Very informative Bob, thanks.
In reading some of the emails from the past few days proves we have a great
resource
in terms of the knowledge from the people on this chat list. Thanks to all who
has shared input.
Great timing from a personal standpoint to, as I have recently been thinking
about alternatives to
standard 9v batteries for use in my setups. Funny thing is the last three
flights I did had more electronics
in a single rocket than I have ever flown in any single flight, 6 different
systems, and not one 9v was in the mix.
All the paring of battery to board was done for me on these systems, the
developer of each of the boards.
I have been thinking how to power my Ozark Aerospace and G-Wiz products, now I
have some good knowledge
to begin with.
pardon my ignorance- what is a FET and it’s purpose?
Thanks again
Jimmy F.
TRA 12524
From: Bob Feretich
Sent: Tuesday, July 14, 2015 12:58 PM
To: tccrockets@xxxxxxxxxxxxx
Subject: [tccrockets] Re: battery for stratologger CF
LiPo (Li-Ion) batteries have a much lower internal series resistance (a few
milliohms) than 9V alkaline batteries (about 1.5 Ohms). This means that the
LiPo can deliver much more current than an alkaline under short circuit
conditions. Popping e-matches is very close to short circuit conditions. (An
e-match is only about 1 Ohm.)
Most e-matches start at 1 Ohms and their resistance increases as the Nichrome
wire gets hot, and finally become an open circuit after they burn. This process
takes a small fraction of a second. But occasionally, they become a short
circuit after they burn.
Flight computers vary in how long they supply power to a pyro port. One second
is typical. This means The Flight Computer's power FETs may be driving into a
short circuit for a second. If the Flight Computer has no circuit to limit pyro
port current and used small power FETs that relied on the high series
resistance of 9V alkaline batteries, then its power FETs will be stressed and
may burn-out.
IMO not providing a circuit to limit pyro port current is poor design. It means
that the Flight Computer (although rated at 9V) will not work properly when
attached to a 9V non current limited power supply.
In case you have one of these Flight Computers, adding an external 4.7 Ohm
resistor would protect your power FETs and still deliver 1.25A (the recommended
nominal firing current for JTEK e-matches) from an almost depleted LiPo battery
(7.4V).
A 15+ Watt resistor would tolerate the short circuited pyro output
indefinitely.
But, assuming the Flight computer turns off the pyro port after a second, a
much smaller 5 Watt resistor should work.
Regards,
Bob
On 7/14/2015 7:06 AM, Steve Sawyer wrote:
I agree with Bob, if you want to turn to later technologies, the 2S LiPo is a
very nice small package - just watch out for the FET rating for firing the
pyros. A 180mA LiPo with a 40C rating means that it can deliver 7.2A into a 1
Ohm pyro load (this exceeds the PerfectFlite 5A FET rating).
I've also had discussions with PerfectFlite - they are fairly clear about
recommending the 9V Alkaline batteries (9V NiMH can also be used from Harbor
Freight).
However, I'm using a 3S LiPo (Turnigy nano-tech 180mah 3S ~40C from HobbyKing
or Amazon). This is very small but as I mentioned, requires the added series
resistors on the outputs. The voltage regulator on the PerfectFlite will allow
operation from either a 3S or 2S LiPo.
Steve
From: tccrockets-bounce@xxxxxxxxxxxxx
[mailto:tccrockets-bounce@xxxxxxxxxxxxx] On Behalf Of Bob Feretich
Sent: Monday, July 13, 2015 6:43 PM
To: tccrockets@xxxxxxxxxxxxx
Subject: [tccrockets] Re: battery for stratologger CF
7.4V LiPo (Li-Ion) will work in 95% of all 9V alkaline battery electronics.
The rated voltages are confusing because that 9V alkaline battery advertises
its starting voltage and the 7.4V LiPo battery advertises its discharged
voltage.
Compare the discharge curves...
http://www.feretich.com/Rocketry/Resources/lipotests.html
http://www.feretich.com/Rocketry/Resources/Pdfs/7v%20Li-Po%20vs%209v%20Alkaline.pdf
Assuming your electronics draw 100 mA of current, the cross-over point is at
30 minutes of use. After 30-minutes, the voltage of a 7.4 V LiPo is greater
than a fresh 9V Alkaline.
A fresh (newly manufactured) 9V alkaline battery starts at 9.25V, drops to 9V
after about 7 minutes of use (at 100 mA), and to 7.4V after about 2 hours. (If
the battery has sat on the shelf for a while, it may start at 8.4V.) See the
discharge curve at the provided link. One alkaline battery was freshly
manufactured the other had one month still remaining before its expiration date.
A fully charged 7.4V LiPo starts at 8.4V and declines slowly to 7.4V. Below
7.4V, the voltage on LiPo (and Li-Ion) batteries drops quickly.
Given that at the end of its rated life, a 9V alkaline battery is only
sourcing 7V, electronics designed correctly to use 9V alkaline batteries will
work fine with 7.4V LiPo batteries.
Regards,
Bob
On 7/13/2015 3:02 PM, Mike Hubble wrote:
Manual says standard 9v is all you need.
On Mon, Jul 13, 2015 at 2:39 PM, Peter Hackett
<peter.hackett@xxxxxxxxxxxxxxxx> wrote:
I've always used a standard 9V alkali battery. Never had any battery
problems with it.
On Mon, Jul 13, 2015 at 2:07 PM, Steve Sawyer <steve@xxxxxxxxxxxxxxxx>
wrote:
I've been using 180mah 3S 25~40C Lipos.
For the stratologger CF required protection of the FETs (i.e.,
short-circuit or 1 Ohm pyros), I use series 3 Ohm resistors on each output.
Lots of flights are possible.
Steve
From: tccrockets-bounce@xxxxxxxxxxxxx
[mailto:tccrockets-bounce@xxxxxxxxxxxxx] On Behalf Of Ben Sandoval
Sent: Saturday, July 11, 2015 11:01 AM
To: TCC Chat
Subject: [tccrockets] battery for stratologger CF
Which battery are you using for your stratologger cf. I'm getting ready to
order a few batteries and I'm still on the fence. Appreciate your input.
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