After replacing faulty glow plugs and starter, i am wondering if my battery is weak

When i first get in the car in the morning, after the glow plug cycle the voltmeter never reaches 12v. Gets to around 11.5v. Just wondering if this could be the issue with my lumpy start for the first few seconds ?

Battery is only 18 months old , it is a vw battery. Think maybe the hard starts for a year have weakened the battery ??

Cheers