icepilot

Hi guys!
I recently installed a Jetcat P180 in my SM F-16. I started with a 2400 mAh NiMh package for the turbine, and for my first flight
on the ice this winter, I was not too surprised when I only got one start before the battery indicator on the hand held display
said Empty. However, now we have 7-10 degrees centigrade, and this weekend I took the plane out for some flying again, first
with a 4000 mAh NiMh package. One start, then an Empty indication. Next day, 2 (!) 4300 LiPo batteries connected with a Y-lead
and kept warm until I was ready to start: No start! All the readings on the display seem right, except the battery power which drops
to almost zero when the preheat starts. Then, I hear the ticking of the valve, and the plug gets hot, but after 15-20 seconds of heating
and ticking, it opens up for the full kero without any ignition???

Yesterday I did not want to ruin the entire day, so changed to gas start. No problem, started immediately, had 3 great flights.

Any suggestions??

Thanks!
Tor

olnico

That's strange Torr.
The preheat phase should not drain more than 15 amps. Most batteries would handle that without dropping voltage too much .

Basically you need to check the impedance of your battery. This is even more important than the capacity in this case. The impedance will tell you "punchy" your battery is. The lowest impedance, the less voltage drop during a big current drain.
You can find very large capacity Nimh packs with very high impedance ( up to 400 mOhm ) in that case the voltage will drop so much during a current drain condition, that it will not be sufficient enough to sustain normal system operation.
What you need to have here is a 10 mOhm impedance pack maximum. Most Lipo packs will fulfil this condition.

Note than pack in series will see their impedance added.
Pack in parallel will see their impedance reduced ( 1/i total = 1/i1 + 1/i2 )

olnico

Now in your case I see two directions to troubleshoot your problem:

1) Check the batteries impedance.

2) Check the current drain of the engine.

1) Some recent charger like the new Graupner Ultra duo plus 30 will tell you the discharge and charge impedance of the batteries. This is also a good parameter to monitor the health of the battery on the long run ( battery log required ).
Note that a lipo battery with a normal impedance level will never catch fire spontaneously. So it is also a good safety tool.
If you don't have such a tool in your workshop, you will have to monitor the battery voltage under discharge condition and see if it remains constant.

2) Since your engine is working normally on gas, you can suspect an abnormal heater condition. Check the current drain during the preheat phase. Anything largely above 15 amps is not normal. You'll need a hall effect amp meter preferably for this job.
Note that the next phase ( preheat 2 + starter clutching in ) will drain up to 30 amps burst during starter engagement. Once again, most modern healthy Lipo bateries will handle this without dropping the voltage.

A343

My Feedback: (4)

Hi Tor
I'd exactly the same problem! Now it works 100%. See my thread in the JetCat Support forum for the solution.
[link]http://www.rcuniverse.com/forum/m_8415279/tm.htm[/link]
Regards Thomas

olnico

Thank you Thomas, very interesting post.

Basically you've upgraded your pack to a high quality one with a 5,5 mOhm impedance. Plus you've used high quality 13AWG harness between the engine and the ECU.
Am I correct ?

Do you have a very sensitive hall effect amp meter to record peak current consumption ? You should read up to 30 amps in that case.

Also note that the Lipo rating does not ensure that the pack will do the job properly

It only says that you will not destroy the pack at this rating. So a 30c continuous 3000 mah pack will not be destroyed at a continuous 90 amp drainage.
However it might drop the voltage significantly to a point where the electronics cannot operate the start any more. Once again, the pack impedance rating is the only significant parameter on that respect.
Too bad that the battery manufacturers don't publish official impedance figures for their cells.

The best source I've found on that respect is Nobs:
http://www.hangtimes.com/giantrxpacks.html

icepilot

Thanks, Oliver - I have also just spoken with Thomas and will make some new cables and try out a few new battery packs.
Here in Norway many of the guys have changed to A123-packs without sending in the ECU for the mod from Jetcat. What
they do is charge the pack to 100%, then drain about 50 mAh off the top, and it works great!

Tor

A343

My Feedback: (4)

Hi Oliver
You are 100% correct about what I've done + an ECU upgrade to v.6.0Q. My "Clamp Amp Meter" is a $35 piece from SEARS, I'm sure it's not of scientific std. but it worked well for my application. Voltage metering is done by high quality meters at battery + at MPX plug on engine with custom made connectors for this test.
It seems like there is a big difference in how well different LiPo brands function after storing them and at what voltage they are stored at. It sure looks like KOKAM's new 30C cell is the way forward for me anyway, with only 0.4V drop during max load with a 13AWG harness compared to >1.2V drop with std. harness and a 6cell NiCd or a 20C FlightPower LiPo.
Regards Thomas

Gonzalo38

Interesting information........

I'm planning to buy a JetCat P160SX turbine with Kero start and is good to know about the possible problems.

I normally use Duralite 4300 mah batteries and if this is not enough, I can always use 5200 mah Fromeco's.

I guess the 13AWG connector is preferred over the typical Multiplez connector and that is a good thing to know. Maybe I should upgrade my other planes too.

Are the supplied cables good enough for the Kero start application or should I plan to change them ?

A343

My Feedback: (4)

Hi Gonzalo38
Don't worry about your P160SX. I'm using std. harness and batteries with my other kerostart JetCats.

P180 + P200 are using another bigger starter and the enginmass to turn, is much higher than the rest of the JC line. Therefore in conjunction with kerostart electric power demand is much higher with these two engines. It seems we have to be careful in selecting batterypacks and considering wire gague.

Regards Thomas

olnico

Once again the capacity of the battery has nothing to do with its capability of providing high currents at a stable voltage. You'll need batteries with an impedance of 10 mOhm or less.

Guillermo Ibanez

My Feedback: (1)

Hi

Anybody knows the impedance fo the Hydrimax Ultra 4200 mAH NiMH 6.0 volts (5 cells) battery?

Gonzalo38

I was trying to find the impedance of the Fromeco and Duralite batteries, but there's no information available.

Guillermo Ibanez

My Feedback: (1)

I just sent an E-Mail to Hobbico.

olnico

Yes, the only guy who publishes the figures for his batteries is Steve Anthony from Hangtime Hobbies:

http://www.hangtimes.com/giantrxpacks.html

The only serious guy around...