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Building lipo packs
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 Posted: Wed May 15th, 2013 04:21 pm
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mwiz64
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I'll say that in powered R/C airplane applications, I used my Lipos pretty heavily. They are pretty big packs (3S 2100mah up to 10S 5000mah) and they are drained hard. I never exceed the MFGs recommendations however and I always charge them with care. I've never had any problems with balance tap equipped packs (even the cheap ones) and balance chargers. Can light use allow you to cheat the recommendations a little? I'm not prepared to give that advice to anyone.

Maybe a better place to ask that question would be these people.

http://thunderpowerrc.com/

Thunder Power has been a leader in Lipo power for R/C use since the very beginning and they continue to supply the highest quality lipos available to the hobby industry.

I wouldn't take their advice and apply it to lesser brand (HobbyKing) batteries however. All Lipos are not created the same.

Last edited on Wed May 15th, 2013 04:27 pm by mwiz64



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 Posted: Wed May 15th, 2013 05:10 pm
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Robin2
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I found this website http://www.rchelicopterfun.com/rc-lipo-batteries.html and while it doesn't answer my earlier questions it does make sense of the need for balanced charging.

When I build a 2-cell pack for my locos with 12v motors I will include the ability to charge the cells separately at the end of (or before) a train-running session. However during a session I reckon I can top up the charge without balancing - unless I can think of a way to wire the batteries in series and in parallel at the same time.

...R

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 Posted: Wed May 15th, 2013 06:10 pm
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mwiz64
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That was a good explanation Lipos for R/C use. It basically agreed with everything I know about them. Again, I wouldn't build a 2S pack from 2 individual cells. Id rather dissemble a matched pack of 2 cells and reassemble the cells in the arrangement I want/need. Id preserve the wiring of balance tap and use a balancing charger.

Lipo fires are for real. Here is a thread on the subject over at RCG.

http://www.rcgroups.com/forums/showthread.php?t=719116



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 Posted: Thu May 16th, 2013 12:34 am
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bobquincy
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Another thing to look for is a protection circuit.  Some packs and cells have them, most do not.  The protection circuit boards run from about $1 to a few $$$, basic ones just limit the current while fancier ones shut off the cell if the voltage drops too low, protect from overcharging, etc.  I would hesitate to use a LiPo without at least the basic protection, it only takes one short circuit to cause big problems!



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 Posted: Thu May 16th, 2013 07:14 am
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1whudson
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Well heck. I asked one question about lipo batteries because of what a clerk told me. It was answered and I am on to other things in order to get my first RC engine project completed. So guys, enouph with lipo theory, what's new with anyone as far as RCing their engines? You know, what this forum was all about. I've just completed my testing on my Athern SD45. I used an S2 7.4 pack and the engine, even at its size, ran very nicely with smooth stops and starts. The installation is kind of sloppy, but I am getting the hang of it as I go along. I can't praise David's Rx60-2 enouph. I've crossed wires twice now and it still operates perfectly. I'll upload a pic in the next couple of days so you can see how a first timer got it done. If not exactly pretty, it's funtional.

Bill

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 Posted: Thu May 16th, 2013 07:24 am
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Robin2
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Dramatic pictures aren't statistical evidence. For every LiPo fire there must be thousands or hundreds of thousands of users who never have a fire.

When you are trying to squeeze a couple of batteries into a tiny space and need to keep them topped up while in use in order to get a decent run time there isn't space for a protection circuit nor is it electrically possible to do a balanced top-up charge.

We need more information about the circumstances that led to each of the disasters that people are so ready to trot out. Maybe the batteries had been abused without any reasonable precautions? (And the "victims" are unlikely to be frank, if they were to blame).

...R

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 Posted: Thu May 16th, 2013 12:37 pm
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mwiz64
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You are right. Dramatic pictures aren't statistics. They are evidence that fires happen, however. The industry has adopted balance taps and balance charging for a reason. The reason is that too many instances to fires were occurring.

The fires generally happen when out of balance packs are charged and one cell is overcharged to the point of failure. I cant understand why balance charging is such a hassle. You gotta connect the battery to the charger somehow. Heck, my PowerLab 8 won't charge without the balance taps connected. Be that as it may, if you buy a a charger that will, at the minimum you should balance them every couple of charges.

Thunder Power sells a balancing device that is separate from a charger or at least they use to. Look for a TP210 balancer.

Maybe a better solution is to figure out how to make a 1 cell lipo power system work. Then all this is moot.

Last edited on Thu May 16th, 2013 12:37 pm by mwiz64



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 Posted: Thu May 16th, 2013 01:22 pm
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mwiz64
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Another thing to consider with lipo use and this is a failure that I have experienced several times. That's when a cell puffs up do to over discharging. This pretty much ruins the battery and if you have it squeezed into too tight of a spot can cause some other issues. In the beginning you should run your system in a fairly typical maner and time yourself. Then check the voltage occasionally. What you are trying to establish is a typical safe run time. As stated in the article linked above, leave yourself a 20% safety margin for long battery life but at what voltage does that occur? That's actually a pretty tough question to answer. I've seen people that set their LVC at 3.1v per cell and have had no problem. At the low current draw these trains are generating that may be OK. I set mine at 3.4v per cell. At no time do you want to run the battery down to less than 3v per cell.... That is puffed cell range. If you timed for 3.4 and you ended up running it a little harder than your benchmark run you'd have some margin built in.

Never run it if you can visibly notice it slowing way down.



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 Posted: Thu May 16th, 2013 03:15 pm
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Robin2
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If you envisage a locomotive sitting on rails which are connected to a suitable charge voltage you will understand how there is no scope for balance charging.

It would be possible to do a balanced charge when a running session is completed. Then the loco can be removed from the track and the internal wiring can be altered. It's not practical to remove the batteries for charging. Neither is there room in the loco for the usual style of charging plugs. (There wasn't even room for a switch - I had to make one).

The batteries I have are rated at 15C for discharge. The motor I have won't draw more than 4C when stalled and my "charger" won't go above 1C, and in practice will probably be at 0.5C.

Based on my experience with lead-acid batteries I am assuming the real capacity of the LiPos is also only about 50% of the sticker capacity.

I am hoping (though not yet tested) that the batteries will seldom go below 70% of full charge as each loco will spend most of its time on the charging track.

...R

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 Posted: Thu May 16th, 2013 03:49 pm
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W C Greene
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My opinion here...r/c flyers and car racers want to get back in "the action" quickly and therefore tend to push the charge which leads to...you know. I have seen it again and again...why anybody would invest a grand or more in a nice r/c plane or car and then have ONE cheap battery is beyond me. A slow, almost trickle charge is OK for Li Pos and since we are running trains here, you should have maybe more than one loco to use while one is being charged. I don't "cotton" to charging from the rails, it seems to defeat the purpose of true wirelessness and the tiny contact area between rails and wheel tread is not a good way to recharge. But again, it is my opinion here. I have been running wireless for many years now and actually do have some experience with these things. Thank goodness we have this forum to inform ourselves about this "niche". I never discuss r/c trains with the guys at the LHS, the train store guys look at me with a "whaaat?" stare and the r/c store guys believe I am running a toy train around the Christmas tree.
Once again, I am very glad to see this discussion going on, even though it is far above my head/understanding. All I can add now is "KEEP IT UP".

Woodie



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