Your question assumes that something is wrong on the basis of your battery health only being at 70%. It's not clear how you're getting this figure, but a battery health of 70% after 300 cycles sounds perfectly reasonable to me if what you're measuring is the number of cycles remaining. Otherwise Battery Health is not usually represented by a percentage value, but in terms of Normal, Replace Soon, Replace Now, and Service Battery.
While it used to be in years gone by that Apple recommended allowing your battery to drain somewhat on a regular basis, this is no longer the case. In other words, keeping your MacBook Pro plugged into an AC power source for prolonged periods will not mess up your battery.
You may want to refer to what Apple has to say about their batteries for more information.
- Apple lithium-ion batteries use fast charging to reach 80% of their capacity and then switch to slower trickle charging to complete the charge.
- Charge your MacBook Pro whenever you want. Many people believe you need to let them discharge 100% before recharging. This is not true.
- Ambient temperature is one of the biggest factors in battery health/life. MacBooks are designed to work at their best when ambient temperatures are between 16° to 22° C (62° to 72° F).
- It’s important to avoid exposing your MacBook to ambient temperatures higher than 35° C (95° F)
- If you're ever going to store your MacBook for long-term without usage, it is best to store it totally powered down and at only about half charge.
Also, some of the latest research suggests that one of the most important determinants of what ruins a battery is time, and that's not something you can do anything about.
Finally, there is no way of safely tricking your Mac into thinking that your battery is connected when it's not. It will cause other issues for your MacBook and, in the event of a power failure, you have no built in uninterruptible power supply.