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The only "dumb"-ish solution I can think of, would be adding some smarts to your EVSE. I've sat in my 38er charging to 100%; I thought a final top-up 80->100% would take 45 mins; it actually took more like 100 minutes, as the final few% were really slow, with the kW going down to about 2Kw at the end. I can only assume the BMS was doing some cell balancing so some cells weren't taking much at all, or maybe cell balancing is a slow, low-current process anyway. Viridian ECUs used in some EVSEs (e.g. I have retrofitted one to a dumb Rolec) support infinitely-variable charging rates, from 6 to 32A, controlled by an external resistor, or applied voltage. So adding say an Arduino + a digipot, and a current-transformer round the mains wire to the car, would be able to measure actual charging current, and so detect when it's doing the final slow-down. It would have to be adjusted to the particular EV of course, but it could be programmed to stop the charge once charging current dropped below 5Kw for e.g.
 

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I've sat in my 38er charging to 100%; I thought a final top-up 80->100% would take 45 mins; it actually took more like 100 minutes, as the final few% were really slow, with the kW going down to about 2Kw at the end. I can only assume the BMS was doing some cell balancing so some cells weren't taking much at all, or maybe cell balancing is a slow, low-current process anyway.
Ideally you'd only be doing that once-monthly balancing charge with the car at home plugged in overnight, so it's speed wouldn't be relevant. Of course not everyone has that luxury, in which case I'd suggest plugging into a Level 2 during a lengthy shopping mall or pub visit to get the balancing accomplished without the agonizing watching and waiting. (Which I know is easier said than done in Covid times, but I'm sure this crisis won't last forever...)
 
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Ideally you'd only be doing that once-monthly balancing charge with the car at home plugged in overnight, so it's speed wouldn't be relevant....
Reason for doing this was I needed 100% to be sure of doing my trip non-stop in poor weather; first time I've done this in my new EV, so learning as I go along. But the point of mentioning this is that the OP wants to be able to stop his charge at say 96%, when his EV has no 90% or other settable-level to achieve this. I'm simply suggesting that an EVSE could be engineered to recognise the shape of the charge-curve of an EV, and so be able to deduce the SOC, despite no comms from the car to report this. My experience of a charge to 100% showed a very recognisable charge pattern of extreme slowdown; I think my 80% SOC charge rate was 14 Kw, decreasing roughly linearly in time down to around 2 Kw-ish.
 

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Discussion Starter #24
Y - H Andy - that's pretty genius and it seems pretty straight forward. You'd think though that one of the charge companies or even the open source group would have figured this all out already. Am I missing something?
 

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Y - H Andy - that's pretty genius and it seems pretty straight forward. You'd think though that one of the charge companies or even the open source group would have figured this all out already. Am I missing something?
I think it's such a low occurrence & a very low priority on the list of EVSE mfrs tbh. They'll be far busier trying to get their smart modes to work etc. But it's eminently do-able fby a group like the OpenEVSE guys.
 

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Discussion Starter #26
I think it's such a low occurrence & a very low priority on the list of EVSE mfrs tbh. They'll be far busier trying to get their smart modes to work etc. But it's eminently do-able fby a group like the OpenEVSE guys.
Take a look at the last comment on this openEVSE discussion thread here.
 
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