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Thinking about Electric?

posted Oct 31, 2010, 10:21 PM by R. Scott Page
Now that the hobby has gone full throttle toward electric vehicles and aircraft we're beginning to see confirmed fuelers looking closer to see "watts" going on with the electrics.  Electric systems offer as much power per weight than fuel making them a viable consideration in any application, but knowledge of internal combustion engines does little to no good when considering electric propulsion.   Electrical system are no more difficult than wet power, but certainly different.   

The difference I want to start with is the "fuel" of electrics - the battery.  Nickel-cadmium (Nicad) were the dry cell of choice until the early 90's when Nickel-metal hydride (NiMH) came along. Lithium ion batteries -- particularly Lithium Iron Phosphate (LiFePO4) batteries are superior to NiMH or Nicads, however their price and availability has limited their adoption in RC hobby.  Research in Lithium ion batteries yielded a new much lower cost battery that could be manufactured in many different shapes -- the Lithium Polymer battery.  

Today the Lipoly offers the highest charge to weight ratio making it the (almost) ideal electrical battery -- in fact it's become the De facto standard rendering Nicad and NiMH obsolete for propulsion systems.   I say "almost: because when charged or discharged too rapidly Lipo batteries can explode or burst into flames.  Rapid discharge can occur from an electrical short or physical damage making proper care and storage of these batteries essential.  While Lipo's are almost ideal, this instability makes LiFePO4 a viable option in some instances, despite their higher cost and slightly higher weight to storage ratio.  Lithium phosphate batteries are in very limited use in the hobby, however a few intrepid souls harvest them from power tool packs to make batteries for their aircraft.  LiFePO4 batteries are probably more useful to power on-board electronics than for propulsion at this point. 

Since I'm going to be looking specifically at electric propulsion for aircraft, when I refer to batteries from now on I'll be referring to Lipo batteries unless I specify otherwise.

Lipo batteries have an operating voltage range from 2.7v to 4.23.  Overcharging Lipo will result in explosion and fire.  Overdrawing them will cause a range of problems ranging from not holding their voltage under load to no longer accept a charge at all.  It's ESSENTIAL that charging equipment for Lipo batteries be designed specifically for Lipo batteries.  
The battery voltage determines the potential RPM's for a motor.  Only the smallest motors can achieve flight speeds with 3 to 4 volts so Lipo cells are combined in Series to form the battery.  Thus a 2S battery has two cells in series --- so it's operating voltage is from 6 to 8.4 volts, a 3S battery then has three cells in series and operates from 9 to 12.6 volts.  Let me clarify again - that a 2S battery with 6 volts of a 3S with 9 volts is a DEAD BATTERY -- as that's the minimum voltage is can have or it will permanently fail.  Also you will notice that 2S batteries are listed as having a voltage of 7.4 volts and 3s batteries of 11.1 volts.  This is the "nominal voltage".  I just consider it the median usable voltage, and for my purposes that's good enough.  

One last word on cell voltage.  Pictured to the left is a very hand device that will give you the voltage of each cell, and then the combined voltage.  This is very useful -- because if any 1 cell drops below that magical drop dead voltage, the entire battery is useless.  If you see that one cell his not at the same voltage as the others in the pack it's essential that you use a voltage balancer, which essentially discharges the higher voltage cells to match the lowest cell.  As a rule I balance every battery every time I charge it.  That may be more than necessary, but at the cost of Lipo batteries, I want to get all of the life out of them that I can.  

To recap, Lipo is the way to go.  Charging correctly with the correct equipment is absolutely essential, never let your lipo cells drop below 3 volts, keep all of the cells in a pack balanced.   

Tomorrow I'll cover how to combine batteries to get either more voltage OR more capacity.  
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