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Model Forum / Radio Controlled / Air Models / April 2004Li Poly, 2 packs, differing voltages
Hi, I have 2 packs of 3S3P 11.1 V, 1500 mAh Kokam Li Poly batteries. My first pack has the individual cells discharged with a Triton charger before initial charge. This pack reads at 12.44 Volts after charging. The second pack was not discharged before charging. The cells were charged individually, and subsequently, the cells read; 11.9 V, 11.55 V and 11.55 V. After they were connected in parallel, the pack read 11.1 V. As you can see, for the second pack, there was a drop in voltage after sitting overnight in parallel. And the difference between the first pack and second pack is about 1.34 V. Would anyone tell me why the difference? Also would discharging in flight allow a better charge to a higher voltage? Thanking you first, Wan Why would you discghare LiPoly cells? I thought you shouldn't. (left mine plugged in and it's trash now) mk > Hi, > [quoted text clipped - 18 lines] > Thanking you first, > Wan > Why would you discghare LiPoly cells? I thought you shouldn't. > (left mine plugged in and it's trash now) > mk A friend of mine discharged the first 2 batteries for me before charging them, though I was told in other posts by many persons I need not do so. However, it was done. Still, why is the difference between the first pack and the second pack? Wan > Hi, > [quoted text clipped - 15 lines] > Would anyone tell me why the difference? Also would discharging in > flight allow a better charge to a higher voltage? Wan- From your description, you didn't charge the cells individually, but rather charged series packs of three. You aren't really going to get an answer to your question until you measure the voltage across each cell. Cell voltage should be very close between cells, say within a couple of hundredths of a volt. If not, the pack is unbalanced. Charge the cells individually to balance the pack (dunno if Kokam provides taps between cells to facilitate this - if not you'll have to provide them). Abel > > Hi, > > [quoted text clipped - 27 lines] > > Abel Abel, Unfortunately, these Kokam batteries came fully assembled in series without any tabs for the individual cell. You are correct in that I charged them in series of three. But that's the way my friends do it. I hate thinking about taking them apart to provide tabs. They were so well crafted from the factory. After resting for several days, the difference in voltage is now about .7 V between the two packs. How does the .7 V difference reconcile with the need to get the cells balanced to within a couple of hundredths of a volt per cell? Thanks, Wan > Unfortunately, these Kokam batteries came fully assembled in series > without any tabs for the individual cell. You are correct in that I [quoted text clipped - 7 lines] > with the need to get the cells balanced to within a couple of > hundredths of a volt per cell? Wan- I'm a little surprised that your Kokam packs don't provide access to individual cells, as I presume Fred Marks was the dealer. Here's a quote attributed to him re LiPo safety: "Before you charge a new Lithium pack, check the voltage of each cell individually. I do this after every tenth cycle there after. This is absolutely critical in that an unbalanced pack can explode while charging even if the correct cell count is chosen. If the cells are not within 0.1 volts of each other then charge each cell individually to 4.2 volts so that they are all equal. If after every discharge the pack is unbalanced you have a faulty cell and that pack must be replaced." See the complete discussion at http://www.rcgroups.com/forums/showthread.php?t=209187 Much more authoritative info there than I can provide - my personal LiPo experience is limited to a 2-cell pack in a 6 oz foamie, and a digital camera. Abel > > Unfortunately, these Kokam batteries came fully assembled in series > > without any tabs for the individual cell. You are correct in that I [quoted text clipped - 30 lines] > > Abel Abel, This is the web site of FMA Direct where I purchased my Kokam batteries. As I said, the packs didn't come with tabs for individual cells. See if you could tell from this web site. https://www.fmadirect.com/site/home.htm I hope it can come through. Is Jim Mcpherson associated with FMA Direct? Thanks for sending the discussion site, After reading and seeing how Li Poly batteries could cause damage, I wondered how modelers