I had a similar situation in a ford galaxy the voltage quickly dropped off and stopped charging a discharged battery after only a few minutes. It went back to Ford 3 times with no fault found. I plugged in my laptop running forscan and completed charging and battery diagnostics and found a fault where the discharged battery stopped charging after a couple of minutes then after around 20 minutes after the engine switch on actively discharged the already discharged battery to the point it threw up numerous DTC codes. It went back to Ford a 4th time with this data and a new BMS sensor and software updates were applied that fixed the problem.