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May 06, 2010, 10:59 AM
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Discussion

7.4v Lipo to replace 7.2v NIMH


Hi,

I currently fly a JP Easy Pigeon glider (aswell as others!!) with a 7.2v NIMH battery in, also this runs via a Jeti 350eco Speedo. I get 20-25mins run time from a 3750 mah NIMH and I must add for a cheap 3 channel glider it is excellent!! I can easily get flights of 40 mins.

My question is.... it has the standard 600 size brushed motor in and folding prop... can I simpy use a 7.4v lipo in it with the same speedo?? They are very similar voltage so the BEC in theory would cut in and allow some time for gliding??!

I have a 7.4v 2250mah LIPO which needs a home and with the reduced weight im sure would make the glider fly even better. Would it be best to use a seperate rx battery? kind of takes the flavour off though as it would create extra weight!

Any comments would be good, also dont suppose anyone has any comments on a run time with this?

I had looked at putting a brushless in it but the motor would be worth more than the glider with it having to be an in runner, not sure i could get an outrunner in!

Thanks Matt
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May 06, 2010, 11:22 AM
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Mark Wood's Avatar
The only differences is that the esc has a much lower cutoff voltage for nimh by default. If you can set it for a lipo cutoff at say 7 volts it should work just fine. The lipo will hold a higher voltage under load too.

mw
May 06, 2010, 11:34 AM
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bartricky's Avatar
I have a Jeti 020 brushed ESC and it worked fine with lipos. It has little jumpers on the side to set brake and voltage cut-off. So ???
May 06, 2010, 11:57 AM
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Quote:
Originally Posted by matada34
and with the reduced weight im sure would make the glider fly even better.
The reduced weight could end up being a problem in the center of gravity area . If your plane was designed to use the heavier NIMH , and you switch to LIPO , you just may end up adding some balancing weight to keep the plane from being tail heavy . This balancing weight could come in the form of the separate receiver pack you mentioned , or you could attempt to lighten the tail by carefully removing any non structural material . See ? sometimes there are other things to consider when making modifications to our beloved flying machines , and that is one of the things that makes the hobby fun - it forces you to really think . Whatever you do , just remember "Nose heavy flys poorly , Tail heavy flys ONCE"
May 07, 2010, 07:27 AM
...design-build-fly-publish...
eye4wings's Avatar
The safe cutoff voltage for 2-cell LiPo is 6 not 7volts and the BEC in your present ESC will still deliver for the receiver as before, although it may get a little warmer because don't forget that the ACTUAL LiPo voltage fully charged is 8.4V.
But having said that it is as close to a straight swop as you'll find.
So I say go for it!... just make sure the ESC has sufficient cooling.
You will have a lighter model (will the battery move forward for balance if required?) and more available power for the launch and climb out.
May 07, 2010, 11:14 AM
aka: A.Roger Wilfong
gnofliwr's Avatar
Don't worry about the high voltage on the 350. It'll handle 10 Nixx cells with onut complaining.

The only issue is that your Jeti 350's low voltage cutoff is probably fixed at Nixx values and is not adjustable. If you fly to LVC (never a good idea on a glider), then you'll kill the LiPo pack in short order. I use several older Nixx ESCs with LiPo packs, but I follow a couple of simple rules:

1) I never over load the pack. I typically use them at 75% of their rated current limit or less.
2) I time my flights and typically land when I've used between 50% and 75% of the pack's capacity.

With these rules, I don't get the full duration out of each flight (but I get enough and always have enough spare capacity for a go-around if I blow the approach). I have a couple of cheap, 2000mAh 8C packs that I fly in my SR Batteries Cutie (geared S400). I get 10 minutes of high throttle tearing up the sky - which uses 1000-1200 mAh. The packs are going on 5 years old, have over 200 flights each and show no signs of quitting.

So, where I'm going with this is that you'll be dropping a lot of weight going from NiMH to LiPo. You can trade back some of that weight for more head room in capacity and not worry about the LVC issue - if you've got two of the 2250 mAh packs, just use them in parallel and you'll have your original duration and shouldn't be getting near LVC. Alternately, use one and land sooner.

- Roger
May 07, 2010, 12:39 PM
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Xpress..'s Avatar
Quote:
Originally Posted by eye4wings
The safe cutoff voltage for 2-cell LiPo is 6 not 7volts
That's okay for you. As for me, I will not let then get below 7v resting.

FYI, NEVER FLY TO THE LVC! It's only there as a completely last resort.
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May 08, 2010, 11:25 AM
...design-build-fly-publish...
eye4wings's Avatar
Xpress, I agree that it is unwise to fly to the LVC. It is there simply to protect the cells from discharging below 3V each beyond which they may not have the ability to recover.
My concern in saying that 6V is the cutoff was to save Matada possible concern as to how to reset the LVC to a higher figure than standard when it would not be absolutely neccessary. With the simple model he has even if he did fly to the 6V cutoff he has a decent glide rate which will allow a safe landing to be made.
We both have experience in larger multi-engined models which dictate a greater degree of caution - especially when using separate packs for each engine.
So I wasn't trying to be contentious - just trying to spare a relative beginner some anxiety.


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