Battery Energy State Hysteresis

[mlbco]

I noticed that the battery energy state has a "hysteresis" behavior if you power cycle the bike while charging or discharging (i.e. riding.) This video shows examples of how you will "gain" energy when charging just by turning the key on/off and similarly you will "lose" energy if you do the same during a ride.

I think the energy level is estimated by the integral of volts*amps when riding (or charging) but every time you power cycle the bike it gets reset based on the actual zero-load cell voltage. I'm pretty sure that if you turn the key off during a ride you will lose about 10% energy (i.e. the software won't let you access that energy in the battery pack because of the estimated charge state.) I'm still studying this but for now, I will not turn off the key during a ride!

Steve

 

[OneLapper]

Interesting. I must have shut my bike off 10 times during my last 40 mile ride. I wonder if your theory would have resulted in a longer ride. I'll test your theory out next weekend.

 

[mlbco]

My theory is that each time you cycle the main power, the energy reading resets to the value calculated from the zero load battery pack voltage. From what I've seen there is roughly a 10% energy error over a complete discharge cycle so you might gain 10% by not turning it off. I think after a certain amount of power cycling the value doesn't change much because it's constantly being corrected to the battery pack voltage-based capacity estimate. I'll test this again tomorrow.

Steve

 

[strider]

I am not a battery engineer, just parroting things I've learned having Teslas for the last 8 years...

Unlike other chemistries, Li-Ion batteries maintain a very flat voltage profile through a huge portion of their SOC (state of charge). That means voltage drops only when the battery is "nearly empty" and voltage rises only when the battery is "nearly full". This is a great thing for operation as it means you get the same performance over a wide range of battery SOC. The downside is that it is very hard for the BMS (Battery Management System) to know the exact SOC when it's in that middle area.

Not sure about Alta's but to get around this, Tesla "counts coulombs". Which just means it keeps track of how much energy has gone into the battery and how much has been taken out and from that information it makes an educated guess about how much energy is left in the battery.

There's a ton of complex stuff that goes into that calculation but at the end of the day it's just that, it's an educated guess and it can be wrong.

Another thing to keep in mind, at least when charging, is that it takes time for the BMS to balance the pack and because of the above noted feature of Li-Ion that balancing can only occur at higher SOC (80% or higher on a Tesla). So it's very common that when a Tesla finishes charging it will report X miles of range. But after 30 minutes it will "lose" range as the BMS balances the pack and builds a more accurate estimate of the CAC (Calculated Amp-hour Capacity).

 

[mlbco]

Here's an explanation that covers the factors you mentioned and also describes the state of charge "learning" function which is based on unloaded cell voltage, similar to what I'm guessing is going on when the power is reset. They mention a 4 hour "rest" period to get an accurate reading from pack voltage!

Measuring State-of-charge - Battery University

 

[Fog 25]

Hopefully @Judaslefourbe will get a explanation from the battery guy at Alta and let us know how the battery functions? Would put all this guessing to rest.
I am finding (thinking) that when you power up the computer will read the battery level and give you 80% of the remaining battery of your usage before going into conserve mode.

 

[Philip]

This is a pretty geeky discussion, I like it. It belongs into the Technical forum. I just moved it.

 

[mlbco]

Today I rode a full charge without turning off the key and I went 10% further than a similar ride yesterday, ending with the same charge state in both cases. It's only one data point but it seems to indicate that you shouldn't turn the key off during a ride if you want maximum range. Of course you are now depleting the battery 10% further than the more conservative energy estimate would allow and the battery life may be affected by this, but you might just make it back to your truck in a bind.

Steve

 

[snydes]

Today I rode a full charge without turning off the key and I went 10% further than a similar ride yesterday, ending with the same charge state in both cases. It's only one data point but it seems to indicate that you shouldn't turn the key off during a ride if you want maximum range. Of course you are now depleting the battery 10% further than the more conservative energy estimate would allow and the battery life may be affected by this, but you might just make it back to your truck in a bind.

Steve

I’m really glad you brought this up as. I’ve noticed this a few times when I only had a few bars left, took a break (power cycled the bike), and then powered back up to see I now had no bars left. Didn’t know why it was doing it, I was going to make a post about it myself.

Great post

 

[OneLapper]

Riding at the state forests we end up stopping at the end of the trails to wait for the group to catch up. We always shut the bikes off to keep the noise down. We'll be back at Pachaug this Sunday morning for another ride. I'll keep the key on this time and see if the range increases. In an ideal world it would be great to make the 60 mile loop on one charge, but at 39 miles I had the empty battery displayed.

 

[Judaslefourbe]

Please note that keeping the key ON does not increase charge. The display shows "more" charge when the key is cycled because of the logic of how the charge is estimated. Basically by cycling the key, the bike is losing the history of the ride and cannot estimate your range based on your recent riding.

