I hope this overview helps folks with what can be a simple resolution to problems caused by a simple source...your battery.
There are many threads on here regarding problems that are electrically related and may or may not be known as such. Many of these issues are on all vehicles, they are just more prominent on Jags (or other higher end vehicles) because of their monitoring system and power requirements.

So, first off you need to pay attention to your vehicle when you start it. Listen for the starter to not quite sound right, perhaps turning the motor slower. Check the brightness on your gauges and lights, watch for flickering or dimming. If your sunroof or windows don't want to open or close, it could be the battery and not a drive motor. If you locks suddenly unlock going down the road, are your blinkers blinking slower than normal, ...look at your clock, is it slow? Things like that could be different issues, but the cheapest easiest and most common issue is the battery and/or its components. This is a first check item, not necessarily the cure.

The battery, at full charge in the car, should be carrying a minimum of 12.8 vdc to 13.6 vdc, checked with a simple multmeter. If you have a really fresh battery, it may be a couple ticks higher. If you had put a charger on your battery, it will probably show 14.4 vdc or up to that. That's what chargers do, they tend to over charge initially. That's the initial difference between a charger and a tender whereas the tender will not charge as high or as fast and maintains that level. Only allowing it to drop approximately 1 volt, then engages a trickle charge. A charger puts lots of power to the battery then shuts off when it reaches its designated max charge. The tender will keep your battery at the voltage level your car is supposed to be at for optimum performing voltage. The tender also charges at a trickle rate so as not to destroy your cells. A charger will deteriorate your battery each time it is applied to it and used, so be very conservative using one.
Let's do some basic checks, things you should do roughly every couple of months or so or if you notice some symptoms listed earlier.

1. ..check the battery itself, look for swelling (it will look a little pregnant), feel the sides for warmth if you haven't started it yet or very warm if you had been driving it. Get it checked by someone if so, it shouldn't be warm/very warm unless there's a problem.
2. ..check for crack(s), leaking, corrosion, loose cables or grounds. If yours is cracked, be very careful removing it, it could burst, recommend letting someone else with proper attire do it.
3. ..check to be sure you have the correct battery, its not uncommon for Joe salesman to throw in an Any Size battery just to sell the car.
4. ..look around the battery and clean out any foreign substances and check for rust where it sits and fix it.
Advanced checks...
5. ..for #1, go to a parts store or a mechanic and have it check. They will put it on a quick diagnostic and let you know if you have a condition in the battery or if its starving from a poor alternator.
6. ..the alternator can be checked by you having the car running and the lights on. Go out and see if the lights fluctuate. Now this fluctuation can be a failing alternator or a bad tensioner or bad serpentine belt....check them all.
7. ..grounds, not only on the battery, but on the body of the car. There are a few, download the electrical guide and check them for rust, looseness or if broken. Grounds are very important and should not be ignored as a possible problem.
Well, now that we've covered those areas, what can we do to prevent or diminish the issues stated. The positive cable is #1, if you don't keep up your battery, corrosion will get into it and you'll be chasing ghosts for just this item not being kept up. When your battery gets low, that's how corrosion can start. A fresh kept battery prevents corrosion....if you want to know more...What Causes Automobile Battery Corrosion? | eHow

Many folks here know from my responses that I highly recommend a Battery Tender (that is company). There are other types, its your choice who you pick. The tender, as stated, keeps your battery at optimum level so after sitting for 2 weeks or 2 months, you hop in your ride and she fires up immediately. I have 2 vehicles (Jag and Vette) I use tenders on and can attest to the reliability of the product. My restore project Corvette sits for months before I start it and it fires on command. Corvettes are probably the, if not one of the, worst vehicles on batteries. So if your job has you travelling, like mine, for excess amounts of time or you just don't drive it that much, get a tender on it.

Let's got to the other end of the spectrum...a dead battery and replacement. So, the car sat for a long time and you don't have a tender or maybe the battery just gave up the ghost and died? How do I start it to get it to a replacement shop? The first thing I implore is by no means should you use another vehicle to jump start it. This not only can harm your car, but the jumper vehicle, especially on any produced after 1986. Not to fail to mention trying to manage an angle to get the vehicles close enough for the cables.
Jump boxes are ok, but are cumbersome and if you've owned one, the hold a charge for only a little while before dying themselves, plus they don't have any safety features.

