Lithium or Not ?

[JOddino]

  ----- Original Message ----- 
  From: BUDDYonRC at aol.com 
  To: discussion at nsrca.org 
  Sent: Sunday, August 31, 2003 12:50 PM
  Subject: Re: Lithium or Not ?


  Jim or anyone else who can shed light on the subject
  Jim Questions on your "Lithium Yes" post
  1. Why is a system with a regulator better? A system without one has one less point of failure. and the radio airborne fail-safe system works unregulated @ 4.8 volts. It does not work at higher voltage since when you get to the fail-safe voltage the batteries are discharged beyond the point of being able to operate. Fail-safe trip point is 4.3 volts on my radio.

  In a pattern plane we want consistency.  That is why we practice and practice.  We want to fly the same perfect flight every flight.  There are some variables like wind and sun that we can't do much about, but we can get consistent servo performance by using a regulator.  Without one the speed and available torque drop along with voltage.  High speed and high torque are desirable things in a servo so we want all we can get without burning up the servo.  The way to get it is start with a high voltage and regulate to the maximum the servo can stand.
  2. I agree
  3.I agree
  4. How is it less complicated than a Sirus charger with NiCads? One hour charge with a Sirus charger and 4 cell NiCad pack and it is fully charged.
  The Sirius charger is relatively complicated.  It detects a deflection in the slope of the charge curve to decide when to terminate the charge.  the batteries must get hot in order to create the change in slope so the whole process is temperature sensitive.  Two bad things can happen.  It can miss the deflection and overcharge or it can get a false alarm and turn off prior to a full charge.  You only have LEDs to tell you what happened and you must trust them.  It is also expensive and the one I have only charges at 500 mA which I do not consider a fast field charger.  It is a good charger but it is a tough task to charge fast and terminate properly under various environments.  Charging Li-Ions is much easier with much less of a chance of a screw up. 

  5. How is the remaining charge predicted? Are they cycled down to assure the capacity the same as NiCad's?
  There is a very close relationship between the loaded voltage and remaining charge and therefore you can use more of the charge with confidence that you are not near the knee of the curve.  You do need to run a discharge curve to know what to expect from your battery just like NiCds.

  6. "Less maintenance. No cycling." If this is true how is the actual capacity of the pack determined?
  You do need to run one cycle when you get them.  I'd also do one every six months to see how they are holding up.

  7. OK
  8. Isn't that true of any system? If not why?
  Let's take your system with a four cell pack.  At high current the voltage at the pack will drop due to the internal resistance of the batteries, there will be a voltage drop in the connectors, wire and switch and you could easily get less than 4.3 volts at the receiver bus.  Therefore people are going to heavier and heavier wire and switches.  We are kind of stuck with the connectors but some folks are using heavier connectors on the battery and switches.  My new plane will have a regulator (which can be inexpensive) at every servo so I don't care if there is a voltage drop in the cabling.  You wouldn't need this in a typical pattern plane where the servos plug directly into the receiver but in the big stuff you can get some long cable runs and multiple servos on one channel.


  Back to my original question, Do Li-Ion packs have a circuit breaker built in?
  Powerflite had them initially but took them out.  Duralite has something on the charge circuit but not on the output as far as I know.  We don't want any packs with a circuit breaker in it, in my opinion.

  Do they Contrary to NiCad's fail open? 
  I am not aware of any open failures but I suppose it could happen as often as it does in NiCds.

  Just trying to sort out the reasons that I should consider going this way over NiCads that I have been using for years without a problem.
  No problem.  Keep asking questions and make your own decisions.

  Buddy 
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