LG K420n Manuel D'instructions page 377

If you need to replace the battery, take it to the nearest authorised
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LG Electronics service point or dealer for assistance.
Always unplug the charger from the wall socket after the phone is
•
fully charged to save unnecessary power consumption of the charger.
Actual battery life will depend on network configuration, product
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settings, usage patterns, battery and environmental conditions.
Make sure that no sharp-edged items such as animal's teeth or nails,
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come into contact with the battery. This could cause a fire.
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