I think that's the problem, but I still don't know how to activate the battery.

C BMU SYS_DETECT NOTES
The BMU SYS_DETECT circuit relies on the system assembly in order to be properly enabled.
During assembly, a metal gasket is placed between the cowling and the top-side of the BMU flex.
The gasket is used to connect the BMU_CWL_DET_L signal, located between J5150.1 and Q5190.2,
to GND. Once BMU_CWL_DET_L is tied to GND, then the SYS_DETECT circuit is ready to be enabled.
The BMU VBAT output will be enabled after the BMU flex and cowling are installed and power is
supplied by connecting a power adapter to either the USB-C or MagSafe Connectors....

I tested only with Q5230, without Q5240 and then swapped one with the other (they are the same). The Q5230 Gate is not activated by U5200. What signal would give the OK for the IC to activate the Q5230 input? It is not activating the MOSFET first because there may be something wrong ahead, like a short circuit.v