lets say a 1200 mah battery yeilds 20 min flight time in a gws zero. One etec 2cell 1200 will take 1 hour to charge so how is this a good ratio? every 20 mins you have to wait an hour ? wouldnt a 600mah nimh yeilding 10 mins flight time and only 20 min charge time make more sence?

I think its becoz lipos are a low maintance type of batt, for example, if I had charged a lipo to fly for today but it rained, you can keep the pack till next week and fly without topping up the charge, meaning that the pack will still be fully charged or lost only a small charge. Also unlike nimh and NIcd batts, lipos dun need to cycle and storage is not a pain.

Also just to note, the wt savings for a lipo vs a nimh or nicd battery is a plus for eflight as the saying goes light is better

You're correct, in many applications, NiMH or NiCd can make more sense. They also cost quite a bit more for larger applications.
I only fly Lithium cells in applications where I desire long flight time that cannot be met with traditional cell chemistry, or weight is a critical factor (i.e. micro indoor stuff). At SEFF, the guys were able to get long pattern or 3D flights with Lithiums, that were previously unattainable with traditional cells. That makes sense. Not for the average sport model that gets 10 minutes on a charge anyway..
..a

do you normally recharge your nimh's at C3 rate ?? i didn't think they would tolerate that kind of rejuicing without venting or just dying young.......... kw

i have been charging them at 2c and that seems to be 20-25 mins

Originally posted by Goleith
One etec 2cell 1200 will take 1 hour to charge

Where did you get this figure? I was of the understanding that 2 hours was the standard charge time.

I have charged mine at 2c without a problem. I just pull them off after they get to around 900 to a 1000 mah. It still takes an hour to completly top it off at 2c because the charge rate slows down alot as it gets closer to charged. The pack doesn't even get noticably warmer than room temp.

Actually it is ok to charge through the constant current phase then disconnect and fly, rather than wait for the timely less effective constant voltage phase to complete. That will take less than an hour while putting 80% or so of the charge in.

In fact the 340 kokams at a 400mA charge have been completly charging with both phases in an hour on my orbit. I guess they have less internal resistance since they do charge much faster. These may be excellent batteries to charge at the field, but I would let them rest a bit before charging.

-Dave
LightFlightRC

"Where did you get this figure? I was of the understanding that 2 hours was the standard charge time." - electron_head

You charge Lipos @ 1C, EH. Therefore, 1.2a on a 1200 mAh pack. 1C = one hour charge time, not two. 1/2C = 2 hour charge time.

That would make sense if the current was the same throughout the charge. Since it tapers off at the end during the constant voltage phase its take much longer than what one might think. The fact is it ussually takes longer than an hour to charge a lithium battery due to the way the lithium charging circuitry works.

Originally posted by WJ Birmingham
"You charge Lipos @ 1C, EH. Therefore, 1.2a on a 1200 mAh pack. 1C = one hour charge time, not two. 1/2C = 2 hour charge time.

That sounds perfectly logical if the battery you are charging has no ESR and the process is 100% efficient. But that is false in both cases.

According to the data I have charging at 1C 65% of the capacity and the current limit phase ends at around 40 minutes then the final 35% of capacity takes twice as long (80 mins) to complete.

Edited to add:

Having said that I have read the technical data on Kokams own charger and indeed they do say that the charge cycle takes 1 hour at 1C. From this I can only conclude that they are using fast top off which pulses higher voltage to maintain the 1C average.

FWIW,

I have been charging a 2S pair of Kokam 1200HD's at 2C rate with no problems. Also, I have been charging them to 4.28V/cell.

In use, after a flight, the cell no load voltage is 3.8

This would suggest I am making little use of the cells, however, on the bench I am consistently seeing 900mah delivered before the same resting voltage is met. Can't explain this.

These particular cells were, for the first 5 charges, charged at the 1C rate and it did take 2 hours+ to hit 4.2V/cell.

The charger I have put together starts out with 4.9V/cell and this is enough to hit the 2C rate for the first 20 minutes or so. I ramp the voltage down to 4.3V/cell once it hits 4.15ish........ this means I get (for my purposes) a full (900mah) charge in almost exactly an hour. (I had had to use an 18V PSU to get this, the LM wouldn't go to 2A at 14V..............)

These cells when in the air are powering a SP400 6V on a 2.3:1 with an APC 8X6 and current draw is maxing out at 8A static.

These cells now have over 150 cycles and are showing no signs of wear, other than abrasions due to insertion into plane!

Scott

Yup, agree with Lightflight.
The mA setup is only valid during part of the charge. Then the amps decrease in order to top the desired voltage. At the end, amp are really minimal, and even close to 0 at the very end. That's why it does not take 1 hour at 1C. Constant amp at the start, and constant voltage at the end, different process than for std chargers nicad or nimh ( that's why we need these special chargers, btw).

Back to the original question:

Lithiums have 5 times the energy density of NiCd, therefore you can fly 5 times as long for the same weight. This, IMHO, is the main attraction of Lithiums because the main gripe of electric fliers in the past has been short flights. Flights of 30+ minutes are possible without having to carry around massive battery loads.

The get comments after every flight of my Mustang w/TP7800-2S4 & 3S4P. One glow guy wants to buy it, and everyone one else loves the sound of my four blader; sounds more like the real thing than any glow powered warbird.

Saved 12oz going from P3000s in SPTs (8 & 10); re-geared so I didn’t loose much power by dropping a couple of volts.

Flight performance is awesome.

Could make one really long flight but I prefer several flights of normal duration. 1st couple of times out I only put 2 flight on a single charge; emptying then a little over half.

Yesterday 3 flights on 5100mah, down to 3.7volts/cell; that’ll probably be standard fare. I only draw 33amps @ WOT, so 2AH give me a very nice flight.


*** Key LiPo advantage for me – not having to peak before a flight! Or charging between flights

Imagine take 3 LiPo birds to the field; do 3 flights on each that’s 9 flights for that day – a very nice day at the field.

Oh did I mention, no need to take my charger or deep cycle battery – planes and TX, that’s all!!!! Better than glow no fueling/refueling, or cleaning.