[KevinT]

Question: What would attaching the BM2 negative wire to the battery negative terminal do? I followed my BM2 directions which told me to do just that. Thx for your help.

The car attempts to monitor all drains on the 12V battery so that it can top it up again (and by the right amount) when necessary. Without a connection through the vehicle -ve lead, the current drawn by the BM2 can't be detected and won't be accounted for. Of course if Hyundai had gotten this system right in the first place, we wouldn't all be installing BM2s. Oh the irony...

 

[KevinT]

Lol, if the internal charging logic worked properly we wouldn't all be hooking up BM2s to monitor our 12Vs in the first place.

 

[KevinT]

Does the car not have some sort of log of what the battery voltage/SOH has been over some recent period? It would be interesting to know just what's done with the sophisticated information captured by the special current sensor attached to the 12V battery -ve terminal.

If the car were continuously logging it, then it would be smart enough to close the power relay assembly and top up the battery from the DC-DC converter the moment it starting getting too low. The behaviours we've all seen suggest that the car only wakes up and takes a sniff periodically to decide whether or not to boost it while the vehicle's off.

This is why I think it was a mistake for Hyundai to play the 'smart charger' game with the DC-DC converter, and they should have just designed it to put out a 13.8V float charge voltage at all times like a good old-fashioned alternator. The car has no idea what's all happened to the battery in the time that it was off and not paying attention, so it doesn't have enough data to know what the battery actually needs to 'smart charge' it while the vehicle is on. In theory smart charging prolongs the life of the 12V battery, but most of us are getting worse life spans from it than Hyundai's current ICE + alternator charged vehicles packing similar infotainment and Bluelink tech. I consider that proof that Hyundai's 'smart charging' approach for the 12V battery in their EVs is a fail.

 

[KevinT]

Parameters gathered by the small BMS box on the 12V battery negative terminal (30 days back) in the attachment.

Interesting, thanks, I'd missed spotting this earlier!

The list of affected models in that document makes me suspect that the module on the negative terminal of the lead-acid batteries found in the classic Ioniq isn't this advanced:

I do like that it actually measures 'dark current', as the BM2 that a lot of folks here have been using only logs battery voltage, not current.

Curiously it doesn't seem to log voltage, although I'd imagine that it has to be measuring that too to determine the state of health. The ratio between the current out and the discharge voltage curve is really the crux of determining the battery's true state of health, and something I'd love to see from an aftermarket logging device like the BM2. That's why I haven't bothered to invest in a BM2 myself, as it only paints half the picture by capturing just a voltage log.