My experience here is with the Biologic Reecharge, not E-Werk, but similar situation....

The issue is NOT voltage, but amperage....

While a cache battery is an additional expense, it really smooths out the charging process, while adding a little weight and inefficiency. Without the battery, every time the current drops (as you slow) the iPhone squawks as the charging starts and stops. This drove me nuts, so I got the battery. With the battery, your dynamo is always working usefully, charging the battery which is charging the phone,etc, and/or running lights.

Hope that helps you decide.