Battery Failure

[myoldjalopy]

Over the last few days my battery has increasingly struggled to turn the engine over. The voltmeter shows a charge of about 12.5v, which seems OK, but when cranking the voltage drops to about 7.5v. A new battery seems to have sorted the problem but what would cause this issue? My experience is that, usually, the failing battery just will not hold its charge but this time it seems to hold the charge but can't deliver it.
Can anyone explain please?

[geoberni]

Well it's been about 40 years since I learnt the theory, and not really used it since, so this will be a bit rusty, but basically....

The traditional Lead Acid car battery is a fickle thing, whose performance depends on a number of factors, including operating (ambient) temperature.
There's lots of chemical reactions going on in a battery.
A battery basically has a life cycle in 3 stages, the initial (formatting) usage where it builds to peak performance, i.e. the first few charging cycles, followed by the actual Peak period, which may be many years, or just a few, followed by the 3rd period of Decline, which may be gradual or quite rapid.
This is all influenced by a number of factors, such as the quality of the initial construction, the severity of use, and the operating conditions, e.g Desert or Arctic.
When you load a Battery, as the current is drawn from it, the voltage will dip. The older and more worn out it is, the greater the dip. it eventually get to a point where it dips so much it still can't deliver the power being asked for it.
In the UK, we tend to get many years from our batteries, but go to Northern Canada, or the deserts of Nevada/Arizona, and those extremes of temperature result in batteries lasting only a few years.


Your past experience of batteries that will not hold charge is probably ones where the degradation of the plates has resulted in one or more cells being shorted out by debris in the bottom of the cell. Lead gets shed within the battery during the charge/discharge cycle.
What you've has here is just general wearing out.

I hope that rambling answers your question.

[myoldjalopy]

Thanks for your reply. Yes, I have certainly known very cold weather to kill my battery before and my batterys in recent years have not lasted as long as they should.......I don't think I'll get any more from Halfords. Although, of course, the better ones will be more expensive. Fortunately, I had a spare lying around that I have kept periodically charged so will see how that fares.

[Shropshiremoggie]

I have a battery ten years old that is swopped between my Moggie and mower . When it is not ' imstalled ' and being used on either it is connected to an Accumate charger / conditioner . I put its long life ( and ability to start both machines ) to being on the Accumate which prevents the plates sulphating .