Batteries used in mobile phones are dangerous if not treated well, they can burst into flames. The electronics in the phone which deals with charging is programmed with the capacity of the battery and all is well ensuring no overcharge. Over a period of time the capacity will reduce and the situation of an apparently charged battery suddenly causing the phone to switch off can occur. This is less likely to be a problem if the battery is regularly run down before charging, when any reduction in capacity is adjusted for in the charging circuit, although occasional boosts of a partly charged battery is ok. Very occasionally a battery losing it's charge can appear to the charging control circuit to be faulty and this gives the flashing of the red light as the battery is given very low pulses of charge which over several hours can restore enough charge without any danger of the battery catching fire. The phone can then be switched on to full charging operation by holding the red switch button next to the SIM card with the end of a ball-point pen for ten seconds and then allowing the charge to complete showing the battery symbol before switching on the phone for use. I hope this helps, I have had years of experience with laptops which use the same battery technology. To recap, full charging cycles are best, occasional boosts are ok, if red charging light flashes wait several hours and press red button for 10 seconds and allow to charge.
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