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Old Nov 27, 2008, 12:52 AM
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Lipo partial charge question

I have a few 3c Lipo's 11.1v 2200mHa. I usually fly for a specific time, or until I notice a slight change in response. Occasionally, I will only fly for 1/2 a flight and put them away. If the battery still has some charge left, should I peak charge it again before using or run out the remaining charge first, then charge? I don't want to damage the battery by over charging it. FYI, I'm using the PZ charger that came with my T-28 Trojan. Thanks, DG
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Old Nov 27, 2008, 01:04 AM
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DEFINITELY charge it first prior to flying. Lithium chemistry batteries do not 'peak' as nickel chemistry batteries do. Rather they are charged to a specific voltage level. You run no risk of overcharging if recharging from a partially depleted state. In fact, your packs will last MUCH longer if they are NOT fully depleted prior to charging again. I try to discharge no more than 80% of a packs capacity prior to recharging, and typically only 60-70%. Packs will run cooler and provide much greater useful life.

Mark
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Old Nov 27, 2008, 10:09 AM
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Thanks for the info. I'll charge them up before flying. DG
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Old Nov 27, 2008, 10:30 AM
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Do NOT treat them like NiMh. Complete discharging is not only not necessary but is damaging. If flown for more than a couple minutes I'd recharge them. This is especially true for that ESC.

That has an e-flite 25A ESC in it. That unit works this way:
[Programmable low voltage cutoff with settings for 2-cell Li-Po (6V), 3-cell Li-Po (9V) or 70% of battery starting voltage]

http://www.e-fliterc.com/Products/De...rodID=EFLA1025

Lipos should not be flown below 3.0v/cell (~3.7v/cell resting). Personally I think the number should be more like 3.3v/cell for best cell life.
70% x 12.6v (starting) = 8.82v (That's ALREADY TOO LOW!)

Now if you flew for awhile to 11.6v, took a break and went out again:
70% x 11.6v = 8.12v (potentially destructive)


I don't recommend you ever fly this lipo/ESC combo to LVC unless you like replacing batteries.

I think this happens to a lot of newer fliers and they end up blaming the charger, the battery, PZ, etc. when the ESC is just not doing a decent job and they don't know better. A number of ESC have this issue-Great Planes, GWS to name a couple more.
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Old Nov 27, 2008, 08:57 PM
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Thanks for the info. What do you typically do, fly it once or twice to LVC to see how long the flight time is and then time your flights from there on? I don't see that the ESC is programmable for say an LVC to 9.5v or anything like that, and it appears that the 70% of initial charge ESC setting could be very bad if your battery was not fully charged to start with.
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Old Nov 27, 2008, 09:10 PM
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Start will a fully charged battery. Data here shows open volts after 20-80% discharges.

http://www.rcgroups.com/forums/showthread.php?t=956764



Charles
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Old Nov 28, 2008, 12:00 AM
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I don't recommend you fly any of your lipos to THAT LVC. You need a decent volt meter to check voltage.

You fly a little, land, check the voltage. Repeat. After some experimentation You'll figure out how long you can fly safely. Here's a approximate table for RESTING voltage per cell:

4.20v = 100% 12.6v
4.03v = 76% 12.09v
3.86v = 52% 11.58v
3.83v = 42% 11.49v
3.79v = 30% 11.37v
3.70v = 11% 11.1v
3.6?v = 0% 10.8v

No, that ESC is not programmable for a reasonable LVC. Most are not. Unfortunate-but true.
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Old Nov 28, 2008, 01:04 PM
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Flydiver, thanks for the voltage chart, do I need to check the voltage on individual cells or can I just use my home volt meter and check the entire pack? If I need to check individual cells, how do you do that and is there any special volt meters required? I've heard of taps on the battery packs, but I don't think my PZ 3s 1800 & 2200 packs have taps.

Last based on what you have listed earlier, you recommend 9.9-10v (resting) as the time to stop flying and recharge vs. the ESC 3c LVC setting correct?

DG
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Old Nov 28, 2008, 01:54 PM
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Quote:
Originally Posted by gregory.da
Last based on what you have listed earlier, you recommend 9.9-10v (resting) as the time to stop flying and recharge vs. the ESC 3c LVC setting correct?
No less than 11.1V resting (3.7V/cell) and preferably higher for a 3S lipoly to extract maximum longevity from your packs.
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Old Nov 28, 2008, 05:05 PM
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Resting voltage will be higher than working (loaded voltage). mrforsyth is right on. Take a look at this lipo discharge graph. Study it until you understand it.
http://www.wattflyer.com/forums/showthread.php?t=41166

That 3.3v/cell is working. Without special flying tools you cannot measure it.
Look for 3.7/cell RESTING (not flying). You do not need to check voltage on individual cells. The main tap will be fine for that.

You have 2 wire groups coming out of the battery, right? The main one is power-check the voltage there. The other is the charging tap-the wires are placed so that individual cells are charged. You can check individual cell voltage there with the pointy end of the volt meter if you are careful not to short anything.

What I've tried to get across is the minimum I expect from an ESC. Your ESC does NOT do even what I consider to be an acceptable minimum. I recommend you consider your ESC doesn't even have an LVC. It's just an empty pit that will suck your lipo dry if you allow it. It's safe as long as YOU don't allow it. Just fly by time. Go no lower than 11.1v combined/resting and you'll be fine. Your home voltmeter is OK. There are some tools that make this a bit easier. A wattmeter is best but some simple plug-in voltmeters can be obtained.
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Old Nov 29, 2008, 02:27 PM
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I fly mine to LVC once in a while to guage capacity but try to fly down to around 50% so I don't have to worry about storage. Means you have to bring a couple extra batteries along but so what. I can also program ESCs to cut out at higher voltage to accomplish the same thing.

Lipos are not as sensitive as many think unless you really run them on the bleeding edge. Storing fully charged has little effect on capacity but rather increases internal resistance which is only a factor when running near max ratings (SUV crowd). I've also discharged way low, even 0v, and tested with little or no damage. How long they stay that way may be the issue. On the other hand, on rare occasions, others have puffed or weakened for no apparent reason. Ya pays yer nickel and takes yer chances...

Quote:
Originally Posted by gregory.da
Thanks for the info. What do you typically do, fly it once or twice to LVC to see how long the flight time is and then time your flights from there on? I don't see that the ESC is programmable for say an LVC to 9.5v or anything like that, and it appears that the 70% of initial charge ESC setting could be very bad if your battery was not fully charged to start with.
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Old Nov 29, 2008, 03:26 PM
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Quote:
Originally Posted by gregory.da
Thanks for the info. What do you typically do, fly it once or twice to LVC to see how long the flight time is and then time your flights from there on?
I think that is a bad idea unless you know what the LVC is set for. Some are as low as 2.7V/cell. Just fly it for what you think would be 1/2 the time you could fly it for and then measure the resting voltage. That will pretty accurately tell you how much is left in the pack and you can go from there. If you don't have any idea of potential flight time. try 2 min to start with. Try to not discharge more than 80% of the available capacity.
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