They’re selling these for $1500 on Amazon Prime, which is a very good price. This is the same battery that Alpina uses in the B3 (they use the i48CE and below is the spec for an i49CE which is just a slightly larger footprint). This is essentially a lifetime battery, meaning the car should go to the junkyard with it. It should perform as good or better than lead acid in almost all situations, with the exception of negative temperatures.
Weight 14.4lbs / 6.5kg [OEM is 55-60 pounds]
Length 13.9" / 353mm [OEM is 15.4" / 393mm]
Width 6.8" / 173mm [same as OEM]
Height 7.7" / 195mm [same as OEM]
Pulse (hot) cranking amps: 2020 [OEM is 900 cold]
Ah: 38
Lithium Amp Hours Equivalent: 114 [OEM is 110]
https://www.braillebattery.com/index.php/braille/product_batteries/i49cx