Topic: Cranking Amps - battery

[Prospect]

Can someone explain the theory behind cranking amps in a battery.  Specifically how a fully charged battery can have 12.7V but low cranking amps.  I'm thinking the acid in the battery is turning to water and not giving all the amps it's designed to give but I still don't get how it can show a full charge.  I've seen this with a number of my older batteries.

[scunny]

I think (dangerous pastime) that the volts are determined by the number of plates and the amps are determined by the surface area of the plates. sulphation of said plates reduces the amount of surface area able to provide a chemical reaction.

[TwoTired]

You can stack button cells to achieve a 12.7V potential.  But, they won't move a large enough quantity of electrons, to maintain that potential while providing current.

It is plate area that makes current.  If the plates are eroded (reduced area), or covered in lead sulphate (insulation of area), there is a reduction in electrons available to flow.

You may observe that the 12.7V potential drops to a very low number when the battery is asked to deliver current.  If a fully charged starting battery can't maintain 9-10V when the starter load (25-35 Amps) is placed upon it, it is usually worn out (plate loss) or sulphated (plate insulation) from neglect.

The electrolyte (turns to water), because the sulphur in battery acid H₂SO₄, deposits coat the plates with lead sulphate, leaving behind more H₂0.

A specific gravity test of the electrolyte will show sulphur loss due to conversion deposition (sulphation) or exhaustion (low state of charge).

[scottly]

If a fully charged starting battery can't maintain 9-10V when the starter load (25-35 Amps) is placed upon it,

Per the Honda manual, the starter load of a CB750 is approximately 120 Amps.

[TwoTired]

If a fully charged starting battery can't maintain 9-10V when the starter load (25-35 Amps) is placed upon it,

Per the Honda manual, the starter load of a CB750 is approximately 120 Amps.

That's the full load the starter is rated for.  And the starting current is variable based on the resistance to turn the crankshaft.  Also sustaining current is less than initial surge.
If the engine is pretty new, has high compression, and the oil and engine is cold, the starter will certainly draw more amps than what I stated.

Of course, if you want to make things very complex for the untrained to understand, then use book values and design for the min/max for each component in the system.

The rating I gave is pretty much an ideal starter load, and if the starting battery can't hack that, it's useless.  Certainly a new battery would be capable of more than 120 amps.  The OP was asking about an old battery.

[scottly]

The rating I gave is pretty much an ideal starter load

25-35 amps is ideal? Or unreal?

[Prospect]

You can stack button cells to achieve a 12.7V potential.  But, they won't move a large enough quantity of electrons, to maintain that potential while providing current.

It is plate area that makes current.  If the plates are eroded (reduced area), or covered in lead sulphate (insulation of area), there is a reduction in electrons available to flow.

You may observe that the 12.7V potential drops to a very low number when the battery is asked to deliver current.  If a fully charged starting battery can't maintain 9-10V when the starter load (25-35 Amps) is placed upon it, it is usually worn out (plate loss) or sulphated (plate insulation) from neglect.

The electrolyte (turns to water), because the sulphur in battery acid H₂SO₄, deposits coat the plates with lead sulphate, leaving behind more H₂0.

A specific gravity test of the electrolyte will show sulphur loss due to conversion deposition (sulphation) or exhaustion (low state of charge).

We're always advising newbies to the forum about the importance of a fully charged battery before doing any diagnostic work. So if a battery is 12.7v but low cranking amps then all the tune ups will be of little use or at least potentially giving false results.

What's the best way to test the cranking amps without taking the battery to a shop?

[crazypj]

The rating I gave is pretty much an ideal starter load

25-35 amps is ideal? Or unreal?

I can't remember load on CB750, but, usual initial current is around 60~80 amps to get crank moving on a good motor then drops to about 40 amps when actually turning.
If it goes over 100amps, you either have a problem or a big bore/high compression motor
It's easy enough to measure, you need  about 2ft of 8 gauge cable with measuring point s 19.1". (I think, been a while)
Measure in millivolts and it's direct reading in amps (just need to move the decimal)
It's about as accurate as a 'cheap' meter

[Jerry Rxman Griffin aka MuthaF'er]

Put it this way, when your battery shows 12.7V but doesn't do #$%* when you hit the starter button it's time for a new battery. Take it to get load tested for CCA to be sure BEFORE you buy the new AGM battery. Good upgrade. More $$$. More CCA's and will probably last longer but probably only if you use a maintainer type charger religiously. I was real excited when my probably 8 year old battery came off my Battery Tender with 12.7V. I really knew better as these batteries will NOT last 8 years. Hopeful beyond reason! Excitement over when I pushed the button. I knew better.

TT, as always, has a great engineer based electrical explanation. He's tha man.