Battery Fact and Fiction
Like most devices batteries have an optimum window of operation on many of their specs. The temperature window on the GP3300 cell is 50ºF - 122ºF.
GP NiMh internal handbook quotes, sic.."By contrast, temperatures above 40º C (104F) adversely affect the cell's chemistry. Nickel Hydroxide decomposes (part of the self-discharging mechanism), and the rate of decomposition increases with temperature. Overall battery life is also adversely affected by elevated temperature, and cells should best be kept below 40º C (104F) during operation, storage, and charging."
Panasonic says "Discharge capacity drops at temperatures below -10C (50F) or above +45C (113F). Such decreases in discharge capacity can lead to deterioration in battery performance. As with the voltage profile, the capacity available during a discharge is dramatically affected by the cell temperature during discharge and the rate of discharge."
This issue is something being explored by the National Renewable Energy Laboratory (ctts.nrel.gov) in Golden, Colorado with regard to Hybrid Electric Vehicles utilizing NiMh as a power source. NREL's Ahmad A. Pesaran, Ph.D. says, "Battery temperature is important. Temperature affects battery operation of the electrochemical system, power and energy. Battery pack thermal management is needed (to) regulate packs to operate in the desired range for optimum performance/life, reduce uneven temperature distribution in a pack to avoid unbalanced modules/pack and thus, avoid reduced performance." The Conclusion? "NiMH is less efficient and generates more heat and appears to be more sensitive to temperature variation (than LiOn sic). So NIMH needs a more elaborate thermal management system."
Yes Virginia, the silly liitle heat sinks are what are called "passive cooling apperatus" and they help keep the pack thermally balanced and operating within the "thermal operating window" just like the the little fins in your PC, cellphone, battery chargers etc.. The combination of radiation from the outer cases and the end caps helps to keep the pack in thermal balance.
To answer another question, no they are not dangerous, they are very small and blunt, they will not not harm you or your car. There is no need to fear them for they are your friends.
Regarding the "PUNCH" issue. First let's define 'punch'. Punch is what you feel when you get an immediate reaction to squeezing the throttle. The blast out of the hole. The opposite is 'flat'. The batteries haven't dumped but they don't kick your car in the butt.
What is it? It is current on demand. The ability to supply all of the current your motor can draw instantaniously without a long spool-up. How do you get it?
LOW RESISTANCE. Remember Ohm's law? E=IxR. To solve for current it is I=E/R.
If your voltage is fixed (and low in this caes) the only real variable that will make a material difference is resistance. An example:
IR of 6 cells = 15mOhm x 6 = 90 or .09 ohms
Voltage of 6 cells = 1.175 x 6 = 7.05 volts
Current (for da' punch) = I=E/R = 78.33 amps available to ESC for motor
OK then, let's solder up those cells. The best pack I have ever received hand soldered had only .1 ohm additional resistance. Tiny fraction of an ohm. So..
IR of 6 cells = 15mOhm x 6 = 90 or .09 ohms + .1 ohm = .19 ohm
Voltage of 6 cells = 1.175 x 6 = 7.05 volts
Current (for da' punch) = I=E/R = 37.10 amps available to ESC for motor
1/10 of an ohm made a difference of half of the current!!
Our typical solder/assembly adds .02 ohms for a total of .11 ohms.
IR of 6 cells = 15mOhm x 6 = 90 or .09 ohms + .02 ohm = .11 ohm
voltage of 6 cells = 1.175 x 6 = 7.05 volts
Current (for da' punch) = I=E/R = 64.09 amps available to ESC for motor
THE PUNCH THING IS ABOUT A COMBINATION OF TEMPERATURE MANAGEMENT (heat sinks) AND THE PROCESS USED TO PUT ALL OF THE CELLS INTO A BATTERY (soldering).
please note that nothing here is discussing charging. That is an entirely separate subject with far more implications to battery health, life, and longevity. The heat -sinks are NOT about charging or soldering.
I hope that helps. Boy you guys must have been brutal on the guy who suggested matching cells was a good idea!