If you have a trickle charger, see if you can charge it back up. I don't drive my cars often so the batteries get tired quickly to the point of slow cranks during startups. 2 days on the trickle charger seems to recondition it and brings it back to normal for me.
Current: E92 M3 DCT, Cayenne Turbo S, Ferrari F430 F1
Past:911 Turbo (996), C32 AMG, S2000, (E36) M3, Supra TT, MR2 Turbo