[emeritus]

In the 1960's I had a Philips EL3585 battery tape recorder. A set of the old blue U2 batteries used to last about 20 hours. When Dad built a simple 9V mains unit for his tranny from a design in I think Radio Constructor ( 8V bell transformer, 1 diode, 1 zener diode, and a 1000 microfarad electrolytic) I tried using it with my recorder. I found it produced significant hum, so I tried leavng the internal batteries connected rather than switched out when using the mains unit. This reduced the hum to an acceptable level, and I found that the battery voltage was higher after disconnecting the mains unit. AFAIR I managed to get about 60 hours from a set in this way. I never ever had any of the blue Ever Ready U2 cells leak in 8 years or so. Unfortunately the blue U2 cells were discontinued and their sealed replacements, as well as being 25% more expensive, were prone to leak with this treatment.