I like Misha's suggestion of swapping out with another battery. If you don't have access to one that fits though, I'd check voltage at both starter nuts specifically while cranking. Steps:
a) Put voltmeter ground on engine, not on battery. Alternate voltmeter positive lead between both starter wires. I am assuming 850 has two starter wires, most (all?) cars do. One is connected directly to battery (larger wire) and one (smaller wire) is going through ignition, fuses, etc. If both are still >12V while cranking, you have a starter issue (I doubt this is the case though).
b) If both <12V, change voltmeter ground to battery ground vs engine ground and check both again. If both still <12V and you charged battery, replace battery.
c) If only one is <12V, report back for more troubleshooting...