Welcome to the Dell Community [NAME]  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
I am going to switch my SATA drive out for a M.2 SSD, I have the drive and 2FFRO and X3DN4 bracket. In Dell's instrutions, it says to replace the 68 Whr battery with a 92 Whr battery. I noticed in the owners manual technical specifications for the batteries, that they are different voltages. The smaller is 7.6 vdc and the larger is 11.4 vdc. Do the M.2 slots require the extra 3.8 volts? On another discussion about installing M.2 SSDs there were pictures of laptops with the 20x80 SSD installed with a 68 Whr battery. Do I need to buy the bigger battery or will it work with the small 68 Whr battery?    
Precision 3520 batteries have different voltage.