[tccrockets] Re: battery for stratologger CF

  • From: James Dougherty <jafrado@xxxxxxxxx>
  • To: TCC chat <tccrockets@xxxxxxxxxxxxx>
  • Date: Sun, 19 Jul 2015 15:57:18 -0700

I always charge my batteries and use a DVM to test them before I fly and I
have never had that problem.

One thing I want to warn people about when using LiPo batteries is never to
short the battery leads, doing so can
cause an explosion if not attended. When running power with LiPo's
(especially a 10C/25C battery) be very careful
with any wires and leads - you have an ARC welder on your hand - NOT a 9V.
Just remember a 10C 2000Mah battery
can discharge 20 Amps!. I've heard horror stories of people handling
hi-current 25C batteries with wet hands.

But, I have had problems where my wiring was not secured and/or when using
a twist-tie it shorts out something
(even an accidental e-match firing). Since then I always try to solder
wires and use heat-shrink and/or sleeving
to protect the wiring, don't leave anything electrical ever exposed!


On Sun, Jul 19, 2015 at 3:45 PM, scott yow <syow13@xxxxxxx> wrote:

This is an interesting discussion..

The issue that I have had in the past is that if the altimeter battery is
just marginal, it may have enough for the altimeter to fire the e-matches,
but have a voltage sag low enough to reset the altimeter. This results in
a great drogue charge but typically a failure in firing the main.

After this had happened - I always now either use a fresh 9v for every
flight, or use LiPos.. however i have been thinking that an overall safer,
better approach could be to wire in a few very small relays into the
circuit, and have a separate battery power the actual e-match circuit….

Eg. the altimeter would just trip the relay, and the relay would then
close a separate circuit (powered by separate battery) to the e-match -
hence isolating the load on the battery for powering the
electronics/altimeter from the current required to fire the e-match.

Has anyone tried this approach? The downside is that it is one more
device in the chain which has the potential for failure..

—Scott



On Jul 14, 2015, at 4:28 PM, James Dougherty <jafrado@xxxxxxxxx> wrote:

Great info guys!
Thank you
-James


On Tue, Jul 14, 2015 at 2:02 PM, Chris Attebery
<chrisattebery1971@xxxxxxxxx> wrote:
Field Effect Transistor. It's the electronic switch that the altimeter
uses
to fire the charges.

I'm an integrated circuit layout designer. I draw FETs all freaking day
long. LOL!

On Tue, Jul 14, 2015 at 1:50 PM, Jimmy Franco <Aldocyber3@xxxxxxx> wrote:
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 <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 <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.







No virus found in this message.
Checked by AVG - www.avg.com
Version: 2015.0.6037 / Virus Database: 4365/10222 - Release Date: 07/13/15

No virus found in this message.
Checked by AVG - www.avg.com
Version: 2015.0.6037 / Virus Database: 4365/10222 - Release Date: 07/13/15


Other related posts: