Mid 90's I worked for a company that dealt with 911 systems. We have a bunch of spare parts in the office and one day lost power. The ups on the phone system started beeping after a couple hours so I wheeled one of the spare ups's back to it and plugged the phone ups into it. Then as that ups would run low it had charged the phones ups up to where it would run things long enough for me to swap in another. Kept the system up for several hours that way.