Question: I have installed a Redmine on CentOS 7, I can run the Redmine application by manually execute the command of `cd /var/www/html/redmine && bundle exec rails server webrick -e production -b 192.168.1.109 &` but I want to set above command can be automatically executed when the contos startup.so the Redmine can be automatically started in when I reboot my CentOS 7.

I have tried put above command into `crontab -e as *@reboot` 

`/var/www/html/redmine && bundle exec rails server webrick -e production -b 192.168.1.109 &*`

but it wasn't worked,and I used the shell,it was also faied.
I really want to know how to rewrite the `crontab -e` to startup my redmine automatically when I reboot my CentOS 7.

Answer: 
In the cases of both tablet and laptop, if you're plugged into AC, power will be coming from the AC outlet. This assumes the AC adapter has enough current output to satisfy the unit's needs. 

If it doesn't then some current will be coming from the battery to make up the deficit. This is uncommon.[1]

In your case, the batteries in your tablets are failing not because they're being used all the time (they're not) but because they get old - most Lithium-ion batteries in most consumer applications have a lifetime of two to five years even without charge/discharge cycles. 

[1] This is the same thing that happens in a car. The only time you're exclusively on battery power is when the engine is off (or, of course, if the alternator has failed). With the engine running the alternator supplies current that both recharges the battery- or, once it's recharged, maintains the charge - and runs the rest of the stuff on the car. If you have something like an enormous aftermarket stereo and/or a weak alternator there can be a current deficit during times of a high electrical load. This means that some of the current needed to run the car will be coming from the battery, meaning that the battery is being discharged. Keep this up for too long and the battery can be drained to the point where the engine will shut down because there isn't enough to run the ignition system, the fuel pump, etc.
Laptop and Tablet charging works the same way. 

Simplifying a bit:

The device needs a certain amount of power - which equates to a particular voltage (or set of voltages) to run. 

If the device is running off battery, the output voltage of the battery will be increased by circuitry to run the device at the required level, however the voltage of the batteries themselves decreases as they loose power (and this is how the amount of charge left is calculated)

When you have a power supply, it needs to provide the correct voltage. If there is enough current it will run the computer. If there is more current available then the computer requires to run it will charge the battery with the excess, and if it's not enough, the battery will provide power to top up the difference.

Where you could be running in to issues are that as batteries get old, some of the cells die, and actually suck power from other devices, causing the power supply to need to provide more power then it can, and the unit could eventually die.

Note that while it's not always cost effective, even devices with "non replaceable" batteries can have their batteries replaced ,- it's just a lot harder to fo (exactly how hard depends on the device, but generally easier to do in laptops then tablets, and in non Apple devices (tablets and cellphones and Apple devices sometimes require the screen to be removed using heat, making the process risky and hard - especially waterproof ones) Most laptops have screws and a case which can be pried open. Pretty much all devices have a connector which the battery actually plugs in to.