Precision 3520 batteries have different voltage.

Welcome to the Dell Community @JerryJa 

The 68Whr should work just fine.

Consider you are replacing a "High Power Consuming" SATA drive with a "Low Power Consuming" M.2 SSD.

That would mean slower battery discharge and longer run time???

Best regards,

U2

The reason for the wording is that the larger battery takes up the space used by the 2.5" drive, so it's required to use the smaller battery with a SATA drive -- with the larger battery, there's only space for the M.2.  It has nothing to do with the battery capacity. 

An M.2 drive will run fine with the smaller battery, though if you're ditching the 2.5" drive, that does free up the option of installing the larger capacity battery, if you want it.  It's not required.

I think it's a stretch to call a SATA drive "high power consuming" and M.2 NVMe "low power consuming".  In fact, with high performance drives, you'll find NVMe usually consumes more power, not less than a SATA drive does.  Neither will the battery discharge rate necessarily change -- though of course with an extra couple of cells, the larger capacity battery will give you more runtime.