Well, I use an item called a antigravity micro start battery booster. They come in different sizes, no bigger than a full size envelope (but about 3/4" thick) and as small as a descent size cell phone only twice as thick. They have safety features where if you try to jump the wrong terminals on a battery, it won't do it, I've tried it. It won't allow the starter to run too long so as not to burn it out. They can also charge your laptop (depending), cell phone or gopro and many come with a flash light that has strobe ability and/or SOS.
They stay charged for months, I received mine Nov 2014, it came fully charged, I never charged it again until Dec 2015 and it had gone down only one bar and that was after jumping roughly 12 other vehicles. I just felt I wanted to charge it. It sits in my work van still fully charged now. They are quite amazing and I anticipate them replacing the wet cells we use in our cars soon. I think this is what Tesla uses in their cars? I'm going to include this YouTube link for the one I have since I can attest to my experience with it. Many more have come out since and they are cheaper, but do your due diligence.

 

I really think this should be in the stickies.

 

Sorry ... but no, absolutely not, not ever.

As for boosting from another vehicle, it is perfectly safe
provided you know exactly what you are doing.

Yes, you have to be careful, but it is otherwise not a
problem.

 

I am open for criticism Plums on the alternator, its a method I've used, please advise and I'll correct it....or was it I forget to put on there, shut off the vehicle first?
That's the issue, folks jump off vehicles incorrectly all the time by either crossing terminals mainly or trying to jump a bad battery and it pops. I personally had this happen as a teenage driver milking a dying battery. I touched the terminal with the jumper and POP, I was poring my coke over my face trying to get it off.

 

The car electronics want a buffer - that's the battery - to prevent nasty spikes and overvoltage. I'd not disconnect the battery.

 

I will agree this is not the optimum test and will be happy to delete that part if you feel strong enough toward it. I was going to get into voltage and amps, but that would defeat the idea of a simple check....so, as I stated, I will be happy to eliminate that as a test process.

....15 minutes later....so I updated it with another process, does that appear more useable as a simple check?

 

For the alternator, it is the same answer as given already by JagV8.

Additionally, you can check the output for ripple by setting the multi-meter
to AC voltage. If you show AC, the diode bridge is suspect.

For the jumping, I think it is your personal experience that makes
you reluctant to jump start from another vehicle.

Your description falls under the caveat of "knowing exactly what you are doing".

When I do it, I attach at the dead battery first, donor battery second,
all the while keeping all clamps separated with my foot.

I also insist on doing it myself. No helpers at the other end of the cables.

I am also dead sure of which terminal is which. Tracing back to a ground
point if necessary.

If you think about it, any type of power donor is a large source of
amperage. It really does not matter what the source is. If you do
it wrong, it is still wrong.

 

"The battery, at full charge in the car, should be carrying a minimum of 12.8 vdc to 13.6 vdc,"

A lead-acid battery should be 12.6 to 12.7V, fully charged, at rest. If you get a higher reading, that is because some surface charge is still on the plates, left there from charging. Surface charge can be dissipated by drawing some current, or just waiting 24 hours.

Here is the best way to remove the surface charge and get an accurate reading :

Engine should be off.
Turn the headlights on for 30s.
Turn them off again, and wait 10 minutes.
Then measure the battery voltage.

Alternatively, if the car has not been run, or the battery charged by other means, for 24 hours, you can skip these steps and just measure the voltage, as the surface charge will dissipate on its own in that time.

I vote NOT to put this is the stickies, as it will result in anyone who heeds the advice blowing their alternator (disconnecting positive terminal) and throwing out a perfectly good battery (because it was less than 13.6V).

 

Ok, we are going at this way wrong. You're attacking the post instead of trying to make it a better post to afford it being posted as a Sticky.
Have you guys lost the aspect of trying to help others here?
I said I will take criticism and make corrections, but to just say no because "I don't like it" makes zero sense for the community.

 

Yes, and by using the antigravity device, it prevents the screw up.