 

[Fog 25]

Please note that keeping the key ON does not increase charge. The display shows "more" charge when the key is cycled because of the logic of how the charge is estimated. Basically by cycling the key, the bike is losing the history of the ride and cannot estimate your range based on your recent riding.

Thanks @Judaslefourbe that’s what I was thinking I might be figuring these things out NOT!!!

 

[mlbco]

Please note that keeping the key ON does not increase charge. The display shows "more" charge when the key is cycled because of the logic of how the charge is estimated. Basically by cycling the key, the bike is losing the history of the ride and cannot estimate your range based on your recent riding.

Energy is definitely not "created or destroyed" by playing with the key in these tests. What is at stake is how much energy the computer thinks is available and will therefore allow you to access. So far, my one test indicates that you have access to more of the battery's energy if you don't turn the key off during a ride.

Steve

 

[strider]

Energy is definitely not "created or destroyed" by playing with the key in these tests. What is at stake is how much energy the computer thinks is available and will therefore allow you to access. So far, my one test indicates that you have access to more of the battery's energy if you don't turn the key off during a ride.

Do we know for sure that you lose access to the energy? Does the bike shut down when the charge estimate reaches 0 or when the BMS detects the battery voltage is at the low cut-off point? If it's the former then you are correct, you are limiting your range by cycling the key. If it's the latter then the range is still there (you can ride beyond 0 miles remaining) but you just don't know how far that is.

To test this we'd either need 2 bikes or for a rider to do the same loop twice. One bike (or loop) would do a mid-ride key cycle, the other wouldn't, and see if the bikes have the same amount of range. Or maybe @Judaslefourbe can tell us

 

[Judaslefourbe]

Of course we are using the actually pack voltage to shut down, not the average showed on the display.

 

[Butch]

I’ve always been skeptical of the indicated charge. I do not usually turn the key off. I have only run the battery down all the way once when it was downhill back to the truck. I did 43 miles that time, though there are a lot of varibles.

Am I correct in that the indicated remaining charge is really the estimated remaining capacity based on current depleation rate since the key was last cycled?

I’d gather more data, but I’m busy riding. Good stuff here.

 

[Mark911]

Both the state of charge and the range to go (I not sure which is shown on the Alta) is not much different than the fuel gauge and the calculated "miles to go" shown on most modern cars. They're both based on various direct and indirect measurements and in the case of range has many additional "assumptions" factored in. Although the assumptions are based on actual use (some moving time scale, motor loading, etc,) they are still assumptions which are extrapolated in an attempt to predict the future.

We typically never run our cars when the little red light is on or the range indicator is flashing "urgent, urgent, you got 5 miles to empty". It's just too unpredictable. I never even get below three bars before I plug the plug on my rides. I know everyone wants to get that last half mile into your rides but the Alta's just not suited for that. At least not yet.

 

[Butch]

Ha! I always run the cages down to empty. I have seen "0" miles to empty many times. I have run out of gas once, a long time ago. A long time ago, when I used to wrench, the fuel gauge in my 1971 BMW 2002 was transmiting actual fuel level in the tank; I had to remove the float assembly for some reason. I have no idea what they do now.
I will run the Shiftless the Alta the end too, as long as it is downhill to the truck. Certianly to blinkey mode. We quit today when Eric got to blinkey mode. I think we could have gotten one or two more runs on Fingers...

 

[mlbco]

I’ve always been skeptical of the indicated charge. I do not usually turn the key off. I have only run the battery down all the way once when it was downhill back to the truck. I did 43 miles that time, though there are a lot of varibles.

Am I correct in that the indicated remaining charge is really the estimated remaining capacity based on current depleation rate since the key was last cycled?

I’d gather more data, but I’m busy riding. Good stuff here.

My tests indicate that your theory is probably correct, i.e. if you don't turn the key off, the system is calculating energy consumed by summing the volts*amps*dt and using this as the energy state displayed and also to decide when the battery pack enters blink mode. If the key is turned off it resets the energy state based on the zero-load battery voltage and not the previous energy-use data.

I was the other guy riding with you today at Metcalf on the MXR and I did not turn my key off for the entire ride. I got 21 miles (on the Trail Tech) which is my nominal range without turning the key off. Next time I'll turn it off at 1/4 energy remaining and see what mileage I lose, but in the past I've noticed that I lose a significant amount of range, roughly 10%.

Steve

 

[Butch]

Interesting how the remaining charge is calculated. No wonder I did not think it behaved linearly.
Good riding with you, Steve. Eric and I did 27 miles before he went into blinky mode. I still have a quarter bar graph left. I think I am lighter on the throttle than you guys.