HP HP-20S - Power and Batteries

Power and Batteries
The HP-20S is powered by 3 button-cell batteries. Expected battery life
depends on how the calculator is used and the chemical content of the
batteries.
Use only fresh button-cell batteries. Do not use rechargeable batteries.
Low Power Annunciator ()
When the low battery annunciator (§J) comes on, you should re-
place the batteries as soon as possible.
If you continue to use the calculator after the battery annunciator
comes on, power can eventually drop to a level at which the display
becomes dim, and stored data may be affected. If this happens, the
calculator requires fresh batteries before it will operate properly. If
stored data has not been preserved due to extremely low power, the
HP-20S displays ALL CLr.
Installing Batteries
Once the batteries are out, you must replace the batteries within
one minute to prevent loss of Continuous Memory.
Assistance, Batteries, Memory and Service 111