Tesla big battery at Hornsdale earns record revenue in September 2019

Matt, FYI: 100% of the necessity for "frequency control ancillary services" in Australia can be blamed on the irregular, upredictable output of the country's 38 wind farms.
Of course, these costs are never included in LCOE estimates for wind power, though customers are paying the bill.

"The 100 MW/129 MWh Tesla battery in South Australia...has earned its highest ever monthly revenue from wholesale markets in September."

Or, framed another way: "In September, South Australian electricity consumers were forced to pay the highest ever monthly cost for the privilege of accommodating intermittent wind energy."

"...while actual power backup applications aren't yet at scale"

There's no evidence grid power backup using electrochemical batteries will ever be remotely affordable or practical. Maybe we should wait until one such facility has proven viable before considering it anything but a naïve fantasy.