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        simple noob question on new lipos cells

#1 clearconcepts Dec 19, 2004 11:27 PM

simple noob question on new lipos cells
 
I have some new bare cells still sitting here waiting to be used. :)

Just wondered what you guys do with new cells before you make them into packs...


1. discharge them all equally.
2. charge them all equally.
3. nothing... just grab your solder iron.
4. cycle them.

I assume nobody really does 3 or 4... but had to ask. I assume that LVC cells would be "safer" than fully charged cells, should an assembly mishap occur, as there is much less energy stored in the cells.

Will discharging the cells adequately balance them? (I plan to have some form of "taps" in my packs, but would prefer to not have to balance on the first "flight".)

Thanks! And feel free to rip me if it makes you feel better. I can take it. :cool:

p.s. I guess there should be a #5.. check the voltage of each cell and see if they are already the same. If so, then they are already balanced? This seems unlikely, but it just occured to me.

#2 RD Blakeslee Dec 20, 2004 07:13 AM

Do no.5 first. They ARE likely to be close, @ about 3.82V or so.
If not, and no farther apart than .3 volts or so, connect them together in parallel for a couple hours, to equalize the voltages. Then,

A. Construct your pack with taps and charge it a little short of your charger's full-charge voltage (that varies from charger to charger). The idea is to leave a little room at the top, for step C, if needed.

B. Measure each cell's voltage through the taps, then

C. SLOWLY charge the low-voltage cell(s), (if any) up to match the voltage of the highest cell(s). The idea is to get the pack balanced at the outset, and observe what it does after that.

D. Fly the pack, then

Repeat steps B.-D. The cells should be within .02 volts or so.

If not, the cells are mismatched and you will have to live with the extra time and attention required by a pack that will require frequent re-balancing.

- RD

P.S. You can avoid all this by constructing your pack for parallel charging-series discharging.

#3 Dave Hederich Dec 20, 2004 07:57 AM

Also be sure you understand all the special considerations required for soldering LiPos before trying it for the first time. LiPos are more difficult to solder correctly and safely than NiCd or NiMH. Soldering mistakes can lead to pack failures, and LiPo pack failures can result in venting with flame.

#4 clearconcepts Dec 20, 2004 09:20 AM

Sounds great... thanks guys.

As for soldering, I will be very careful not to overheat the tabs. And these cells have nickel tabs.

Seems like it will be more difficult the shorter the tabs are, so I may just leave the tabs long for my first experience. From reading comments of others about soldering I don't expect too much trouble. Some have said things like hold the iron to the tabs "no more than 5 seconds"... wow, I don't think I need 5 seconds to solder a thin tab, I think that's way too long. Even SubC cells don't take more than 1-2 seconds.





I plan to make "sub packs" of 1s2p or 1s3p and then use these in series so it would be very easy to parallel charge them if I wanted to.

The only problem with parallel charging these is that I would need several chargers or a several hour charge. The cells are 4000mah and I will be using packs of 3s2p or larger. My charger is good for 8A.

Still, a 3s2p pack would equal charging 24,000mah in parallel. Maybe I need a couple more chargers.

BTW, is a slower-than-1c charge better for the cells? Like if I charged the "24AH" pack at 1/3c 8A... is that actually better for them anyway?

#5 Dave Hederich Dec 20, 2004 09:33 AM

There have been technical papers written that suggest that charging Li cells at <1C can slightly extend their useful life. The way I fly, I am usually in no rush to charge my packs, so I use a 0.7C rate. But it's really the discharge that makes the big difference in cell life.

(Edited to remove inappropriate response to parallel charging)

#6 clearconcepts Dec 20, 2004 10:54 AM

Sorry to sound so confusing....

#7 hall woo Dec 20, 2004 11:14 AM

JST Balancing Connectors: Female Pin Socket
 
I like the JST connectors for center tap balancing .

The TANIC packs , for example a 3S2P pack come with 6 JST FEMALE PIN SOCKET, so U can check balance on all cells

#8 Dave Hederich Dec 20, 2004 11:28 AM

Quote:

Originally Posted by clearconcepts
I know my 3s2p will be 11.1/8000mah. ;)

Sorry to sound so confusing....

My stupid. I'll go back and edit. Your original message was perfectly clear. I was speed reading and not in the full comprehension mode. Now, if your initials were RD, I'd have known without having to read the message that you were planning to parallel charge. ;)

#9 clearconcepts Dec 20, 2004 11:48 AM

Yeah, I'm being brain washed here by someone... :eek:

#10 RD Blakeslee Dec 20, 2004 01:15 PM

Not me! Not Me!


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