Thursday 1st August - a pattern is emerging

I'd like to thank Bellinger for suggesting I keep this diary. So far I've found it very useful and it has actually changed my view of what is wrong with my car.
Before recording my range I thought I had an intermittent problem that meant about once a week I either had the Propulsion Power Reduced message or lost 10 miles of range whenever I stopped with less than about 15 miles remaining. Now that I've been recording it I realise that it isn't intermittent at all, it seems completely repeatable.

Here's what I think is a summary of the behaviour:

• If I park anywhere with more than ~15 miles range remaining the range estimate remains pretty much steady give or take a mile or two.
• If I park anywhere with less than ~15 miles range remaining I lose 9-10 miles from the range estimate and only get about 8.8kWh total from the battery.
• If I park anywhere with less than 9 miles remaining I lose all remaining range and get Propulsion Power Reduced when I start the car (which isn't surprising if I suddenly have -5 miles of range)
• If I keep driving over that last 15 miles I get up to 10.8kWh from a full charge, but also drop into Propulsion Power Reduced when the ICE starts because the battery has drained too far.

Here's today's log:
Same as yesterday, the initial range estimate is 49 miles:

 13 miles drive to work and I'm doing slightly better than the estimate:

Lunchtime and the range has dropped slightly over the morning. I go out for lunch driving a total of 12 miles though the range estimate isn't going down as fast as expected:

 

Time to leave work, and the range estimate has crawled back up another 5 miles, in fact at this point 25 miles covered plus 32 miles estimated would be 57 miles total range.

 We need to get some shopping on our way home, so we leave the Volt showing 13 miles range remaining, 8.2kWh used. If I could actually achieve that range I would get about 51 miles from a single charge. That would be nice.

 ... but not going to happen as I return to find only 3 miles remaining.

Total battery capacity used 8.8kWh which is in line with all the other times that I've seen this without any part charges to confuse the issue. I am consistently getting 8.7-8.9kWh out of a full battery charge.

I think I have a duff battery.