The nominal voltage of Nickel coupled cells (NiCd and NiMH) is based on the electrochemical couple which is around 1.20V. The actually running voltage depends on the load mostly but the age of the cell also reduces running voltage. Cell manufacturers can also add additives to both the electrodes and electrolyete to increase voltage, usually at the expense of another performance attribute (this is one of the reasons why IB4200 cells have such high voltage but they dont last very long in terms of cycles).
For Lithium ion and lithium polymer cells, the voltage is based on the average voltage when the cell is discharged at the 5 hour rate and this is typically 3.6V or 3.7V (Some of the cylindrical 18650 cells developed for power tools are now 3.8V) at this 5 hour rate.
Personally, i think lithium polymer is the wrong cell technology for RC cars, the 18650 (18.0mm diameter and 65mm height cell) which has been used in notebooks for the past 10+ years is now finally making it's way into powertool although the cells are only around 1.5Ah in capacity due to their need to deliver high currents.
A 2S2P 18650 battery pack would be ~7.4V 3000mAh and could be manufactured in either stick or saddle pack, have built in electronics for over voltage protection and over discharge protection (like a power tool battery). The current situation in which cells rely on the user to protect them is not a good one. It would not be entertained in the "real" world of consumer electronics and OEM electronic products. I wont let a lithium ion battery leave our factory without adequate protection and customer who want unprotected cells are told to buy them from someone else...
There are great LiIon possibilities for RC cars but i think the technology should be adopted from the powertool market in the same way as NiCd and NiCd was (because if it was not for powertool, the Cs cell would not exist).
The last bit of my rant is about transport. Did you know that transporting Lithium ion cells or batteries is being controlled more and more, they have to undergo many safety tests before they can be transported on aircraft. Any modification to these cells or batteries woudl mean a re-test. Anyone who buys cells and makes their own battery and then takes them on an aircraft woudl be breaking international law. Pre-tested, assembled battery packs are the only way this can go in the future.
N
|