any merit in running dual battery packs ?

[Ed Alt]

Ron:
It's been a few years, but I've done tests with NiCad packs, taking a 4 cell pack and measuring the current dump into it from a 5 cell pack, in order to simulate what happens when you have a 1 cell short in one 5 cell pack in parallel with another 5 cell pack (no regs, no diodes).  As you say, the situation is very tolerable.  For the typical pack sizes we'll use in a pattern bird, and depending on the state of charge in both packs at the time that the short happens, you can expect to see something on the order of 100 ma flow at the beginning.  Could be as high as 120, might not even hit 80, it will depend on these and some other factors (wire & switch resistance, load imposed by the flight pack etc).  

What you then see is that the failed pack starts to come up in terminal voltage fairly quickly, causing the "charging" current to ramp down.  Over several minutes it will typically drop down to around 40 to 50 ma.  It will again depend on the flight pack loads, state of charge etc. how much it comes down, but the key point is that it does not represent any kind of catastrophic current drain from the good pack to the bad one.  The extra load is roughly equivilent to wiggling the throttle servo a whole bunch more than you should.  The trick to survival is checking both packs under load before each flight.  You will probably not notice the failed cell otherwise.  You might go all day with no harm, you might, maybe plant it if you have the failure occur early in the day, fly a bunch, not check it and drain the good pack at a slightly higher than normal rate all day.  I bet you already knew this, but I figured it might be worth mention to anyone thinking about trying this.  I've used it very successfully for years on the bigger birds. Using regs again though, just for the consistent response all day long.
Ed
  ----- Original Message ----- 
  From: ronlock at comcast.net 
  To: discussion at nsrca.org 
  Sent: Monday, January 24, 2005 1:39 PM
  Subject: Re: any merit in running dual battery packs ?


  I wanted redundancy in the power system for my pattern birds, but without ANY new potential points of failure.   My confinguration is:
  Two 700mah 5 cell batt packs.    Each pack direct to a switch, direct to Receiver.
  No regulators, diodes, etc.   
  I'm more concerned about connector, switch, and solder joint failures in the pack, than failure of a battery cell.  This config gives me redundancy in those areas.
  When things go normally, I have 1400mah available with only a slight weight gain over a single larger pack, switch & wiring.

  I understand cells more often fail open, than short.   But in case of a short, (the worst case for this config) the good pack must fly the plane, and charge the "bad" pack for duration of the flight.  After research & discussion with others, it's my belief the good pack will tolerate the load of charging the bad one (which won't be at a very high rate) and finishing a flight.  

  A potential failure point is me.  (darn humans!)  My duties as crew chief include checking both batts before every flight, and turning on both before flight.

  Later, Ron Lockhart
    -------------- Original message -------------- 

    In a message dated 1/24/2005 7:19:07 AM Eastern Standard Time, hitesh at salt.ac.za writes:
      Hi,

      Now that we all using high powered digital servo's with incredible holding power etc - is there any merit in running 2 battery packs, say 1 Ah each as opposed to 1 high capacity pack thereby eliminating the single point failure ? If I did want to run 2 packs, is a diode necessary to prevent 1 pack from possibly charging the other if 1 pack were to go bad ?

      Cheers,
      Hitesh

    Hitesh, a 1500 to 2000ma battery is all you need. There's little advantage to redundancy unless you are planning to fly more than 5-7 flights (res) on any given day. But then again, I use NiMH on the airborne and only Sanyo packs, which have proven extremely reliable for several years now.  As such, a redundant power source isn't really necessary in pattern models and you can avoid some unnecessary weight build up. 

    What has been done in large models is to add another battery and switch harness to an unused channel on the  receiver, which provides adequate redundancy. The extra weight on such models isn't as critical as it is in pattern models. Both switches "on" at take-off please.

    Of course, if you just have to have the extra battery and you have enough weight margin and room, the above is one way to do it. There are other ways, but this is simple and reliable.

    MattK
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