Any switcher with heavy enough contacts can be arranged to do the job, but Tim's answer is the simple one that works.
Relays with contacts that can carry the full battery load tend to be thirsty when operated, and will tend to negate any benefit. Switching to a smaller capacity reserve battery to get you back "might" be viable, but really it is simpler to notice that performance has drooped a bit and bring it in. Having a switchable reserve is probably a temptation too far.
Edited By Malcolm Frary on 23/08/2020 17:04:44