Design help for "smart switching" battery charger for DIY speaker. BMS?

[Bassblaster505]

Greetings, ive been working on and off on this big Bluetooth speaker that runs on a 12V 4.5Ah SLA battery (it was free and holds a perfect charge lol). The plan was to toss together and get rid of loads of parts i have laying around and put them to use. The unit runs off an older car head unit with Bluetooth that will power a pair of 4" woofers and Dayton Audio tweeters. Its RCA preout feeds a separate mono TPA3116D2 amplifier thats sole purpose in life is to feed a 2.5Ohm 8" subwoofer. Not gonna lie i basically ripped the design from the "Blast Box 200" that Parts Express sells.  Ive attached a block diagram of how the unit would be wired up, pay close attention to the power delivery section, as the 12V SLA will charge through a center negitive external charger that disables power to the rest of the unit when charging

That original design was over 2 years ago and today i would be interested in using a "smart" (i dont know the actual name) battery and charger setup. In a perfect world i would like to run a 4S2P 18650 pack and have ONE single DC jack that will power the unit, charge it and play from battery power on its own. I Look at big name Bluetooth speakers for inspiration, Such as my Harman Kardon Studio 5 but i dont know HOW those circuits work. Is it just a BMS? or is there more to it?

Sorry if made myself sound like a complete newbie, I've been into audio and analog circuits all my life, but ive never been big on battery power or its charging setups

[viperidae]

You're probably going to want a low voltage cut off, SLA batteries have their life shortened every time you discharge them completely.
The capacity is also dependant on discharge current. A 4.5ah battery is only 4.5ah when discharged at 225mA. If you discharge it at 4.5A, expect something more like 30 minutes.

[Bassblaster505]

There is a low voltage cutoff in the block diagram, but im into the idea of ditching the SLA all together and running a lithium cell pack and that "smart charger" deal that im not sure if its just a BMS or what