SRAM Red batteries - 2 contact vs 3 contact?

[tripmaster]

My SRAM Red derailleurs/batteries that came with my 2018 Cervelo have 3 contacts -- but bought some used SRAM Red 11 derailleurs and they have 2 contacts.

Are the 3 contact batteries backwards compatible?
My batteries look like https://ciclimattio.com/en/p/sram/et...QaAj4zEALw_wcB

but the just purchased deraileurs are missing the third contact. I swear my current mechs have 3 contacts...

[Dave]

My sram axs batteries have three contacts and so do my two chargers, but the derailleurs only have two contacts. That would suggest that the third contact has something to do with charging. If the batteries charge up, I wouldn’t worry about it.

[djg21]

As an aside, I thought I was purchasing SRAM AXS/eTap batteries on eBay when I needed a couple spares, and got a great price. The batteries ended up being after market knock-offs. For the price I should have known. They were less than half of the cost of SRAM batteries and came with a charger.

Initially I was angry at myself for not reading the item description more carefully. But I have been pleasantly surprised. I have used the batteries for a few months now, and they work perfectly well, and apart from the missing SRAM logo on the battery casing, they are identical.

They all are 3-contact by the way.

[tripmaster]

My current derailleurs are 2 contact. Shows you how faulty memory can be!