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