I have an old device, the Sony TC-D5M cassette recorder that uses two 1.5v size D alkaline batteries. Wanting to be a little less wasteful I’ve found 1.5v size D Li-ion rechargeables that are recharged by USB (specs included in photo).
Would these be safe to use as an alternative to alkalines in such an old device? Is there any concern that it could damage the device?
These are designed to replace standard D size batteries, they output 1.5 V which is what you would get from an alkaline or carbon-zinc battery, so they should be fine assuming they work as advertised.
A lithium-ion cell has a nominal voltage of around 3.6 V, so these must use some sort of internal regulation to get 1.5 V, probably a small buck converter, about the only danger would be if the regulator failed.
