My dad left his parking light on his car when leaving it standing for a few days. The result was a few days later, the battery had an OCV of 0.09V - i.e. completely dead. We were able to jump start it and run the engine and get the battery up to 12.54V. We then put it on a battery charger for 8 hours to see if we could resurrect it; but how likely is it that it will be okay? He's worried that he'll have to spend £70 on a new battery.

 

Give it at least 24 hours of charging. 8 hours will not be enough to fully charge it I think. It is important to charge a car battery to 100% as soon as possible after such incident.
I also charge car battery once a month anyway. That will prolong the battery life time. I the car is only used for small trips. The car dynamo will never be able to charge that the starter motor drained. A lead-acid starter battery love to fully charged all the time

 

Also whether or not it recovers depends on the age/condition of the battery.
A long slow charge ( what you already have done) is better than a fast high current one.

 

That battery has been permanently damaged. It might last the summer but on the first cold snap it might leave you stranded. Your Dad should start saving money for a new one.

 

I haven't seen one in a while, but you can buy discharge limiting devices that will isolate the battery instead of letting it totally discharge. I think they acted pretty quick because you were supposed to be able to still start the car after a draining experience. I never figured out how they interfaced with the KAM.

 

That battery has been permanently damaged. It might last the summer but on the first cold snap it might leave you stranded. Your Dad should start saving money for a new one.

Some damage has been done. I can agree with that. But as I said you may get some extra time from it by charging it regulary. And start saving now would be a good idea. But no reason to get a new battery now if it still take some charging.

 

nsaspook may be right with the current crap that's manufactured. However, batteries from 30-50 years ago (ones I have more familiarity with) would usually handle a few such events with reasonable aplomb. It was pretty common for people to leave their headlights on, so the cars/batteries need to be able to deal with such things. Usually, you just had a friend jump the car and drove around for a while and things were back to normal. You stuck a charger on it later if you could (this is much more important in cold weather, but I grew up in the Bay Area of California and it doesn't get cold there).

But any battery's life will be lessened if it is discharged completely -- that's a well-known fact. Since we have an RV trailer, I keep multiple RV batteries charged in the garage. When I'm going somewhere with a car with a battery I'm not real sure about, I toss in one of these spare RV batteries with my jumper cables. This has gotten me out of a jam a few times.

You might have your father get that battery now. You can run some wiring to the trunk and make or buy an adapter to let it stay charged from the alternator, but not allow discharging (probably some MOSFET thing to avoid a diode drop). I did this on our first trailer's battery -- it worked well; after driving, we'd have a fully charged battery for camping.

 

You can still get batteries that can take massive abuse. Forklift and golf cart (traction) batteries have thick lead plates that can shed the crystals from a total discharge and be brought back to life but a typical 'CAR' battery is designed to deliver a high cranking current for a short time so the normal design is with thin lead plates and separators with a large surface area for very low internal resistance and large current flow. A single total discharge can warp these plates from sulfation and make the possibility of an internal short on a separator very high on recharges plus with the lead plates so thin there's no new metal to reform so it remains permanently sulfated with loss of capacity. I've killed a lot of cells testing Solar charging systems with cheap car batteries in the last year until I got a set of Costco GC2s.

http://rollsbatteryne.com/docs/A Sulfated Battery.pdf

 

The car battery will fail completely in a short time. It WILL fail. Maybe not today, maybe not tomorrow, but soon. Most likely when you can least afford to be stranded is the time it will choose to die. Starting batteries CANNOT be allowed to completely discharge. The way they are designed, as others have said, means they will suffer physical damage from the complete discharge and subsequent recharging.

Replace it now, or very very soon, BEFORE it does.

 

"Jump starting" is very hard on the battery, as there is a very high charge current into the battery, which causes internal heating, which causes more chemical activity. Once heated, the large thermal mass of the battery requires a LONG time for the heat to dissipate through the battery enclosure.

If at all possible, auto batteries should be charged from a 6A or less charger until they reach around 14v when at room temperature (25°C/77°F), and then "float charged" at around 13.6v-13.8v indefinitely.

Battery plates are more fragile than they used to be; the demand for higher cold cranking amps in small packages with lower weight has caused battery makers to use thinner plates. The thin plates are not as tolerant to abuse as those made in years past.

 

"Jump starting" is very hard on the battery, as there is a very high charge current into the battery, which causes internal heating, which causes more chemical activity. Once heated, the large thermal mass of the battery requires a LONG time for the heat to dissipate through the battery enclosure.

If at all possible, auto batteries should be charged from a 6A or less charger until they reach around 14v when at room temperature (25°C/77°F), and then "float charged" at around 13.6v-13.8v indefinitely.

Battery plates are more fragile than they used to be; the demand for higher cold cranking amps in small packages with lower weight has caused battery makers to use thinner plates. The thin plates are not as tolerant to abuse as those made in years past.

We used a 4A 12V charger which has an open circuit voltage of 14V. It took a few more turns to start this morning but seems to be okay. I have told dad that he will probably need to replace it soon, but he ignores me... what can you do.

 

Another thing to keep in mind is that a bad battery can cause your alternator to go out. A bad alternator can cause your battery to drain. It's a vicious cycle. My mother had this happen; the battery in her truck went dead so she recharged it but it kept going dead so she bought a new one. Worked ok for a few days then the new one started going dead. took it to the mechanic; turns out the alternator had gone out due to the excessive load of the previously bad battery & wasn't charging the new battery, so she got a new alternator. ran good as she drove home from the shop, but then wouldn't start again. The new Battery was bad from being discharged so many times. jump started it and limped over to autozone and they ran the little scanner and said that both the new alternator and the new battery were bad. They recommended that anytime you replace the alternator, you replace the battery at the same time. that's what we did. So, 3 batteries and 2 alternators later the truck ran just fine. good thing we got the warranties on the parts so we only had to pay for them once each (get the warranty).