Overcharge and battery are just stupid OP. Just toss a Tesla in between and it will completely solo the game. Then over time just build up more overcharge+ruby+battery groups to have more single target dps and take every single camp fire and event for flat -CD and flat +DMG and you have 0.1s CD on all your units and effectively infinite dmg and healing. Since this seems to default into endless mode. I just got bored at station 59 and peaced out.
Had I realized it kept stats on merge and merged +1 heal mystery boxes with my battery carts, it would have probably been even sillier. But it really didn't matter, beyond the first few seconds of a fight. Nothing could ever actually get close enough to deal damage. Also ended on like 1700 gold, because there was just nothing to spend it on. Since there is no overflow, so beyond a point you can't merge anymore without ruining your setup.
Oh and since battery carts are triggered, not cast. You can just throw all the +0.5 CD from events on them, and it will not impact the rate at which they can be triggered. So that event has no downside.