Siemens U15 - Manual (Page 169)

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Battery quality statement
The capacity of your mobile phone battery will reduce each time it
is charged/emptied. Storage at too high or low temperatures will
also result in a gradual capacity loss. As a result the operating time
of your mobile phone may be considerably reduced, even after a
full recharge of the battery.
Regardless of this, the battery has been designed and
manufactured so that it can be recharged and used for six months
after the purchase of your mobile phone. After six months, if the
battery is clearly suffering from loss of performance, we
recommend that you replace it. Please buy only Siemens original
batteries.