any merit in running dual battery packs ?

[Larry Caldwell]

In November, after some discussion with a friend who is build a giant 
scale project, I decided to do a few simple test regarding parallel 4 
cell packs. The plan was a two part test, one for batteries, the other 
to see where the receiver and servos actually say "no more".

The batteries were both 600mah Sanyo packs, not new but in good shape.
Starting from a freshly charged condition, each pack was discharged at 
300 ma for one hour, to simulate a bit of flying.  The packs were then 
connected thru a y harness, and the voltage measured with a 300 ma load. 
This condition produced about 4.8 volts.

One of the packs was then reconnected with only 3 cells in series, and 
again the parallel connection was measured under load.  Initially I 
measured about 4.4 volts.  This was disconnected, and a current 
measurement was made.  Initially there was about 180 ma flow into the 3 
cell pack, without the 300 ma load.  The current did begin to drop off a 
bit, but slowly.  That added to the receiver & servo load would probably 
drain the small pack fairly quickly.

The test of the receiver and servo voltage provided a surprise.

The receiver was connected to the variable supply with 1 each Futaba
S-3004, S-9001, and S-9202 servo. A standard Futaba R-127 receiver was 
used. A recently calibrated Tek voltmeter was connected across the 
supply output

Both the receiver and the servos would operate at 3.0 volts.  As you
started down from there approaching 2.9 volts, some chattering began in
the 3004. shortly below that the 3004 failed to rotate. at 2.8 volts,
the 9001 and 9202 would still rotate, but erraticly.  At 2.75 volts, no 
sign of life from any servo.

As the voltage dropped below about 4 volts, you could easily detect the 
slower speed of all of the servos.  Torque was not measured, just a 
comparative check by holding the servo arms.  The coreless 9001 seemed 
to maintain a higher value of torque as the voltage dropped.  The 3004 
seemed to get pretty weak as the voltage was lowered below 4 volts.  It 
would be interesting to have a simple way to measure the torque as part 
of the test.  I think if you had this condition in flight, it would show 
easily in control response, giving at least a little time to get down.

I haven't done this with 5 cell packs, but you would expect a better 
loaded voltage.  It may be that 5 cell packs with diodes between the 
switch and the receiver would be the best setup.

Your mileage may vary.

Larry Caldwell
Brookland, Arkansas



Ed Alt wrote:
> 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 <mailto:ronlock at comcast.net>
>     To: discussion at nsrca.org <mailto: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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