Model A1398 EMC 2881 Rated 20V

That model uses the following (from Identify your Mac power adapter - Apple Support )

85W MagSafe Power Adapter with MagSafe 2 style connector

• 15-inch MacBook Pro models introduced in 2012 through 2015

That is available from Apple online in the U.S. -- Apple 85W MagSafe 2 Power Adapter - Apple

Apple power adapters provide power ONLY after a negotiation with the computer. The computer ask for the power adapter's capabilities, the power adapter responds, and if appropriate AND the Mac wants to charge now, it commands a power level and accepts power.

When charging is complete, the Mac stops accepting power. Your Mac will NEVER overcharge, because the computer is commanding the process, and only when needed.

The corollary is that asserting 20 Volts power against the interface does NOTHING. Every usable power source must be capable of smart charging. In this case, using MagSafe2 power delivery protocol.