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Shimano’s compatibly chart says you can only use EW67A with an external battery, and explicitly rules out your combination:
“SM-EW67-A-E (Junction [A]) is not compatible with SM-BCR2 and SM-BTR2.”
https://productinfo.shimano.com/#/com?cid=C-472&acid=C-473
It still might work in practice, but you’re paddling uphill.
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Good point. I was thinking I'd got a Di2 system with an EW67 junction box working with an internal battery, but thinking back that was an external battery that I'd hacked to turn into an internal battery.
It appears from this that the EW67 used to work with a BTR2, but Shimano broke it when they stopped 10/11 speed mismatched systems. Presumably on the basis that with an EW67 and an internal battery you've no way of charging the battery without disassembling everything.
Brommers
How can I get a sm btr2 battery to work with an older external di2 system? It works fine with btr1.
Is it a firmware issue or will it just not work? Ew67 junction, everything else recent ultegra level.