GetOffYourGas said:
lorenfb said:
Using the relationships from the previous posts (1C & C/2):
Power = I^2 x R, where I is the motor current and R is the impedance of the battery (typically 60mohms @ 70 deg F)
Then at 1C I equals about 60 amps and battery Power = 216 watts.
Then at C/2 I equals about 30 amps and battery Power = 54 watts
The rise in battery temperature is a function of the thermal resistance from the battery to the chassis.
The actual battery temperature over time is a function of the chassis temperature which is a function of
ambient.
camasleaf said:
Google battery 1c meaning
"Charge and discharge rates of abattery are governed by C-rates. The capacity of a battery is commonly rated at 1C, meaning that a fully charged battery rated at 1Ah should provide 1A for one hour. The samebattery discharging at 0.5C should provide 500mA for two hours, and at 2C it delivers 2A for 30 minutes."
So we are talking about two different things. Allow me to elaborate. Your numbers are looking at different C rates for the
same battery. I was comparing the C rates for two
different batteries. Like, for instance, a 30kWh Nissan battery and a 60kWh Tesla battery.
Let's assume both are 360V.
1C of a 30kWh battery (360V * 83.3Ah) = 83.3A
C/2 of a 60kWh battery (360V * 166.6Ah) = 83.3A
Both batteries output the same amount of
current, despite having different C rates, due to the different
capacities.
P = I^2 * R
Assuming again that both batteries have a 60mOhm internal resistance, they
both have a power dissipation of 416W.
Consider a 60kWh battery as being two 30kWh batteries in parallel. The result also affects the effective output
impedance of the combined batteries (a 60kWh resulting battery). It's now 1/2 of the single 30kWh battery.
The combined battery can obvious supply 2X the current output of the original 30kWh.
So driving at the same speed for either a single or parallel 30Ahr (60Ahr) battery results in the same power consumption,
as would be expected. Also, the total heating effect of the 60Ahr battery will be less than the single 30Ahr battery,
i.e. the lower output impedance of the 60Ahr battery.
Example - 2 parallel 30kWh
P (for each) = (I/2)^2 x R
P (for combined (60kWh)) = 2 x (I/2)^2 x R) = I^2 /2 x R = I^2 x R/2
That is P = 1/2 the 30kWh battery losses for the 60kWh battery.