Dictation. 
Now I will try my best to be accurate and polite. First of all, I don't have a Microsoft surface Pro 4 device. However, the method generally used to calibrate the battery life on most Windows-based machines is something like the following:
The first step is to completely charge the battery up to its maximum and then wait a little bit longer to make sure it really is at the complete 100% charge.
Next, run your device without the charger plugged in for as long as you like or you can and see when it shuts down by itself. The time from your start until the end will be used later to calibrate the battery meter.
Now one would think that there is some method they have to track r the battery charge at any time.
Unfortunately, that is not the case. It is extremely difficult to know the exact charge of a lithium ion battery due to a number of factors. There are variations in the manufacturing process even on a batch of batteries all made at the same time. In addition to that, the manufacturers will sometimes make adjustments to the manufacturing process and the next batch of batteries will be slightly different.
Yes, this is getting to be a long-winded explanation. But there's a reason for my doing this. Trying to cite all the information you might need on this would take days of work. Most of the work on rechargeable battery systems actually was started before the Internet came into existence and much of the manuals and books on the subject are probably out of print or have not been digitized. So that means I have to do this as an ad lib. without a lot of specific links to explain these ideas. As I mentioned earlier, it is the amount of time that it takes your laptop to discharge the battery that is used as the basis for calibrating the battery charge. You might expect that it would be the voltage level. That is not a reliable indicator of battery charge. I won't detail all the reasons for that, just say that the battery voltage at any given time is not a correct reflection of the batteries current status When the voltage is below the full charge point but still above the threshold of cut off, all you really know is the battery is not fully charged. In other words, it is not a clean noise free line or curve or even a zigzag of any kind. It's a blurry area. Take tests of say 10 different batteries you could end up getting 12 different charts.
That is not a joke. At least a couple of batteries will be inconsistent and will not do the same discharge every time.
Now then, about the time to discharge. This is a predictable measure of your battery status much of the time. If the mean average time to discharge for your particular battery being used in a way that you normally use it is six hours, then we can estimate that after about two hours of use from full charge you would still have maybe four hours of time left.
Now this more. The steady concentrated discharge is not what really happens in the real world. The way you use it and some other factors will cause the discharge current go up or down and not always in a predictable manner. The best we can do is take the average and use that as a way to estimate the discharge cycle of the battery in real world conditions.
By the way, I have to do this by dictation because I cannot type this fast. So be patient with me you see some glaring grammar errors. Now back to topic.
The Microsoft form has some information about how to adjust the time on your device so that it shows an accurate representation of how much time is left on your battery. Please note that it varies not only with what computer you have an what battery you using but also your personal habits as to how you use the computer and whether not you would actually use it in a constant station uncloaked the battery forced the system to shut off.
Another issue that clouds the matter is the way batteries respond to constant discharge and what we call intermittent service discharge. The intermittent service discharge will give you a longer total discharge cycle than the constant discharge scenario. And, as mentioned, even the constant discharge is not really very constant because it goes up and down. Even so the battery discharge life is extended if you turn it off for periods we know you're not going to use it and then turn it back on again. Also, there certain power saving features that can be used, such as some form of hibernation or sleep that are just as good or even better than turning the power off for. Time.
When you plug your laptop back into the AC power, the battery begins to charge immediately and any voltage readings on the battery do not give a clear indication of how rapidly the battery is charging. It's possible to try to estimate that, but the estimation is rather poor. I mean very poor, and I'm not trying to be vague. It is really, really very poor.
Not here's a link to a Microsoft form where some of this is discussed in some detail and it's more specific to the type of computer you are using.
https://www.microsoft.com/surface/en-us/support/hardware-and-drivers/battery-and-power?os=windows-10&=undefinedSurface pro 3
https://answers.microsoft.com/en-us/surface/forum/surfpro3-surfhardware/surface-pro-3-calibrate-battery/0876958f-7f9f-4f8f-a040-7724ff1fd0acNot Microsft.
http://www.windowscentral.com/surface-book-and-surface-pro-4-battery-drivers-out-of-datehttp://www.surfaceforums.net/threads/what-is-your-minimum-battery-discharge-rate-while-idling.20612/IMO the above link is very relevant,but it is about the 3, not the 4 and it does not give a clear answer to the issue.
So is it impossible to have a reasonably accurate battery meter? No, I did not mean to say that it is impossible. But it is just not easy. If Microsoft is strongly motivated they could come up with a better way of demonstrating your expected battery life. It is a challenge to come up with a good algorithm for determining the approximate discharge rate of the battery while it is being used. In my opinion, Microsoft apparently does not think this feature is terribly important at this time.
I hope that this rather drawn out explanation makes it clear that I did not intend to be very harsh or sarcastic. My intent was to say that this issue is not easy to resolve even though the technology behind the thing has been out there somewhere for years and years. It is just hard to estimate the discharge time of a battery that is currently in use and is subject to unpredictable changes in load as well as other factors such as temperature.
In some industrial applications, where things are more predictable, it is possible to estimate the discharge time of rechargeable batteries. In some industries it is vital that the operators know ahead of time how long the batteries are going to last. But a good example of this would be if you're in a submarine. You definitely would want to know how long your batteries you going to last when you're underwater.
However, Microsoft apparently does not think that the surface Pro needs that much attention at this time. So they provide you with an indicator that does not work the way most people would expect it to work.
If you want to continue on the subject, please let me know and I'll come back and do some more research or say some more words that might be of some help.
EDIT: Corrections made.
