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Help: Li-poly discharge question

Old 11-22-2003, 06:35 PM
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roger- Michigan
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Default Help: Li-poly discharge question

Posted this in "batteries" got no response. Perhaps this is a better forum:

For use on Li-ion (converted new cell-phone batteries) and li-poly battery packs in my indoor electric airplanes....

Knowing that these cells are particular to minimum discharge voltage, at the expense of a little weight, I am building a small circuit board that can monitor voltage through discharge and turn on an LED when the pack reaches "X" voltage to signal it is time to land. I've got the circuit built up and it is very stable at room temperature (triggers EVERY time at "set" voltage - no drift - with a lab power supply)

So to those experts out there (Red, please, included!!!): What would be a good reasonable voltage discharge threshold that would cover both types of cells?

My gut says a safe minimum would be 5.0V for a two-cell pack, but I've read somewhere that you can safely push these cells to 2.25V/cell, so something possibly closer to 4.5V/pack would be appropriate.

I'll post the circuit if someone asks, and perhaps a photo of the hard-wired prototype board to give an idea of the size/weight....

Thanks in advance...

Roger
Old 11-23-2003, 08:44 AM
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dhurd
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Default RE: Help: Li-poly discharge question

Roger,
It think most of the Lipoly monitoring devices out there indicate when the cells reach 3.0v-2.8v per cell. This way it gives you time to get the plane on the ground without damaging the cell. I don't use a monitor myself as one of the features of Lipolys is that they have a very flat discharge rate until there almost done then there is a rapid drop in voltage. It is fairly noticable that you now have to carry full throttle to maintain level flight so thats when you land. I have yet to damage a cell by discharging it too much. The only exception to this rule of thumb might be if the plane is able to fly at a very low throttle setting (such as a floater type plane) you may not notice the decrease in performance.
Just my 2 cents worth.

Regards,
Dan Hurd
DWE
www.smallrc.com
Old 11-23-2003, 08:52 PM
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roger- Michigan
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Default RE: Help: Li-poly discharge question

Thanks Dan.

I ended up building 4 of the devices this weekend (see photo). Through tolerances in the resistors, diode, etc., they range from 5.7 to 6.0V settings. The original design called for a trimpot for fine adjustment, but that would have made it bigger and heavier.

As it is now, it weighs about 4.5 grams, measures 1.25" x .375", and has an LED which I think will be easily bright enough to see across the indoor soccer field where I fly. Best of all, I bet all 4 cost less than $15 to make, and I have enough spare parts to build at least 2 more! Not a bad weekend!

Roger
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Old 11-28-2003, 01:40 AM
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Default RE: Help: Li-poly discharge question

Im always open for to much information than not enough.

Post your plan's and circuit. it would be nice to see a construction guide and also a parts list and source list

This is a goos idea and the design could be used for other low voltage test conditions. Like a low voltage warning light that can monitor your car battery voltage and turn on at 10.5 vdc. It could help some one get home from a days charging and flying.

Uses are up to the end user but the design would be nice to see.
Old 04-13-2004, 10:41 PM
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Airboy
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Default RE: Help: Li-poly discharge question

Hey I just ask for such a device under a new thred called Lip-poly moniter. Please do includ all information of ckt. Airboy
Old 04-14-2004, 07:38 AM
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Mike Taylor
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Default RE: Help: Li-poly discharge question

Roger,

That is great that you did this yourself, but WHOA! is that ever heavy! That 4 1/2 grams is what 1 1/2 more batteries, another motor and gearbox, or two of the biggest actuators would weigh...
Bob Selman (http://users.joplin.com/~bselman/) has a couple of these on his site (see LBD) that weigh ~0.5 gram for $10.
HomeFly (www.homefly.com) has one for $7 at 120 mg (1/8 gram)

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