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I have a bluetooth speaker (UE Megaboom) which I constantly use EVERYDAY in my workshop. The OEM charger is extremely fast and can charge the included 3400mA battery unit in less than 2 hours. But what I learnt is that any Li-Ion battery life can be extended to beyond the normal 500 cycles by simply charge it less than 4,2V. Industrial/solar cell Li-Ion batteries usually only charge up to 50% - 80% of their rated capacity to extend the life to beyond 3000 - 5000 cycles. My old Vaio laptop also has a built in software to limit the charging to only 50% or 80%.
So, I use my 250mA shaver micro USB charger constantly plugged in and turn it on only when I need to use the speaker. This way, the charge capacity kept constant between 50% - 60. I have this speaker since 2015 and the battery life seems still in a very good condition. When I go travelling, a full charge (100) can still lasts for a full week with normal uses (3-4 hours/day normal volume level in the hotel room).

- Clemence