Why is it that the Ipad Pro 12.9 cant get below 5 % of battery or it will never charge again?

Dead batteries can’t be “fixed.” Even when you stop using the iPad at 5%, the battery continues to drain for background activities. Modern batteries are very resilient, but the one thing that can damage them, especially if slightly older, is draining them to zero.

http://www.apple.com/batteries/maximizing-performance/

If your iPhone, iPad, or iPod touch won't charge - Apple Support

When to charge your iPhone or iPad

Yes it is possible they are defective. But it is very harmful to today's batteries to go to zero!

They like to be recharged when they hit 10 - 20% range. They will also usually have a longer overall lifespan

if they're charged in that drainage range, that is as long as the battery doesn't have some manufacturers defect.

I would highly suggest you start charging at 10%.

Are you a very heavy user? Did you frequently or routinely discharge those to very low states (say <10%) before charging them? Did you use them in high heat environments?

My iPad Air 2 is now three years old and while its battery capacity is clearly down noticeably from new, it is still good for 5-6 hours of continuous use. My 12.9" iPad Pro is coming up on 2 years old and still gives me a full day of use. However, it is plugged in far more than it is used on battery (and most battery sessions with it are only a couple or few hours at a time).

Depending on your use and usage pattern, its certainly possible to kill off a lithium battery in a fairly short period of time. That's why estimated or design life span is measured in full equivalent discharge/charge cycles and not time.

Hmmm...

I regularly (every month or two months) let my iPad Pro battery drain down to zero with no issue.

I have had my iPad Pro for nearly two years and not had an issue with the battery charging back up from zero.

None.

So, I am not sure about your reply to the OP.

It's possible that the OP got two defective iPad Pro models at different times.

I think that was the old Apple premise I was working from.

I guess it's better late than never to change old habits.

Hopefully, I haven't shortened my Pro's battery life too signifcantly..

I've done about 8 to 9 zero discharges to my Pro's battery in the nearly two years I have had this iPad.

You cannot drain any Apple lithium powered device to zero charge. The device will automatically power itself off when it reaches minimal safe discharge. No lithium battery will recover from a truly 100% discharge. And, when the device does auto power off, it is recommended to charge it as soon as possible, to avoid the battery further draining to a dangerous and unrecoverable discharge state (any battery auto discharges slowly even when there is no active power draw on it).

But repeaetedly draining the battery to the point of auto shut down will degarde it faster than shallow(er) cycles and more frequent charges or partial charges.

In the past, Apple used to recommend the monthly “full” discharge (to auto power off) as it was necessary to keep the battery meter calibrated to the battery’s naturally diminishing charge capacity. That was the sole purpose of the full recommended full cycle. Other than that, it did nothing but slightly increase the rate of battery aging. But, Apple has not recommended that process for about 2 or 3 years now as the current battery meter and battery control firmware is self calibrating as the battery ages.

If you’re now at two years of use and the battery is still performing alright, I wouldn’t worry about a thing. It’s just not a necessary process anymore is all. It’s not even an Apple thing per se. It’s just the smarts in SMART Lithium battery technology has gotten a lot better and now can accurately monitor capacity without any user calibration steps needed.