No experience, but I do note the smaller ones seem to be just as expensive as the official Mahle extender, i.e. about £400-500 for around 200Wh.
The larger one (500Wh) looks like better value, and you could probably do day-long rides before charging.
However, the translation to English says they're 'exhausted'. Made me chuckle anyway.
- at around £380* for 250Wh., that's almost 50Wh. more than the Mahle one and £170 cheaper. I've read that it works differently - in parallel with the main battery rather than being in effect a powerbank. I also think the smaller one looks a lot neater than the Mahle X35 one.
*edit: around £400 with carriage. I don't know if the price they quote includes UK VAT and taxes.
Well, Volabike do say they use top quality cells, LG or Panasonic.
It's not exactly 'cutting edge' technology is it ? Providing there is anecdotal evidence 'out there' of tests and satisfied users I would be happy to purchase. Not that I need an extender at this time.