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In two cases, I experienced that a device does not work, or does not work correctly, with rechargable batteries, while one-way batteries do the job. Can that be, and if so, why?

The two experiences where:

A radio-controlled wall clock designed for I think one AA battery. The initialization failed -- no matter how long I waited, it always did a round-trip to 12:00, then ran to the correct time, then returned to 0, and remained there forever.

A bathroom scale designed to use four AA batteries. It simply does not work, as if there were no power source at all.

The batteries that worked in both cases were plain vanilla AA batteries. The recharchables I tried were of the various kinds I have in my household, for example, Ansmann2100 mAh NiMH AAs.

The charger I use is a Cellcon "Super 1 Hour Easy Bi-MH/Ni-Cd Battery Charger Model C-250". It has a nice LCD display indicating charging status, and progress, and I always waited until it say "Charged" for all four AAs I insert.

Of course, I tried brand-new, then older, but verified-to-work rechargeable AAs, but the result was always the same.

I must admit that I am a software guy, and I can plug & play hardware, but if something does not work, if is not a software problem, I am completely uninterested in searching for a solution myself, because usually, I fail in doing so :\

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    \$\begingroup\$ What are "ex-and-hop" batteries? You may also want to use the term "rechargeable battery", as not everyone is familiar with the term "accumulator". \$\endgroup\$ – Hearth Apr 5 at 11:24
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    \$\begingroup\$ "Ex-und-hopp" is a very informal derogatory German expression for "short term use then discard". \$\endgroup\$ – JimmyB Apr 5 at 11:38
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An alkaline cell has a nominal voltage of 1.5 volts when new.

A NiMH cell has a nominal voltage of 1.2 v when charged.

Your equipment might be expecting a higher voltage than the NiMH can supply.

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NiMH rechargeables have a lower voltage (1.2-1.3V) than a new alkaline or whatever normal battery (1.5V) from the start, see e.g. https://de.wikipedia.org/wiki/Nickel-Metallhydrid-Akkumulator

Alkaline batteries' voltage decreases during use and may reach 1.2V after some time, too. So, a fully-charged NiMH may look to a device like a half-or-more discharged alkaline.

Some devices are ok with running off 1.2V (which drops even more as the battery discharges) instead of 1.5V, and some just don't.

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