survived? Anyway, out of curiosity, I charged the same 3S3P Kokam battery pack again and it came up with a new reading of 12.64 V. I can't explain it, since it was quite different the first time. Wan > > > Unfortunately, these Kokam batteries came fully assembled in series > > > without any tabs for the individual cell. You are correct in that I [quoted text clipped - 52 lines] > > Wan Sorry for coming back right after my last post, but here is another site that offers Kokam batteries with tabs for testing individual cells; http://www.lightflightrc.com/indexHTML.htm I could've kicked myself for not doing more research. For a few dollars more I can have the better battery! "Too late for crying over spilt milk" Fred Marks is the one I should've asked if he was associated with FMA Direct or with LightFlightRC? Wan <quoted text snipped> >Sorry for coming back right after my last post, but here is another >site that offers Kokam batteries with tabs for testing individual [quoted text clipped - 8 lines] >Fred Marks is the one I should've asked if he was associated with FMA >Direct or with LightFlightRC? Wan- Don't kick yourself, just fix it. I know you can, as I have had a first hand look at your workmanship. Fred Marks is the 'FM' in FMA Direct. Abel > <quoted text snipped> > > [quoted text clipped - 19 lines] > > Abel Thanks for the compliment, Abel. I didn't know FM means Fred Marks. In that case, since he is a proponent for testing each cell once in a while, why did the batteries I purchased from FMA Direct come with tabs for the indivdual cells, as I found in the other web site (LightFlight RC)? I didn't mean anything by it, just being curious. Anyway, I replaced my ESC's with the new programmable ones that senses the cut off point for batteries and allow time to come down before harm to the batteries. Progress! Isn't it great? Wan >>>Unfortunately, these Kokam batteries came fully assembled in series >>>without any tabs for the individual cell. You are correct in that I [quoted text clipped - 52 lines] > > Wan The cahrger should finish charging at somethwere near 12.60v. Thats the resting voltage of a fully charged 3s pack. When fully discharged the pack should still show at LEAST 9v, prpbably nearer ten. Packs are normally shipped half charged - somehing like 11v -11.5 volts or so for 3S. > >>>Unfortunately, these Kokam batteries came fully assembled in series > >>>without any tabs for the individual cell. You are correct in that I [quoted text clipped - 62 lines] > Packs are normally shipped half charged - somehing like 11v -11.5 volts > or so for 3S. Thank you TNP, that helps a lot. However I don't know why my post has not appeared after more than 12 hours. I will copy and paste it here again; Abel, Sorry for coming back right after my last post, but here is another site that offers Kokam batteries with tabs for testing individual cells; http://www.lightflightrc.com/indexHTML.htm I could've kicked myself for not doing more research. For a few dollars more I can have the better battery! "Too late for crying over spilt milk" Fred Marks is the one I should've asked if he was associated with FMA Direct or with LightFlightRC? Wan Wan, > However I don't know why my post has not appeared after more than 12 > hours. Probably due to some glitch between your ISP's servers & your PC. It _DID_ appear on..... > Subject: Re: Li Poly, 2 packs, differing voltages > Date: 24 Apr 2004 00:54:32 -0700 -- Jim L. https://sourceforge.net/projects/virtual-access/ Using - Virtual Access(OLR), ZAP 4.5, & WinXP Pro w/SP1 > Wan, > [quoted text clipped - 13 lines] > https://sourceforge.net/projects/virtual-access/ > Using - Virtual Access(OLR), ZAP 4.5, & WinXP Pro w/SP1 Jim, The post that was on, Date: 24 Apr 2004 00:54:32 -0700, refers to the FMA Direct web site. Whereas my recent post on, Date; 24 Apr 2004 18:26:08, refers to the LightFlight web site. Thanks for your commentary, but I am confused. Perhaps it WAS my ISP that caused the problem? I know we're getting off topic. Still, I'm trying to deal with the delay, 3 to 9 hours is long enough already. Wan wanjung@toast.net (Wan) posted message ID<7387603e.0404250450.62506d31@posting.google.com>on 25 Apr 2004 05:50:55 -0700 >> Wan, >> [quoted text clipped - 27 lines] > >Wan It's simple. You're posting through Google. 12 hour delays are normal. If you want real-time, you need to post through your ISP or an independant news service. |
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