Why is the Twizy slower in cold weather?

The obvious answer is that the cold temperatures mean the chemical reaction in the battery is slower.

But, can anyone with a better understanding of motor controllers than me explain how this is different from the battery only being, say, 30% charged?

At first I thought it was because in the cold this is all the power the battery could push through the motor’s coils, but (a) I’m sure if you stuck a spanner across that cold battery it would still push an insane number of amps before it exploded and (b) the “powerbox” would do absolutely nothing in cold weather… which I’m sure isn’t the case.

Then I thought maybe it was a simple mark-space/PWM modulation scheme in the controller (like a domestic light dimmer *edit - not an exact analogy, plus the battery is DC…) which didn’t compensate for the reduced input voltage… but then the Twizy would get slower and slower as you drove, through the normal discharging of the battery.

Presumably then, the motor controller modulates power (not just voltage OR current) in a more sofisticated way. So back to my question which is just, how is cold weather different from partial state of charge? Can anyone enlighten me, just because I’m interested! Thanks!!

Oh wait… I wonder how much the colder tyres affect things…?

Try Wet roads that is as bad if not worse than cold temperatures. Water weighs a lot when trying to lift it up and move it.

That’s a very good point!! I’ll see what happens next time it’s dry, (and cold…) if that ever happens.

Do you reckon there’s no change with temperature in the torque reaching the wheels? I could easily believe it.

My view is the cold reduces the Battery capacity. The torque is the same it is the wet roads that have the effect on making feel less powerful. But range will be effected by both. Warm wet days are as bad.

Interesting! I hadn’t really noticed it in the summer but I guess by definition it doesn’t rain as much and I was probably enjoying the sun rather than pushing on to get home before my core temperature drops to zero… haha

I just found a Michelin document that states that tyre rolling resistance generally increases by 0.6% for every degree celcius you drop from 40C down to 10C. Below 10C the increase in resistance is more pronounced. This is at 2.1 bar. So… if that’s true, rolling resistance from the tyres could be maybe 15% higher in “cold” UK weather compared to UK summer conditions due to temperature alone. That also assumes a maintained pressure, which, I’ll be honest I’ve not checked this week so it’ll be lower now the temperature has dropped…but that’s another matter!

Given that most of the resistance on Twizy comes from it’s spectacularly un-aerodynamic shape (certainly at the 40 - 50mph I’m driving at) I would imagine 15% rolling resistance increase isn’t as significant as it sounds, and your suggestion that it’s lifting the rain water that bogs it down is still a more likely main factor.

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I think that You should go back to cells behavior. BMS measure voltages on the cells. Depend on those voltages there is a power limiting map. Unfortunately most of all BMSes measure only real voltages…The true is that it should be a OCV voltage (voltage with drop compensation, which is equal to DCIR x current). So if YOu have bigger current or bigger DCIR the drop is bigger

This is the same behavior as slowing down if You have SOC < 45%…

And now the root cause. The cell has DCIR much bigger in lower temperatures. eg if You have 1miliOhm in 23C deg it could be 12 times bigger in -10C deg

Air density (and hence drag) is approximately linearly related to temperature in Kelvin, I think, so there is an 8% difference in drag between +25C and +1C, all other things being equal.

Lithium batteries don’t like the cold.

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Lithium Ion cells work by electrically charged Ions making their way across the separator and going into the carbon matrix in the other plate. When its about 0c those ions dont migrate so effeciently. But worse, if you try and discharge heavily at or around 0c yu stand a chance that not only will you get an ion go across, but that the copper plates over the hole in the carbon. Result is you lose locations for the lithium ions to go to, and the ah capacity is reduced permanently.
So to get the best performance in cold weather. First charge as soon as you finish using the vehicle, while the cells are still at least slightly warm. Try and cover the vehicle , especially try and reduce underfloor airflow, as that is directly over the battery pack. This will ensure the pack is as charged as it can be. Dont wait until cheap electricity tariffs kick in when its really cold. The money saved isnt worth it for the relatively few days a year that we have such extreme conditions.
When using the car in 0c or about, always use the vehicle in a careful way for the first few miles. Dont accelerate hard. Thats what can cause the permanent loss of capacity.
But above all if you can get the vehicle out of the wind , and in garage if at all possible. If not consider some form of cover.
I work on a number of Citroen C1 conversions, and these have a small pack, so getting the best out of them is important. On these I suggest to customers, that they turn the electric heater on for 20 mins before they use the vehicle this pulls about 40 amps through the pack, not a high discharge, but enough to start the cells warming up. Then when you drive off, turn off the heater as much as possible. Not only is the car warm, but thecells are warm and will perform better, still take it easy for the first few miles, but this sequence does seem to help.

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Ha, that’s a neat trick.

So it seems from what everyone’s said, the answer to my question is “it’s cumulative”.

I’ve found the following top speeds achieved on an incline near me:

Warm & Dry: 51mph
Warm & Rainy: 50mph
Cold & Dry: 49mph
Cold & Rainy: 47mph

Obviouly I’ve no idea about wind direction, maybe I more often get a headwind when the weather is bad.

That’s an interesting point! So it’s that the control doesn’t compensate for this resistance, it just uses voltage to calculate power. So it’s not that the battery couldn’t deliver more juice in cold whether if the controller “demanded” it, but that the controller “thinks” more power is flowing than it really is because the resistance has gone up.

So this is unlike a lower SOC because although the voltage on the cells comes down with SOC, the resistance stays the same (for a given temperature)?

No the controller will deliver what you ask it to, but if the cells cant deliver the current and voltage then the controller cant deliver the power you require, so it will try to deliver greater current , and thats when the damage gets done. As the current goes up the voltage goes down, if it goes too low then the cells will get permanently damaged if heavily discharged under load. Thats where the BMS will hopefully step in and cut back the controller in an attempt to limit the damage of low temp high current, low voltage discharge.

the limit of the max regen and discharge current comes from BMS. I don’t know exactly what is a power map in original BMS, but the 0x424 CAN frame on 2nd byte has value of max recuperation power (scaling 500W for LSB, 8 bit value) and on the 3nd byte the maximum drive power (current taken from the battery) with the same scaling

I have my own battery and BMS so I know my power map. Of course it cooperate with Sevcon max values. If You have DVT Customer ans IXXXAT this could be adjusted too because minimum value is used from both: Sevcon limit, BMS from battery limit)

Ok thanks folks! I think I get it now, I’ll repeat back what I think you’ve just told me about the battery/controller side of things :grin:

So the BMS scales back the power map at low temp via 0x424 on CAN.

And in practice SoC (so long as it’s above a few percent) has negligible effect on performance at normal temps because the maximum power allowable by the BMS matches or exceeds that allowable by the Sevcon on wide open throttle?

i.e. V-min and I-max do not get clipped in normal operation so full “throttle” is always available in warm weather (even as cell voltage falls with SoC) but when the weather gets cold the BMS starts calling for scaling back of the power map in the Sevcon to avoid damage to the cells.