Motors making louder noise after a lot of slow regenerative braking.

[Spivas]

So we live near the Smoky Mountains which is the perfect place to take the Tesla for the day. There is a motor trail (Roaring Fork Motor Nature Trail) behind the tourist trap city of Gatlinburg, TN. It's a 5.5 mile one way loop that winds drivers through the forest and I'd say has an elevation change of a few hundred feet from start to end. The average speed along the trail is probably around 10mph or less if you're taking the time to view the scenery and occasional black bears.

Both times that we've taken the Tesla (2022 M3LR), towards the end of the trail, the whine of the motors got much louder and had a different pitch to them which I mistook for a distant emergency vehicle siren without the change in tone. When I gave it some "gas" along parts of the trail where it was flat or uphill, the noise mostly went away but came back when going downhill even slightly.

It kinda freaked me out the 1st time and figured something was wrong with the car that I had to make an appointment for but by the time we got back into Gatlinburg, the noise went away. It was almost like the motors were dealing with too much regenerative braking even though we were under 10mph the entire time. Is this Tesla's version of what happens to the brakes of ICE cars when using too much brake pedal going down long mountain roads?

 

[STBR0510]

Could have been hearing a lot of things handling all the energy going down hill. Systems pulling heat away from motors, inverters making a whining as it dumps all the energy into the battery. Long distance down hill I noticed extra motor and inverter noises as well as the heat pump working in my Model Y

 

[KenC]

What was the SOC level, when you were hearing the whine? I'd guess that it was dumping excess regen energy, but that's only a guess. You should try it again to double-check.

 

[Spivas]

What was the SOC level, when you were hearing the whine? I'd guess that it was dumping excess regen energy, but that's only a guess. You should try it again to double-check.

I'm sure I'm gonna smack my forehead when you tell tell me what SOC means but, what does SOC mean?

 

[STBR0510]

I'm sure I'm gonna smack my forehead when you tell tell me what SOC means but, what does SOC mean?

Lol, state of Charge.

 

[AlanSubie4Life]

It was almost like the motors were dealing with too much regenerative braking even though we were under 10mph the entire time. Is this Tesla's version of what happens to the brakes of ICE cars when using too much brake pedal going down long mountain roads?

Sort of, I guess?

You’ll hear this noise (which is very similar to Supercharger preconditioning noise) when you regen too much. The car is automatically warming the battery by running the motors in a heat-generating state, which makes this noise. It’s common at a given SOC (even not that high, can happen at 60-70% no problem) for regen to become limited slightly (it often will not show the message, but you might see a tiny change in the regen bar if you are paying close attention).

Anyway, this is likely what you were hearing. It’ll happen more quickly at a high SOC, and at sufficiently low SOC (depends on temperature too, of course), you probably won’t hear it at all.

So the heating is a way to both warm the battery to allow more regen, and the few kWs of heating also serves to slow the car (slightly - not much!) even if you don’t have full regen at the time.

By “too much” I don’t mean you regen so much so that you appreciably increase SOC…the tightening limits are not due to the SOC change…it doesn’t work that way exactly. The battery just cannot accept too much charge for too long; it has to be given time to rest. This is why heating is so important at Superchargers. Note that regen power can be very high, quite easily. For example a regen-only stop in 10 seconds from 60mph would be an average of 65kW or so (obviously that is an average, the peak is higher, and you might not stop that fast), and that is not when going downhill. And there may be tighter limits applied to regen than Supercharging since it happens so often.