Some car battery questions...

I have a Jaguar S-type, with a gearbox problem. As a result of that it’s had a long time sitting, not being driven. (except the occasional short two minute trip to a shop). Recently I went to try it and it wouldn’t start. The symptoms were battery flat or too low to start the car.

Tonight I took it out and plugged it into my draper automatic 12 volt battery charger. It showed 60 percent good!. Disappointed, I wondered if this battery (20 mh compared to the previous one that was 90mh) might just not be powerful enough to start this car when it’s as ‘low’ as 60 percent.

Another alternative is, the draper automatic charger isn’t very accurate (one Amazon reviewer said it was telling him his dead battery is full)

Question 1 is :is it feasible for a basic/budget 20mh battery at around 60% to be unable to start a car with a 3 litre petrol engine?

Question 2: Assuming I do manage to charge it, and it does turn out to have been too low, how long should one run the car to cancel out the drain caused by starting the car?

In other words, If after tonight I want to stop the battery draining again I obviously need to keep turning the car on. So what is the minimum amount of time I should leave the car running, on each of these occasions to keep the battery ‘alive’?

Question 3: If I’m just testing the ability to start the engine, how long should I keep it running just to undo the drain caused by testing it?

well, assuming the meter on your charger is accurate and not totally worthless like most of them, then yes. a fully charged, six cell lead acid battery should have about 12.75 volts available. if it’s fully discharged, it’ll read 11.8 to 12.0 volts, which is a hell of a lot more than 60%.

if the battery is salvageable, then you should really recharge it using a proper battery charger and not the alternator of the car (especially one with British electrics.)

do it the right way and get a proper battery charger. lead-acid batteries are pretty tolerant of everything except excessive mechanical shock and being drawn down completely flat.

Get a Battery Tender. It’s a little trickle charger that you leave plugged in all the time. It is better for the battery and the car is always ready to go, even if you only drive it a couple times a year. This is what is used on classic cars and other garage queens. They aren’t very expensive.

What type of meter is on the battery charger? As jz78817 said, the voltage doesn’t follow a linear rate with the remaining battery life. The meter on some chargers accounts for this. If you have one of these fancier meters then you may indeed have 60 percent of a charge on the battery. If you want to check it yourself, then read the battery voltage with a fairly accurate voltmeter and look up the discharge curve from the battery manufacturer’s web site. If it is reading 60 percent voltage, then chances are the battery is completely shot. Lead acid batteries self destruct chemically if you let them discharge too much.

I also agree with the above advice. Get a proper battery charger/tender. Running a car for a short amount of time isn’t healthy for the car. The engine and exhaust pipes don’t get hot enough, so water vapor in the exhaust can condense and rust out your exhaust pipes. This doesn’t happen when everything gets up to its normal running temp because any water just gets heated back into steam and expelled out with the rest of the exhaust.

A good battery charger may be able to salvage your battery. Some charging algorithms are better than others and can help break down sulfated plates.

If a vehicle sits a long time, the battery will run down due to parasitic losses. Some cars have more of these than others. A weak battery may still turn the starter, but not have enough juice for the electrical system to produce a spark.

Alternatives are:

Take a cable off the battery when sitting.

Get the biggest battery in terms of CCA that you can fit.

Have a battery maintenance charger in place.

Or be prepared to sit and wait while the battery recharges. This is a poor idea because a severely discharged battery can become damaged and may never return to service.

Also, replace the battery regularly, depending on quality, up to four year’s use.

Thanks guys. A lot of info to take in here.

I don’t think the battery was fully discharged. It just wouldn’t let the car start fully start. It would make some noises as if it was trying.

As mentioned in the OP I put the battery on charge last night. I put it back in the car today and the car started up fine. I know that this doesn’t necessarily mean I can forget about the battery though. I’ll read through this thread again after I’ve posted this reply. I just wanted to mention that I’d got the battery back in the car today and the car started up.

FYI the battery is not that old. Perhaps somewhere between 1.5 and 2.5 years old.

Just a quick note about this.

A lead-acid battery does not have to fully discharge to be damaged. Discharge it more than 50 or 60 percent or so and the battery starts to suffer damage, especially if it remains discharged for a longer period of time. The damage is cumulative, so deep discharging a car battery will tend to make it die a very early death.

Most car batteries are a bit over-sized for what they need to do. You want that extra safety factor in there so that the car will start even on cold days when the battery isn’t putting out as much current and the engine is harder to crank, and you want a bit of a safety factor in there for the battery aging as well. So, even though you may have damaged your battery a bit, you may not need to replace it. It may still function well enough for your car for quite some time. Of course, it’s also possible that you’ll need to replace the battery very soon. You’ll have to see how it holds up.