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[ColoPaul]

At about 7:30AM, ambient of 21F/-6C, with my bike cold-soaked in a unheated garage to ~35F/2C, I rode 21.8 miles (~35 minutes).  I plugged in at work, and the kill-o-watt registered 3.2 kWH to charge to 100%.   By 4:00PM, with the ambient up to 62F/16C and some sunshine on the bike, I rode the exact same route, and plugged in at home.  This time, only 2.4 kWH.   No wind today, no significant change in elevation, just different temperatures.
In the cold it took 33% more power to go the same distance!  I knew there was an effect, but I never really measured it, pretty dramatic.  Where does the extra power go?

"Cold temperature increases the internal resistance and diminishes the capacity. Batteries that would provide 100 percent capacity at 27°C (80°F) will typically deliver only 50 percent at –18°C (0°F)."
http://batteryuniversity.com/learn/article/discharging_at_high_and_low_temperatures

So I guess this means that the extra 800 Wh that was used in the cold went into heat dissipation in the battery through the "increased internal resistance" mechanism?  I find this a little bit hard to imagine as that's a lot of heat - like a hair dryer running for 40 minutes.  I would think the battery pack would have been pretty toasty at the end of the ride, even with the ambient of 21F/-6C.

Does anyone have any insight into this?  I'm curious to learn more.

[Richard230]

I have no idea for sure, but I imagine that the cold just prevents the battery cells, due to their chemistry, from producing, or storing, the usual amount of kWh.  A lot of things slow down when they get cold.  I think that is why senior citizens like to live in Arizona and Florida.   

[protomech]

The bike will probably be a little more inefficient in cold temps (including because air density increases), but I would be surprised to see 33% more energy used.

It's possible that the batteries balanced while at work, which will use a little more energy. It's also possible that, beyond the cold, your evening ride was a little different from the morning ride - small change in wind, riding at higher speed or with more traffic, etc.

Try recording several days and see if the energy usage pattern is consistent.

[ColoPaul]

It's possible that the batteries balanced while at work, which will use a little more energy.

Good thought, but...   I have found on charging the kill-o-watt matches the battery gauge almost exactly.  For a ZF6, 1bar=500W.  And for the ride described above, the number of bars used was very close to the kill-o-watt readings.

Try recording several days and see if the energy usage pattern is consistent.

Yes, it's quite repeatable.  I remember this from last winter too.  Very noticeable power & capacity loss.  Recently, it has been cold, and I was wondering if there was actually something wrong with the bike, but all it took was a warm afternoon to prove all was well.


Back to the "where did the extra 800Wh go?" question.  The battery gauge registered the "missing" 800Wh.  If the missing wattage was dissipated in the battery due to the "higher internal resistance" mechanism, how would the battery gauge know this?   It would not have seen extra amps leaving the battery, but I guess it might be able to tell from the voltage measurements?  Or maybe the gauge algorithm looks at pack temperature during usage and automatically compensates for internal resistance?   However the gauge did it, it seems to work very well.  Interesting.

[Justin Andrews]

If the missing wattage was dissipated in the battery due to the "higher internal resistance" mechanism, how would the battery gauge know this?

The BMS most likely has a temperature table, which allows it to adjust for the temperature while performing the SoC calculation.