Using a modified non-proprietary battery for CPAP
A CPAP is a Med device, and therefore the Batt used for backup is rated as such.  FDA approval governs Medical equipment for health reasons.  A Back-up Batt for a CPAP must remain @ 92% rated voltage for approx 6 hours,  your example Batt would not hold this constant drain.  I am a 20 year CPAP user, Elec Eng Tech, retired.