How to keep "Battery Saver" mode enabled, even when charging

How can I keep "Battery Saver" enabled, even when my device is charging? I travel a lot on multi-day excursions where I don't have access to power, so I charge my phone with a battery bank. It seems silly that Android automatically disables "Battery Saver" mode when my phone is charging. Just because my phone is charging doesn't mean I'm not interested in saving power. How can I force "Battery Saver" mode to remain enabled, even when charging my Android device? Ideally I'd like to do this without installing any closed-source software.