Battery level versus acceleration?

[kirkp]

I believe this question may have been asked before, but I couldn't find it.

Does anyone know if there is any difference in the maximum acceleration of a P85 depending on the battery level?

In other words, will the car run a faster 1/8 or 1/4 mile with the battery at 100% versus say 85%?

I've taken my car to the 1/8-mile drags the past couple of weeks, and I do a full range charge before I leave my house to 260 miles, and then by the time I get to the track 30 miles away, I'm down to 234 miles and about 90%, and then by the end of the night I'm down to about 224 miles and 85%. These are not timed events, so I can't judge for myself if the car is getting any slower as the night goes on.

I've been limiting my practice runs to keep as much charge as possible for the main event, but if it doesn't really make any difference, then I can stop worrying about it. On the other hand, if it does improve times by a few tenths of a second, I might try recharging at my father-in-law's house about 4 miles from the track in the future.

(Oh, and I apologize for the duplicate post. I tried to fix a typo in the title of the post, and it created a new thread. :redface: If a moderator can delete the duplicate, that'd be great.)

 

[dsm363]

mod note: no problem. Deleted the other thread.

 

[dhrivnak]

I have had a chance to run more than 20 1/4 mile runs back to back and I saw less than .1 sec variation. So in my case the driver is a bigger variation than charge level at least a charge over 30%.

 

[kirkp]

I have had a chance to run more than 20 1/4 mile runs back to back and I saw less than .1 sec variation. So in my case the driver is a bigger variation than charge level at least a charge over 30%.

Thanks dhrivnak, that's really helpful. Do you happen to remember about what level of charge you were working with during these runs?

 

[dhrivnak]

Thanks dhrivnak, that's really helpful. Do you happen to remember about what level of charge you were working with during these runs?

I started about 85% and finished the day a little less than half.

 

[David99]

The voltage of the battery depends on the state of charge. The difference between full and empty is around 20%. That should in theory be measurable. But the inverter and motor management might compensate for it by allowing a higher current draw at lower voltage. There are other factors that make a difference. Battery temperature for example. Tire and road temperature influence grip. Motor and inverter temperature. All these things make a difference. Not a big one, but they all play into it so it might be hard to isolate to what extend the state of charge has an influence on max power. Just doing two tests at different state of charge alone won't tell you much.

Very high current draw from the battery is never healthy for the battery. Especially at low state of charge you have to monitor the voltage carefully to avoid damage to the battery. That's why you see the orange power limiter show up when the battery is low.

 

[kirkp]

I started about 85% and finished the day a little less than half.

Perfect, that's just what I was looking for. Sounds like I should work on getting my own reaction times more consistent before worrying too much about the state of charge! Thanks again.

- - - Updated - - -

The voltage of the battery depends on the state of charge. The difference between full and empty is around 20%. That should in theory be measurable. But the inverter and motor management might compensate for it by allowing a higher current draw at lower voltage. There are other factors that make a difference. Battery temperature for example. Tire and road temperature influence grip. Motor and inverter temperature. All these things make a difference. Not a big one, but they all play into it so it might be hard to isolate to what extend the state of charge has an influence on max power. Just doing two tests at different state of charge alone won't tell you much.

Right, the consensus seems to be that a 100% full battery pack could be slightly faster than a partially depleted pack. But it seems that the few people that have done consecutive timed runs don't notice a very significant drop-off in performance, at least above 30-50% charge anyway. And as you say, back-to-back differences of a few hundredths of a second might just as easily be attributed to battery pack temperature increases or other factors as compared to voltage drop.