Predicted effect of moderate temperature range on battery

[michael]

I have been noticing some reduction in available energy from the expected 19.5 kWh, and I was wondering whether it represented battery degradation or possibly some other effect. The most likely seemed the frigid (50 to 60 F) temperatures we have been experiencing here in Los Angeles.

I was curious about the expected effect on battery capacity over a moderate range. Panasonic publishes

https://www.panasonic.com/industrial/includes/pdf/ACA4000CE254-NCR18650A.pdf

the data sheet on a battery that I believe is somewhat similar (not identical) to that in the FFE. According to posts on the Tesla forum, this particular battery is very similar to that used in the Tesla.

Refer to the chart labelled "discharge characteristics (various)" This shows the discharge at various temperatures, and conveniently at 25 C (77 F) and 10 C (50 F). It also happens to be at the 1 hour rate, which represents freeway speeds.


At 25 C (77 F)

End point = 3000 mAh
90% discharge (which is probably about where the FFE reports end) = 2700 mAh
Midway to 90% = 1350 mAh, where the Voltage is 3.45
Approximate usable energy = 3.45 x 2.7 = 9.3 Wh


At 10 C (50 F)

End point = 2750 mAh
90% discharge (which is probably about where the FFE reports end) = 2475 mAh...call it 2500
Midway to 90% = 1250 mAh, where the Voltage is 3.4
Approximate usable energy = 3.4 x 2.5 = 8.5 Wh

8.5/9.3 = 91%

So these particular batteries (and by proxy, the FFE battery pack) loses 9% of capacity between 77 and 50 F. If we assume the nominal usable capacity is 19.5, it wouldn't be surprising to have only 17.7 at 50 F

The above assumes that the FFE batteries act in a manner generally similar to the particular Panasonic batteries examined, and that the TMS doesn't become active at or above 50 F.

Does this seem reasonable? Anyone see a flaw in this logic?

 

[davideos]

I posted some data a while back when I recorded about getting under 18kwh of energy from my battery. Temperature outside was around 50f when I took measurements. One thing I am not sure of is how much energy is credited back to the battery on regen. The power generated or the power assumed to go back into the battery...taking into account losses in the battery.

I was intending to wait for warmer weather to test again. For the time being I will accept you explanation and not worry about premature battery degradation. If I call it PBD perhaps the drug companies will come up with a drug to solve the issue...and hopefully without a side affect where the tires may fall off or something

 

[michael]

Right...that was my reason for considering the issue. By my reckoning, that's still within normal and the reduction may well be attributable to temperature alone.

 

[michelleannB]

The National Transportation Safety Board (NTSB) will hold a forum on lithium ion batteries in transportation on April 11-12 in Washington, DC. The forum is intended to educate the NTSB on how lithium ion batteries are used across transportation modes. The forum is free and open to the public and will be broadcast via webcast.