Brad330l

My Feedback: (1)

Hi all,
I still hav'nt made up my mind on a perminant set up for my twin ingition power supply so up till now i have been using a 1400 mah nicad (AA) for my twin ignition and charging after two 12 min. flights.
Now i have a 2500 mah Nimh (AA). I hav'nt tryed it yet so what can i expect? Is the discharge rate going to be too much for them to handle? Will i be able to top them up at the field quickly enough and not ruin them? I have a 'Swallow' charger that i will be using.

I know Lipo's and a reg is the ideal but we have limited choice in OZ and i hate getting hit with import tax.
Like i said this is a temp set up,,,, just want to know how you think it will go.
Cells are Sanyo and ignition is on my ZDZ80 Twin.

Thanks in advance for any ideas

Brad

JNorton

My Feedback: (2)

Brad the voltage put out by the pack is the same - so the discharge current draw will be the same. No disadvantages here just longer battery life. Charge the 2500 NiMh cells at 250 mA for 12 to 14 hours and use them. Do this for at least the first three cycles before you attempt to fast charge them. This is called form charging the batteries and is very important for NiMh to reach their rated capacity. I am not familar with a Swallow charger.
John

Personally I do not believe that a LiPo set up is ideal for ignition. You need a regulator and the cells are much more prone to failures if not properly taken care of. This includes making sure they are fully charged for storage. If you put them away for 6 months they can self discharge below 2.6 volts and self destruct.

Geistware

My Feedback: (16)

One 1200maH pack should last you for about 60 minutes of run time with no problem. If you are charging after 30 minutes with two packs then something else is wrong.

Brad330l

My Feedback: (1)

Thanks Guys.
I will be sure to 'Form Charge ' my new pack before using it.

What kind of draw would one be looking at from a twin plug ignition?

Would a single 1400Mah AA NiCad really last an hour?

Regards

Brad

JNorton

My Feedback: (2)

Brad,

Measure the current draw of the battery powering your ignition and divide it into the battery mAh rating. As an example if it draws 1 amp then 1400 mAh / 1000 mA = 1.4 hour

John

Brad330l

My Feedback: (1)

John,

Sorry for seeming so dumb and putting up with me.

If I start out at say 5.5v with my pack and run it to say 4.8 to 4.7v is that the 1400 or 2500 or what ever Mah for the Battery?
The usable capacity? Flat is flat right and i have run my engine till it misses.

Another way,,,,,1400/1000 = 1.4 hours,,,, is that down to 4.8v or so.

Thanks again

Brad

Geistware

My Feedback: (16)

No pack capacity is run at .5c load to 0.9volts.
Pack capacity means nothing other than a rule to masure the health of the pack.
Fly your plane until it is down to 4.8 volts with the engien running or a 1 amp load.

Ignition units draw at different rates but I would say a twin would draw from 800ma to 1000ma of current.

JNorton

My Feedback: (2)

Geistware is right my bad. Using manufacture rated capacity the voltage drops to .9 or 1 volt per cell ie 3.6 or 4.0 volt on the low end. Sorry for the confusion.

A better approach would be time the plane on the ground (paying attention to cooling) until it starts to miss, and give yourself a safety factor.

Here is Sanyo's web site for battery charge and discharge graphs. You can use these to help determine capacity if you know the type of cell and the draw.
http://sanyo.wslogic.com/

John

EDIT -> I truly believe I'm still asleep! Running the plane on the ground till it misses is somewhat impractical, it would work but.... If you just take the current drawn by the ignition divided into the rated battery capacity and then halve the result you should have a good time to work with. 1400 / 1000 = 1.4 hours / 2 = .7 hours or 42 minutes.

Brad330l

My Feedback: (1)

Thanks Guys, for the lessons and input.

Looks like I have ended up with a simple, light, cheap and effective system.

Manual switch with charge jack $12
Optical/electronic switch $55
2500Mah NiMH AA pack $28
And that's Australian $$$$$$$!!!!!!!

I will be able to get my four or so flights of 15mins with no worries.

Have fun and all the best.

Brad