my previous batch function wasn't always in parallel. This one seems to work properly and foces the parallelism plus gives you an option on the parallelism settings.

Batch2.cs

    public static IEnumerable<T1> Batch2<T, T1>(
    this IEnumerable<T> list,
    int batchSize,
    int maxParallelism,
    Func<IEnumerable<T>, IEnumerable<T1>> action)
        {
            int i = 0;
            return (
                     from item in list
                     group item by (int)i++ / batchSize
                         into part
                         select part.AsEnumerable())
                                .AsParallel()
                                .WithExecutionMode(ParallelExecutionMode.ForceParallelism)
                                .WithDegreeOfParallelism(maxParallelism).SelectMany(x => action(x));
                          

        }