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Building lipo packs
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 Posted: Tue May 14th, 2013 04:29 pm
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1whudson
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Hi All

I was told by the sales clerk at my local hobby shop that it wasn't safe or a good idea to build my own battery pack from assorted lipo batteries, e.g. three one cell lipo's to make a 11.1v battery pack. Is he right? Are we limited to buying rather than making?

Bill

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 Posted: Tue May 14th, 2013 05:11 pm
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Robin2
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I am interested in this question also, as I have been planning to join 2 cells to make 7.2v packs.

There seem to be plenty of articles on the Web about how to do it - which doesn't prove anything.

I hope that if the batteries are neither violently discharged or charged (both of which would be normal with model aircraft) there shouldn't be any problems. I don't plan to leave my batteries unattended while charging.

...R

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 Posted: Tue May 14th, 2013 08:43 pm
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mwiz64
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Here is the deal with building your own packs versus buying them. Factory made packs are made of matched cells. Why is that important? Because you want your cells to discharge at the same rate. If one has a slightly greater internal resistance than the other the pack (cells) gets out of balance quickly. Besides a degradation of performance, packs that are out of balance are more likely to fail. Lipos batteries that fail can cause fires.

If the reason you want to build your own pack is to get a custom shape consider buying a premade pack and disassembling the pack and re assembling in the new shape.

MOST IMPORTANT! Never charge a multi celled LiPo battery without a balance tap and a balancing charger. This is generally where out of balance packs cause fire... during charging.



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Mike
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 Posted: Tue May 14th, 2013 10:13 pm
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1whudson
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Your thoughts are right when it comes to my reasoning. I want a configuration that will compliment the little space I have in a given engine model. I certainly don't have the knowledge that should be required to take one apart and rerap it to my liking. With that it mind, can anyone recommend a company that makes lipo's by the millions so that I am more likely to find a shape/power that meets my needs? In my case, my needs are to run a 12v motor in an HO engine for 2 to 3 hours before needing a charge. If that can be done in a one cell lipo, even greater! I did a test today with David's Rx60-2 board and it operated the engine fine, but that was with a s2 lipo at 7.1v. I getting ready to try the same engine with a 1 cell lip at 3.4v just to see if its possible.

Bill

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 Posted: Wed May 15th, 2013 02:58 am
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mwiz64
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Bill,

I have a friend that buys 2s lipo packs from Hobby King. He disassembles them to make 2 separate 1 cell packs. I personally have taken a pack or two apart as well. The Hobby King packs come with steel tabs where the wires are soldered. Just carefully liberate the cells from the shrink wrap. Then cut the wires and splice in a new piece. Are you competent at soldering wires? That's all you need.... And the smaller Hobby King packs are dirt cheap... Like a couple of bucks each.

Check it out...

http://www.hobbyking.com/hobbyking/store/uh_listCategoriesAndProducts.asp?idCategory=86&LiPoConfig=2&sortlist=&CatSortOrder=desc



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Mike
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 Posted: Wed May 15th, 2013 03:04 am
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mwiz64
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Here is another possibility. If you want to step a 1cell lipo up to 5v you can use one of these. Guys use these in tiny RC planes to get 5v power to their receiver and servos. Should work fine in this application too.

http://www.dimensionengineering.com/products/lvboost

Last edited on Wed May 15th, 2013 03:09 am by mwiz64



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Mike
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 Posted: Wed May 15th, 2013 03:08 am
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mwiz64
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If you decide to go the 1 cell route, these are an interesting shape and size.

http://www.hobbyking.com/hobbyking/store/__582__85__Batteries_Accessories-Lipoly_Round_Cells.html

I've used the 1200 and 900 and 650 myself in my small gliders. I have to believe a 1200 on a 5v inverter like the one from Dimension Engineering that I posted above would provide several hours of opperation on an HO locomotive.... But I've never tried it myself.



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Mike
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 Posted: Wed May 15th, 2013 07:31 am
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Robin2
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I'm struggling with this ...

I don't understand how two batteries wired in series can discharge at different rates. Surely if a motor is drawing 200mA at 7.2v the current must flow equally through / from both batteries?

With Lead-Acid batteries (for example) there would be nothing wrong with wiring a 6v battery and a 12v battery in series to get 18v.

I can easily see how batteries wired in parallel could discharge unevenly.

...R

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 Posted: Wed May 15th, 2013 11:44 am
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mwiz64
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One battery has a higher internal resistance than the other. (Think of that as a partly clogged pipe....friction in the line.) Because of this it charges a little slower, it discharges a little slower, it has a little less capacity, its voltage sags a little more under load, it also gets a little warmer when charging and discharging. Why? I don't know. I'm not a battery engineer. Why can lead cells handle this mismatch while lipos cannot? Again, I don't know. What I do know from my many years of using lipos and reading about them is that what I'm telling you is true. In this case we are not talking about large numbers of difference. They are very small. Remember when discharging a lipo it goes from full charge at 4.2v per cell to basically dead at 3.2v per cell. One lousy volt. Suffice it to say this battery chemistry is a sensitive one and lead must not be...or Nicd or even regular old Alkaline batteries.

In the beginning of lipo use in RC this issue with lipos wasn't readily understood, even by the battery pack manufacturers. We didn't have balance taps on our multi cell battery packs. "We never needed them with any other batteries so why would we need them here?" Unbeknownst to us that was a problem of epic proportions. We ended up having some users that had some catastrophic fires in their homes and cars. Since that time we've learned. Now we have balance taps on all of our multi cell lipo packs and we have computer controlled balancing chargers to make sure the cells stay in balance through the whole charge or they shut down. These chargers also bring out of balanced cells back into balance... to the best of their ability. This has brought the safety of lipo use up to an acceptable level.

I know some people here dont balance charge their lipos. They just charge them slowly and figure they are OK. I hope it never costs them too much but to me they are just tempting fate.

Last edited on Wed May 15th, 2013 02:32 pm by mwiz64



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Mike
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 Posted: Wed May 15th, 2013 04:10 pm
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Robin2
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Mike, I don't doubt for a moment that your advice is safe and sensible.

However it's also inconvenient. And it's potentially expensive - if an alternative approach is also sufficiently safe.

I wonder how often it is necessary to check the balance of a pair of cells in series? And if after several checks they are always found to be in balance would it be appropriate to extend the time between checks?

Does the risk of imbalance reduce if the batteries are not discharged fully between recharging?

Is the risk of imbalance reduced if the battery is discharged gently - say 30% or less of its max rate?

...R

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