/ Brake lights on electric vehicles

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Climbing Pieman on 12 Nov 2017
Was told recently that at least some if not all of the latest electric vehicles (incl hybrid ones) are programmed to put on brake lights automatically when the foot is lifted from the throttle (without even applying the brakes).

Reason being that there is no coasting ability and they decelerate as soon as power removed, and some of the latest vehicles are very quick indeed in deceleration partly due to regeneration mode.

The brake lights programmed that way are to warn others that they are slowing since it will be quicker deceleration than traditional vehicles.

Could explain why a hybrid car I followed recently appeared only to accelerate or brake with no steady speed maintained. Brake lights were on/off so frequently I did find it somewhat distracting though. Certainly got the car noticed!

Any EV uses know?
Jamie Wakeham - on 12 Nov 2017
In reply to Climbing Pieman:

In my Mitsubishi, there is variable regen which you can select with paddles by the steering wheel. It goes from B0 (zero regen, freely coasting) to B5 (maximum possible regen, feels like quite significant braking).

It illuminates the brakes only on B4 or B5, which makes sense as they're the settings which will slow you as quickly as actual braking.

Sounds like the person you were following was on pretty high regen, when the road conditions would have actually called for a lighter setting.
deepsoup - on 12 Nov 2017
In reply to Jamie Wakeham:
> Sounds like the person you were following was on pretty high regen, when the road conditions would have actually called for a lighter setting.

I didn't realise they do that.

It'd hardly be surprising if there are people out there driving around without selecting the appropriate regeneration setting, given how many drivers can't get their heads around the much simpler concept of turning fog lights on and off again (!) according to the conditions.
Post edited at 13:15
BnB - on 12 Nov 2017
In reply to Jamie Wakeham:

My petrol estate has a coasting mode which makes great use of momentum to increase efficiency. I wonder which is more efficient, coasting to travel further, or regen to increase range?
Jamie Wakeham - on 12 Nov 2017
In reply to BnB:
The former, given that generator->battery->motors won't be a 100% efficient process. Buy both are a lot better than burning KE as heat in your brake pads!

In fact, when you hit the brake pedal, it did as much as it can with regen before it actually ended the traditional brakes. There are anecdotes of people getting impossible life spans for their brake pads, as a careful driver will more or less never use them.

The coast on B0 is astonishing; you get much further than you would even on a regular car with the gear box in neutral.
Post edited at 13:21
wintertree - on 12 Nov 2017
In reply to Climbing Pieman:

I once followed an early Tesla Modle-S that had brake lights coming on all the time under what I presume was very gentle regeneration - less aggressive than me coasting down in my high compression diesel. It was borderline dangerous (in terms of training the follower to ignore its break lights and by being very distracting.)

I don’t think our Leaf puts its brake lights on under no-throttle regeneration.

I’ve been musing on this lately - I think the brake lights should indicate the rate of deceleration crossing a threshold and not what a pedal or regeneration is doing. So many different vehicle behaviours out there now, and deceleration is what people actually care about.

> Could explain why a hybrid car I followed recently appeared only to accelerate or brake with no steady speed maintained.

It took me about 100 miles or so of driving our Leaf to get the hang of holding a speed to within 1 mph in a 30 or 40 zone without a dangerous level of staring at the dashboard. The lack of engine revs/vibration/sound, the instant responsiveness of the throttle and the fact the throttle pedal is mapped to your full 0-max speed range (as a single gear vehicle) all conspire to make speed holding difficult. Ironically I use cruise control on my 3-series (more to avoid accidentally going over speed and to free my attention from dial watching for the road) but I don’t in the Leaf because the controls for it are not very ergonomic. Actually they’re outright crap compared to the BMW paddle.

What I think is really needed is a non-linear response to change in throttle position so that small movements during cruise have a smaller effect. Or a “sticky” throttle with haptic feedback creating soft stops every 10 mph when required.

Mind you I did adjust but I made a conscious effort to train myself for the new car. As EVs become widespread I’m not sure everyone who learnt on ICE cars will.

I had a test drive of an i8 and that had a nice approach to the EV speed awareness problem - a little colour heads up display showing your speed.
Post edited at 13:52
Climbing Pieman on 12 Nov 2017
In reply to wintertree:
Interesting thanks. I did wonder of EVs had a cruise control. Also I did think it must be hard to maintain a speed by pedal and this was partly why the brake lights were so frequently on and off as the driver was continuously going on and off throttle to control the ave speed camera enforced 40 mph.

Thanks to other replies.
gethin_allen on 12 Nov 2017
In reply to Jamie Wakeham:

> The former, given that generator->battery->motors won't be a 100% efficient process. Buy both are a lot better than burning KE as heat in your brake pads!

> In fact, when you hit the brake pedal, it did as much as it can with regen before it actually ended the traditional brakes. There are anecdotes of people getting impossible life spans for their brake pads, as a careful driver will more or less never use them.

>

> The coast on B0 is astonishing; you get much further than you would even on a regular car with the gear box in neutral.

I wonder how this will play with classical brake pads and disks ? The issue I see with this is pads getting glazed and then not working when you need them most.
wintertree - on 12 Nov 2017
In reply to BnB:

> My petrol estate has a coasting mode which makes great use of momentum to increase efficiency

Most modern ICE cars actually go further than this - they stop the alternator output when accelerating and cruising, running the battery down to power the vehicle’s electronic systems. Then they aggressively re-charge the battery as part of coasting and braking. It’s sort of a poor-man’s weak hybrid.
wintertree - on 12 Nov 2017
In reply to gethin_allen:

> I wonder how this will play with classical brake pads and disks ? The issue I see with this is pads getting glazed and then not working when you need them most.

A generator used as brakes becomes less effective at lower speeds, so the traditional hydraulic brakes and pads are still used multiple times on every journey - even if you make excellent use of regeneration - whenever the car stops.

You could use a synchronous motor/generator to brake to a stop by feeding it backwards - this would waste more energy and overheat the poor bugger but it might make a nice redundant braking system.
Post edited at 23:43

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