with the data) on my website.

Finally - and if people will forgive a minor plug, I cover this topic in some depth in my book 'Motorhome Electrics'. It's all there - but you may find it tells you more than you had in mind knowing!
Trust this of interest
Collyn Rivers

You raise a very interesting point and I do not know the answer.

I suspect that the CCA is measured with the battery (atypically) fully charged and that the CCA will be lower at the typically lower percentage of charge.

There is a further point actually. A battery typically increases in capacity over the first few months (and many a battery maker relies on this to justify promotional claims). It then goes downhill ever after. I'd be interested to know if CCA is measured when the battery is made - or at the short term peak that enables some of the makers to claim 100 amp.hour knowing that it's only 90 amp/hour as sold and will only be 100 after being cycled a few times.

I'll approach a few contacts amongst battery manufacturers and see if I can obtain a definitive answer.

Re sealed batteries and under-bonnet use - it's very common to use sealed starter batteries up here in the Kimberley - and there's not many places as consistently hot.

I've used them successfully in several vehicles including our Troopy and diesel 2.8 HiLux - (oh yes - and also in the OKA) but was initially wary of them. That is of course arguing from the particular and should not be taken as necessarily applicable more generally.

Collyn Rivers

Wok

Here's a bit more info and a few thoughts re CCA etc. Bill Darden confirms that the US CCA (commonly used here) is defined as performed with a fully charged battery. This means that the test begins at (say) 12.7 volts and (by definition finishes at 7.4 volts). If the test starts at (say) 12.6 volts then the battery will drop to 7.4 volts sooner. The CCA must therefore be less by a substantial amount.

It would thus seem that (as with solar modules) starter batteries can only deliver the quoted output under standard conditions that are more favourable than typical conditions.

What I'd suggest is this: Buy a physically large battery that has the CCA that I suggest (below). This will almost certainly have adequate Reserve Capacity. Do not waste your money on buying more CCA than you need.- Reserve Capacity is more important.

How Many CCA?
Quoting from my own book: "[n Australia] a petrol engine needs about 80-90 CCA per litre. A diesel needs about twice that".

Re 'sealed batteries'. I apologise for not making the distinction - not least because I've just updated Motorhome Electrics re exactly this! The term 'sealed batteries' originally referred only to gel cells - and was briefly extended to AGMs. Conventional lead acid batteries that were 'maintenance free' (in that the electrolyte could not be topped up with water) - were called just that.

In recent times however maintenance-free batteries are also commonly called 'sealed' by their makers. My post referred to maintenance-free batteries and contraty to expectation mine exceeded their advertised life-span.

I routinely fit new starter batteries within a few months of their designed lifespan however, so I do not know by how long they would have exceeded their claimed life. I suspect however that, as battery life is closely related to temperature, that they will last longer down south.

Wok - re the PL 20 and Arrid. I have just (like yesterday!) become aware of a new brand of dc-dc converters that seem more flexible than the Arrid - and also cheaper. I will drop you a private email when I know more. From what I have seen they will solve a whole swag of voltage-related problems.
Trust this helps
Collyn Rivers