Do all EVs lose performance as battery charge drops?
Discussion
Hi all,
I have an 2019 E Golf and I'm generally very pleased with it.
One thing I have noticed is that the available power drops when you hit something like 30% battery charge. Meaning you have less performance available for acceleration. I get that the car is suggesting you should drive more frugally at that point to preserve the available charge, but there does not appear to be a way for the driver to say "I don't care - just give me all the power"
30% is still a fair amount of charge so it's not like the car is limping along with a few miles left.
Interested to know if this behaviour is common to all EVs or just something the E Golf does?
Cheers
Ian
I have an 2019 E Golf and I'm generally very pleased with it.
One thing I have noticed is that the available power drops when you hit something like 30% battery charge. Meaning you have less performance available for acceleration. I get that the car is suggesting you should drive more frugally at that point to preserve the available charge, but there does not appear to be a way for the driver to say "I don't care - just give me all the power"
30% is still a fair amount of charge so it's not like the car is limping along with a few miles left.Interested to know if this behaviour is common to all EVs or just something the E Golf does?
Cheers
Ian
Yes, they all will. As a battery discharges the voltage will drop. Power = Current x Voltage. To keep the Power up, most systems will draw more current from the battery. However once the battery voltage drops low enough, the system will have to draw too much current to maintain the power and too much heat will be generated. Therefore it limits the power output !
The previous reply is true, but it also depends on the how much power the inverter and motor are designed to make. It's quite possible that the battery pack's power output at cut-out voltage is still enough for the design of the other parts of the system. The difference in voltage between fully charged and discharged is probably less than 10%.
For example, my Zoe doesn't make a lot of power even when fully charged (80kW) and it appears the battery is capable of providing that much power at all charge levels. Even when the battery is low, the OBC shows 80kW output at full 'throttle'. If you take it below 10 miles range it does force you to ECO mode but at there is a 'kickdown' type switch behind the throttle pedal which will override and give full power.
For example, my Zoe doesn't make a lot of power even when fully charged (80kW) and it appears the battery is capable of providing that much power at all charge levels. Even when the battery is low, the OBC shows 80kW output at full 'throttle'. If you take it below 10 miles range it does force you to ECO mode but at there is a 'kickdown' type switch behind the throttle pedal which will override and give full power.
Edited by SamR380 on Thursday 26th March 10:56
There are three reasons the performance of an EV drops as battery State of Charge (SoC) falls towards the lower end.
1) For an electric motor, the power it can make is directly proportional to the voltage being fed to it. A battery at a low SoC has a lower voltage, so the motor ultimately makes less power. This is a reasonably small effect, in the order of up to a 10% reduction typically. If the system is calibrated to make the maximum power at full battery voltage, then it will make less power when discharged. However, most manufactures actually leave a bit of headroom at the top, and therfore hardly have to derate at all when the battery SoC falls
2) A battery has internal resistance, and a minimum voltage below which it cannot be allowed to drop. Hence, if you pull high current from the battery, it's voltage falls. As the batteries nominal voltage falls with falling SoC, a point may come where you can no longer allow the motor to pull "full" current from the battery because that would result in a battery undervolt situation. Hence, as SoC falls, ultimately traction motor power might be limited in software to protect the battery, even though the motor itself could be capable of making more power
3) Battery wear/degredation is a complex function of factors, including battery temperature, SoC, and the current being pulled from that battery. "high" wear occurs at high current and at low voltage, especially on a cold battery. Under these circumstances the Battery Health Monitoring System (BHMS) may put in place a short term current limit, that effectively limits the power the traction system can pull from that battery. In the UK, where it's generally single figure ambient temps overnight, and the battery soaks down, you may find that the BHMS limitation is causing a power reduction, especially if you have only driven a short distance at low speed and the battery hasn't warmed up much
Ime, with a fair number of EVs, i can't say it's ever been a significant problem, especially as most EVs so far have had enough performance that you rarely actually need full throttle in daily driving. The simpler the battery conditioning system, and the smaller (in terms of energy storage) the battery is, the more significant the derating at low SoC is likely to be. In most cases, the car just feels a "bit flat" rather than being actually significantly slower to accelerate, kind of like a ICE on a hot day or with poor quality fuel in it........
1) For an electric motor, the power it can make is directly proportional to the voltage being fed to it. A battery at a low SoC has a lower voltage, so the motor ultimately makes less power. This is a reasonably small effect, in the order of up to a 10% reduction typically. If the system is calibrated to make the maximum power at full battery voltage, then it will make less power when discharged. However, most manufactures actually leave a bit of headroom at the top, and therfore hardly have to derate at all when the battery SoC falls
2) A battery has internal resistance, and a minimum voltage below which it cannot be allowed to drop. Hence, if you pull high current from the battery, it's voltage falls. As the batteries nominal voltage falls with falling SoC, a point may come where you can no longer allow the motor to pull "full" current from the battery because that would result in a battery undervolt situation. Hence, as SoC falls, ultimately traction motor power might be limited in software to protect the battery, even though the motor itself could be capable of making more power
3) Battery wear/degredation is a complex function of factors, including battery temperature, SoC, and the current being pulled from that battery. "high" wear occurs at high current and at low voltage, especially on a cold battery. Under these circumstances the Battery Health Monitoring System (BHMS) may put in place a short term current limit, that effectively limits the power the traction system can pull from that battery. In the UK, where it's generally single figure ambient temps overnight, and the battery soaks down, you may find that the BHMS limitation is causing a power reduction, especially if you have only driven a short distance at low speed and the battery hasn't warmed up much
Ime, with a fair number of EVs, i can't say it's ever been a significant problem, especially as most EVs so far have had enough performance that you rarely actually need full throttle in daily driving. The simpler the battery conditioning system, and the smaller (in terms of energy storage) the battery is, the more significant the derating at low SoC is likely to be. In most cases, the car just feels a "bit flat" rather than being actually significantly slower to accelerate, kind of like a ICE on a hot day or with poor quality fuel in it........
