[Peter Harris]

I have always assumed that when I take my foot off the accelerator pedal causing the car to brake that this was all due to the back EMF generation charging the battery. However, now I am wondering.

I have just got back from a weeks holiday when the i3 has been sat on the drive unused. As expected, the damp and salt on the roads has caused some corrosion of the disk brakes so the brakes are noisy until the rust is polished off. This is normal with any car I have owned.

What I was not expecting was the same noise to occur when taking my foot off the accelerator (and not pressing the brake pedal). This seems to indicate that the brake pads are actually being applied when regeneratively braking. Has anyone else noticed this or is this normal expected behaviour?

I have seen another thread where someone complained of excessive brake pad wear when they only use the brakes rarely. Could this be the reason?

 

[Lexden]

The i3 uses an element of normal braking when at a full SOC, and the foot is taken off the throttle. If you think about it, there is nowhere for any regen power to go. The car's systems also apply normal braking from time to time to prevent the brakes etc from seizing through nil use.

FWiW, I am in the unusual situation after 3 years of ownership that the thickness of the pads on my car's front brakes has increased since they were last checked 6 months ago. The only explanation offered was that the i3's brake pad thicknesses are notoriously difficult to measure.

 

[gnomen]

While I accept that regen may not work when at full SOC for practical reasons, I do have a theoretical question. Once the car has been driven off there must be a decrease in battery charge, and since regen will restore less than was taken out, when is there never available capacity for regen to work?

 

[DBMandrake]

While I accept that regen may not work when at full SOC for practical reasons, I do have a theoretical question. Once the car has been driven off there must be a decrease in battery charge, and since regen will restore less than was taken out, when is there never available capacity for regen to work?

To understand the reason for that you'd need to know a bit about how Lithium Ion cells charge.

Charging the cells causes a temporary increase in cell voltage which is greater the higher the charge (regeneration) rate.

At the same time a Lithium Ion cell can't be allowed to exceed the maximum charge voltage (typically 4.1 to 4.2 volts per cell) even for a moment as the cell can be damaged by excess voltage. Battery management systems are really strict about not exceeding the maximum cell voltage on Lithium Ion cells.

This is the reason why the charge rate tapers down on a rapid charge - by the time you get much past 80% the battery cant take the full charge rate without the voltage going too high, so charge rate (current) is reduced to keep the voltage in range. The closer you get to 100% the lower the charge rate to stay within the safe voltage range.

By the time you get to say 99% charge the maximum charge rate possible might typically be less than 1kW.

So if you set off on a full battery and drive a mile down the road causing the charge rate to drop to 99% there is theoretically enough "room" in the battery for the regenerative energy from stopping from 30mph, however at a maximum charge rate of under 1kW the deceleration would be so low as to be barely noticable.

Moderate regeneration on a car like my Ion is in the order of 15-25kW, on a car with strong regeneration like an i3 I'd say it's probably in the order of 50kW - no way can a battery at 99% state of charge take a regenerative charge rate of 50kW or even 15kW and hence at that high state of charge you have little or no regenerative braking as the battery simply is not in a position to accept that high charge rate.

To reach full regeneration the battery state of charge has to be low enough that the battery can charge at the full regenerative charging rate, which depending on car, battery size and design, how much regeneration the car can do etc could be anywhere between about 80 and 95%.

One minor complicating factor in this is that if you have a resistive cabin heater turned on - which is typically up to about 5kW on maximum, you may be able to achieve about 5kW of regenerative braking even with a battery that is full or nearly full - as the energy is going into the heater instead of the battery.

I notice this effect on my Ion where with the heater on full at 100% charge I still get a relatively small amount of regeneration, however with the heater off at 100% charge I get none at all.

This was brought home to me yesterday when I drove the car for the first time in months with the heater turned off and wondered why I had no regeneration for the first couple of miles, before remembering that's how it behaved last summer!

 

[Lexden]

There has been a lot of discussion about friction brakes on various i3 forums. All I can say is that when car is showing 100% SOC, it retains this SOC for quite a while before it starts to fall away. During this time, I will invariably hear the grind of the friction brakes when coming to a stop or slowing down without use of the brake pedal: less so in Winter as heating; lights etc may be in use.

battery fully charged...regen overcharge? - BMW i3 Forum

FWiW, regen braking on the i3 is no longer what it once was following a software upgrade 18 months or so ago. This was partly aimed at reducing loads on the engine mounting bolts, and reducing the possibility of a driver backing in to the car behind in the event of an unplanned ACC disconnect (an all too frequent occurrence). Prior to the software change, an ACC disconnect at 70mph was an attention-grabbing event if the accelerator pedal wasn't guarded.

Friction braking is also used more frequently when Traffic Assist is used in a stop/start situation in traffic.

 

[gnomen]

Thanks for the explanation, Simon. Get it.

I thought I read somewhere that the i3 does not operate to the full extent of its raw capacity, in order to preserve the safety margin you describe. But perhaps not. Based on your explanation it would be more efficient for all of us to only charge the i3 to, say, 80% capacity. I don't think there is even a way to specify that.

 

[Peter Harris]

Thanks, thats interesting to know.

 

[Lexden]

Thanks for the explanation, Simon. Get it.

I thought I read somewhere that the i3 does not operate to the full extent of its raw capacity, in order to preserve the safety margin you describe. But perhaps not. Based on your explanation it would be more efficient for all of us to only charge the i3 to, say, 80% capacity. I don't think there is even a way to specify that.

Standby for Jack’s input. 100% charging is needed, or so I was told, to balance the cells.

This is an interesting blog on i3 battery life:

BMW i3 Long Term Battery Capacity Report: Better Than Expected