Do not ride with frozen batteries!

[Berty]

On the last week of December I drove my fully charged KS16 (840Wh, fw 1.23) for a 4.3 km (2.7mi) to have a drink with my friends. BTW everything happened outside. That evening it was -4 degrees Celsius (24.8 degrees Fahrenheit). In no time four hours passed by - enough for batteries to cool down. On a way home the wheel was performing as usual. Despite the cold it had no problems with inclines and I am 88kg (194 pounds). Half way back I was riding full speed (29-30km/h) on a flat road when I accidently drove into the small hole. The wheel immediately stopped balancing and I crashed hard.

Aftermath could be much worse: Few scratches, broken shoulder and ongoing physiotherapy. I have good chances for full recovery in a year time.

I could have:

• predicted the batteries will not perform and drive slower,

• taken my headlight with me and avoid the small hole,

• gone with the car,

but I did not. I am sorry.

I hope my experience can prevent an accident or two.

[esaj]

Sorry to hear about your crash and subsequent injuries, hope you recover well.

Based on what I've read, the discharging capability (maximum currents and the total amount of energy that can be discharged) of li-ion starts to go down below freezing temperatures, most will still function ok(ish) for lower discharge currents (I've seen numbers like 0.2C and 1C mentioned in some sites, but these would likely be too low for most wheels), but very low temperatures causes the internal resistance to go up and the maximum capacity to (temporarily) lower, and will lead to massive voltage sag with high discharge currents. Although it's pure speculation, I suspect that hitting the pothole, the wheel tried to increase the current a lot to correct the position, and caused the voltage to sag too much for things to stay powered or the batteries simply couldn't deliver enough current (ie. the wheel ran out of torque) due to higher internal resistance.

Vee (EUC Extreme) used (and probably still uses) separate battery heating-system during winter to prevent the freezing temperatures from affecting the batteries (at least as much). Plus at least at some point he also used custom set of Li-poly -batteries, which are said to handle cold weather better (ie. their internal resistance doesn't rise as much, and is in general lower than Li-ion's anyway).

 

As all drivers in cold countries know, a warm battery cranks the car engine better than a cold one. Cold temperature increases the internal resistance and lowers the capacity. A battery that provides 100 percent capacity at 27°C (80°F) will typically deliver only 50 percent at –18°C (0°F). The momentary capacity-decrease differs with battery chemistry.

The performance of all batteries drops drastically at low temperatures; however, the elevated internal resistance will cause some warming effect because of efficiency loss during use. At –20°C (–4°F) most batteries stop functioning. ...  Specialty Li-ion can operate to a temperature of –40°C but only at a reduced discharge rate; charging at this temperature is out of the question. 

Matched cells with identical capacities play an important role when discharging at low temperature and under heavy load. Since the cells in a battery pack can never be perfectly matched, a negative voltage potential can occur across a weaker cell in a multi-cell pack if the discharge is allowed to continue beyond a safe cut-off point. Known as cell reversal, the weak cell gets stressed to the point of developing a permanent electrical short. The larger the cell-count, the greater is the likelihood of cell-reversal under load. Over-discharge at a low temperature and heavy load is a large contributor to battery failure of cordless power tools.EV drivers are being made aware that frigid temperature reduces the available mileage. This loss is not only caused by heating the cabin electrically but by the inherent slowing of the battery’s electrochemical reaction, which reduces the capacity while cold.

http://www.batteryuniversity.com/learn/article/discharging_at_high_and_low_temperatures

 

Another thing worth mentioning is that you shouldn't charge your batteries while they're still cold (less than 0...few Celcius degrees). If you like riding during winter, allow the batteries to warm up at room temperature for a while before charging them:

Many battery users are unaware that consumer-grade lithium-ion batteries cannot be charged below 0°C (32°F). Although the pack appears to be charging normally, plating of metallic lithium can occur on the anode during a sub-freezing charge. This is permanent and cannot be removed with cycling. Batteries with lithium plating are more vulnerable to failure if exposed to vibration or other stressful conditions. Advanced chargers (Cadex) prevent charging Li-ion below freezing.

Advancements are being made to charge Li-ion below freezing temperatures. Charging is indeed possible with most lithium-ion cells but only at very low currents. According to research papers, the allowable charge rate at –30°C (–22°F) is 0.02C. At this low current, the charge time would stretch to over 50 hours, a time that is deemed impractical. There are, however, specialty Li-ions that can charge down to –10°C (14°F) at a reduced rate.

http://www.batteryuniversity.com/learn/article/charging_at_high_and_low_temperatures

 

[Addwyn]

@Berty, I experienced the same injuries as yours ten months ago...

I wish you  a fast and stable recovery, without any sequelae.

 

 

[steve454]

7 hours ago, esaj said:

massive voltage sag

Ouch!  Faceplant!