Ok thanks all for the very helpful responses.
As Max_Torque said it's not really big deal for me - available performance is still plenty good enough but it does take away the grin factor somewhat
It just came as a bit of a surprise when it first happened as I don't recall seeing it in the marketing docs for the car. I guess some drivers who are used to getting 100% performance from their ICE car regardless of the amount of fuel in the tank may not be all that happy about it.
Thanks again
Ian
As Max_Torque said it's not really big deal for me - available performance is still plenty good enough but it does take away the grin factor somewhat
It just came as a bit of a surprise when it first happened as I don't recall seeing it in the marketing docs for the car. I guess some drivers who are used to getting 100% performance from their ICE car regardless of the amount of fuel in the tank may not be all that happy about it.
Thanks again
Ian
As an electronics engineer with some familiarity with motor drives and lithium ion batteries I am surprised by this. Certainly my experience of high rate drone batteries, e.g. "20C" rated - can give up all their energy in 3 minutes - have very flat discharge curves and very low internal resistance. They don't loose much power capability as they run down.
I've not noticed any loss in performance with my Leaf which is regularly discharged down to 25% and below.
I would expect the motor/drive to run an constant current from rest up to "base speed" where power reaches maximum, then current will fall in proportion to speed keeping power constant. Only at very high speed would I expect the battery voltage to come into it. I think the Leaf is limited to 93 mph.
So in normal driving it feels like the Leaf is always regulated at exactly it's 80KW irrespective of state of charge and actual battery voltage.
If any one has details of the exact topology of the inverter used I would be interested to see it. I believe the motor is permanent magnet synchronous. A tare down by Mike's Electric Stuff on youtube of a Zoe inverter is interesting, but the motor appears a bit old school with a wound field, not permanent magnet, which I can see could make the inverter simpler as the field can be weakened easily at speed. How the Leaf does the equivalent of field weakening I don't know.
I've not noticed any loss in performance with my Leaf which is regularly discharged down to 25% and below.
I would expect the motor/drive to run an constant current from rest up to "base speed" where power reaches maximum, then current will fall in proportion to speed keeping power constant. Only at very high speed would I expect the battery voltage to come into it. I think the Leaf is limited to 93 mph.
So in normal driving it feels like the Leaf is always regulated at exactly it's 80KW irrespective of state of charge and actual battery voltage.
If any one has details of the exact topology of the inverter used I would be interested to see it. I believe the motor is permanent magnet synchronous. A tare down by Mike's Electric Stuff on youtube of a Zoe inverter is interesting, but the motor appears a bit old school with a wound field, not permanent magnet, which I can see could make the inverter simpler as the field can be weakened easily at speed. How the Leaf does the equivalent of field weakening I don't know.
As has been said above, it depends.
As a cell discharges its voltage drops and consequently the peak power it can provide drops. However EVs do not let you discharge the pack fully so in practice there will be a minimum charge and hence minimum power limit before the car stops completely to protect the battery; if that power limit is less than the maximum power the motor is capable of using, performance will drop off as the battery drains; if it is not, it wont.
So the less powerful a car is and the more battery capacity the manufacturer has kept in reserve to protect it, the less performance will drop off as the battery depletes.
As a cell discharges its voltage drops and consequently the peak power it can provide drops. However EVs do not let you discharge the pack fully so in practice there will be a minimum charge and hence minimum power limit before the car stops completely to protect the battery; if that power limit is less than the maximum power the motor is capable of using, performance will drop off as the battery drains; if it is not, it wont.
So the less powerful a car is and the more battery capacity the manufacturer has kept in reserve to protect it, the less performance will drop off as the battery depletes.
granada203028 said:
How the Leaf does the equivalent of field weakening I don't know.
Leafs field weaken like most modern motors by phasing the current from the q axis into the d axis, ie moving a proportion of the total current from creating torque to using it to "squash" the magnetic field in the air gap, and hence effectively squash the Back EMF.So this technique can prevent the motor emf from exceeding the battery voltage and can be done so efficiently without high circulating currents and losses?
What would happen if I put the Leaf into neutral at high speed? Presumably then all stator drive stops but it is still connected to the inverter and the motor is still spinning fast permanently connected to the wheels, and still generating a high emf.
What would happen if I put the Leaf into neutral at high speed? Presumably then all stator drive stops but it is still connected to the inverter and the motor is still spinning fast permanently connected to the wheels, and still generating a high emf.
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