Why does my speaker have a reduced playtime?

Hello everyone,

I have a problem with the battery playtime of my Harman Kardon Onyx studio 4.

On max volume it lasts barely 1h30min whereas it used to last at least 3h (it should last a theoretical 8h of playtime on normal volume).

Thinking that the battery life had ended, I replaced it with a new one and charged it as recommended but the problem persisted.

I replaced the battery with another new one thinking that the other may be defective but the problem is the same.

When plugged to a wall outlet, the speaker works for as many hours as needed, it charges as well, so I don't think there's a problem in the power input. The battery lasts for more than 3 hours on normal volume.

I have another same speaker but newer and the battery lasts for more than 3h on max volume.

What could be the problem? Could it be the charging circuit ?

Many thanks!