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R1T Launch Edition '22
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I've ended up using the mobile charger quite often as I've been spending time in a few places that have NEMA 14-50 plug. What I can't figure out is how Rivian determines the default charge speed. I notice a lot of time the charge speed defaults to 16A, but other times it remembers my last setting and usually there is a max but the max is lower than 48 A and it tends to vary. Yesterday when I charged up it went up to 34 A before the "+" button was grayed out. Today it was at 30A. This was on the same circuit both times which has a 40A breaker (so technically I shouldn't go past 32 via the 80% rule). But somehow the charger seems to sense this and limit current draw. Is it somehow detecting a drop in the line voltage? Once or twice I noticed at home my 30A circuit it would let me go all the way to 48A and actually defaulted to that before I turned it down. In short I can't figure out how it is determining default and max current. It's clearly not random but seems largely unpredictable and in some cases unsafe - I have never been able to charge at 48A anywhere so I was surprised to see that current be the default at least once. I didn't wait to see if it would throw a breaker or reduce the current on its own.

I've also used a J1772 (Nissan Leaf branded) and it would only let me go up to 30A charge speed. I assume this max speed is somehow communicated via the J1772 standard. It was 30A consistently every time I used the charger.
 

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In theory the EVSE is supposed to communicate its available charging rates during the handshake with the vehicle at the start of the charging session, but ultimately it is the vehicle which requests a charge rate (presumably something at or below the max rate offered by the EVSE, but not higher).

In EVSEs that can plug into multiple receptacles there is often some sort of resistor or chip in the wall-end plug dongle to let the EVSE know what type of circuit it's plugged into. I suppose it's possible that the Rivian EVSE is doing something fancier but that seems uncommon.

I can't explain the behavior you're seeing - it sounds like either the EVSE is not properly communicating its capabilities (the Rivian bundled mobile charger should never report that it can charge at 48 amps as I understand it) or the truck's software has some bugs.
 

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If you are technically inclined, this webpage explains how an EVSE indicates the maximum charge current it can give.
 
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