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Li Po tests

Old 01-16-2009, 11:31 AM
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TedMo
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Default Li Po tests

I have been attempting to determine the condition of my li po batteries and getting some strange results. My method: Charge pack up on FMA 4S or 10S chargers to done. Connect to E-Flite 25 motor with 11/8 prop. Connect to Astro meter to read max MA/ Watts, run at various throttle settings to obtain run time. Some maintain normal good reading for reasonablt time, about 7 min. Packs vary in amount of MA and C rating and number of cells, 3/ 4. Several pulsate when at high throttle and if I reduce throttle setting stop pulsating, then go back up give better reading? To me it appears like there may be bad cell? Seems improve when pack gets slightly warm? Some however just don't test well. Any thoughts on my test procedure or results? Relatively few maintain excellent reading throughout test. Quite disappointing. Testing about 15 different cells none very old.
Old 01-16-2009, 03:24 PM
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jzrf6c
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Default RE: Li Po tests

Have you ever balanced the cells? Does your charger balance cells for you? Have you tried to recycle the cells (discharge then charge ) with your charger? Sometimes you can get good information on specific cells this way. This takes the unknow load (your motor) out of the equation.
Old 01-17-2009, 10:21 AM
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TedMo
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Default RE: Li Po tests

Yes always balance the cells. When I charge on my FMA cell pro 10S I am able to read each individual cell during charge. This has usually shown me which cell/cells may be below others. They do seem to come up and be equal to others but overall pack doesn't equel others in performance. Annoying since I can't determine why several have gone bad so quickly.
Thanks for responding.
Old 01-17-2009, 11:08 AM
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TedMo
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Default RE: Li Po tests

I think I have answered my own questions regarding deterioration of my packs. Apparantly I have run packs down to the cut-off point before landing thereby killing one or more cells. Don't know how to set higher cut-off (if possible) perhaps best bet is to have flight timed and quit earlier. Haven't been getting much flight time though so perhaps been overpropped?
Wondering now if possible to rebuild packs with any of the good cells. realize that I must use only same size and kind of cells to do so.What are thoughts?
Old 01-19-2009, 06:18 PM
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jdetray
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Default RE: Li Po tests

Battery makers routinely overstate the "C" ratings of their batteries. Or perhaps it is fairer to say that the claimed "C" rating is achieved with a discharge regime that bears little resemblance to the way we actually use the batteries for R/C flying.

So if you are depending on manufacturers' "C" ratings to determine if a battery is suitable for a particular power system, you may be asking too much from the batteries. I often see the recommendation to assume that the "C" rating is actually 80% or less of what the manufacturer claims.

The pulsing at high throttle is almost certainly the ESC's low voltage cutoff (LVC) kicking in. Many ESCs pulse the motor to let you know that battery voltage has gotten low. That is your signal to land immediately to avoid over-discharging the battery. When you continue running the motor at a lower throttle after hitting LVC, you are further discharging the battery, which can lead to permanent battery damage.

With some ESC's you can program the LVC voltage. Programming the ESC for a higher LVC gives the batteries a greater margin of protection, while also shortening your flight time. With some ESCs, the LVC voltage is either fixed or is set automatically and you can't program it.

As for being overpropped, you wattmeter can help you. Your motor, ESC, and battery each have a maximum current or power rating. Your wattmeter can tell you if any of these components are being pushed close to their maximum ratings. As with batteries, it is never a good idea to run a motor or ESC right at the limit of the manufacturer's claimed ratings. The manufacturers of low-cost motors are notorious for overstating the power handling capability of their motors. If the motor is getting too hot to touch, you are probably overpropped, no matter what the wattmeter says.

Building packs with the good cells from various damaged packs is a bad idea. The cells all have different histories and so are not likely to be well matched, even if the cells are of the same capacity from the same manufacturer. What you can do if you are knowledgeable enough and very careful is to remove the one bad cell from a pack, converting a 3S pack to a 2S or a 4S to a 3S.

- Jeff

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