Just to add to the general forum knowledge about batteries.
Batteries follow ohm’s law (well they would, wouldn’t they).
So all batteries have an internal impedance/resistance. When current flows there will be a voltage drop at the terminals - ohm’s law innit.
As a battery gets older, assuming it doesn’t have a fault, that impedance goes up. So for the same current flow, the terminal voltage goes down until one morning you come to start the car, and the combination of internal impedance and current load result in insufficient battery capacity to start.
As a battery has a negative temperature coefficient, this has an effect too.
When I worked in the battery shop, on the first really cold morning of winter, my boss would come in rubbing his hands and saying “battery weather!”
He was right.
Now, here’s the rub that a lot of people don’t get.
When a battery impedance goes up, for a given charge current, the voltage will go higher than for a lower impedance. So the battery will appear to charge more quickly. Well it hasn’t. Not only has it got a lower capacity but because charge rates are governed by the battery voltage, the charge current will be lower for a (partially) discharged battery - it will take a lot longer to reach its full but limited capacity.
So when I hear someone say the battery must be good as it charges up really quickly, I’m always sceptical, and recommend a proper battery test (preferably on a fully charged battery with a load tester)
Also whatever the state of a battery, unless it has a fault - a short circuit cell is an example - the ‘fully charged’ voltage will be dictated by the charger or voltage regulator. This will usually be around 14.0-14.5.
The above is a simplification and doesn’t cover smart charging, battery faults, battery technologies, temperature etc.
hth
