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Thread: 18650 battery packs

  1. #1
    Enlightened
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    Default 18650 battery packs

    Firstly, if there's been a thread on this before, I apologise. I can't find any good information on the subject anywhere on CPF.

    I'm planning an LED bike light build for long distance riding and I want to run it off an eight cell 14.8v (4s2p?) 18650 Lithium Ion battery pack. Maybe even something larger for better run time. The ones I've seen appear to be ready built out of unprotected cells, but with a protection circuit board built in.

    (Incidentally, if anyone knows of a good UK supplier of this sort of stuff, please let me know!)

    From what I can tell, most Lumicycle brand batteries and various other modern proprietary LED bike light systems use packs made up of 18650 cells in this way. They come with an intelligent charger and you just plug in one cable and off it goes.

    Now I've read the do's and don'ts about 18650 Lithium cells and I'm well aware that if you string a load of them together in this way, they can get unbalanced and eventually you'll end up with one or more cells dropping in voltage with the others going up to cmopensate. Best case scenario; your battery's cronked in 20 charge cycles. Worst case, things start to go boom-boom, right?

    I know the RC people who use packs like this run balancing chargers and tap into each cell to balance them individually.

    So when I look at a pack like this from Battery Junction, being sold with just one lead to charge it an no way of balancing it, do I just plug it into a charger like this and hope it'll work?

    Or do I pull the pack apart and solder in wires for a balancing charger?

    How the heck do I make the best job of this, whilst prolonging the life of my batteries and not setting fire to myself in the process?

    Your help would be hugely appreciated!

  2. #2
    Flashaholic
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    Default Re: 18650 battery packs

    Hi Charlotte,
    Yes you can buy the battery pack and charger and simply plug them in to each other. The pack has a pcb built in to it but it doesn't specifically mention whether it provides a balancing function so it's worth dropping them an email to clarify. If the pcb doesn't provide balancing I wouldn't bother ripping it apart and wiring in balance taps - just find a pack that does what you want.

    http://www.batteryspace.com/index.as...OD&ProdID=4459

    That pack might need a socket soldering onto it too. Their advertised postage charges are an absolute rip-off tho, but if you ask them they will ship via USPS mail for around $20.
    I tried to find a UK source for Li-ion packs but in the end I gave up and bought some loose 18650's off ebay, and a seperate pcb and built it up myself.

  3. #3

    Default Re: 18650 battery packs

    Steve6690, the pack you suggested: http://www.batteryspace.com/index.as...OD&ProdID=4459 only balances to +- 0.2V. To me, I would still consider that somewhat unbalanced. Do you know if the PCB you bought balances to a tighter tolerance?

    Semiman

  4. #4
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    Default Re: 18650 battery packs

    I think Wildwood on here does Packs in the UK.
    Smudge on STW also does packs and chargers in the UK -Singletrackworld

  5. #5
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    Default Re: 18650 battery packs

    Quote Originally Posted by SemiMan View Post
    Steve6690, the pack you suggested: http://www.batteryspace.com/index.as...OD&ProdID=4459 only balances to +- 0.2V. To me, I would still consider that somewhat unbalanced. Do you know if the PCB you bought balances to a tighter tolerance?

    Semiman
    This is the one I bought :

    http://cgi.ebay.com/ws/eBayISAPI.dll...m=330277825676

    0.2v seems a lot to me too. After charging I let my pack rest for 30 minutes and when I last tested the cell voltages I found differences of no more than 0.02volts.

  6. #6
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    Default Re: 18650 battery packs

    Quote Originally Posted by steve6690 View Post
    I tried to find a UK source for Li-ion packs but in the end I gave up and bought some loose 18650's off ebay, and a seperate pcb and built it up myself.
    Hmmm - I'm tempted to do that, I really am. That way, I can put them together in a physical configuration that is easiest to strap onto the bike's top tube. What worries me is how it's done. Do you just solder all eight of them together as 4s2p and then whack the PCB in front, between the batteries and the fly lead?

    Or is there some cleverness going on inside the commercial packs that balances each individual cell through the protection PCB?

  7. #7
    *Flashaholic* LuxLuthor's Avatar
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    Default Re: 18650 battery packs

    Take your time and keep reading. Most Li-Ion cells out there are Lithium Cobalt chemistry which absolutely should not be soldered. The best packs are ones that have separate balance tap leads to keep cells balanced closely with each other.

    Second best is to use a PCB circuit like above, and try to get cells that have tabs welded on them, then solder to the tabs.

    Third best is to use a safe Lithium chemistry (see pinned topic thread at top of this section) cell which eliminates the fire danger, but at reduced storage capacity.

  8. #8
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    Default Re: 18650 battery packs

    This type of question comes up all the time, and you are one of the only people I have ever responded to who was even responsive/aware to the concept that these packs need to be balanced properly. You have obviously already put a ton of research into this and I can tell that just by reading your question here that you are probably further along that you think you are on the "curve" of understanding everything you need to know..

    You are right, that any LiCo pack that doesn't properly isolate each cell (or groupings of parallel cells) and properly balance it, will just result in an out of balance pack after a few dozen cycles that will probably lead to a lot of damaged cells, and *could* lead to an unwanted explosion.

    The solution to that "ideal" "holy-grail" of LiCo pack configurations is semi-complicated, but I think you'll do fine...

    As LuxLuthor said, please don't solder directly on LiCo cells, they don't like heat, ideally, having them welded is best, but you could get by with tabs, and only soldering on tabs instead.

    Any grouping of cells that is in parallel, can be treated as a single large cell. Taking your example, a 4S2P pack, it's basically a "4 cell" pack. The pairs of cells that are in parallel are like "3.7V 4400mAH" "cells." The balance taps just need to be there for each point in-between these parallel groupings...

    So, for a 4S2P pack, the charging jack would be 5 pins. One on each "end" of the pack, and 3 balance "taps."

    My suggestion is to look for a good hobby style charger that has numerous pack size charging capability for LiCo/LiPo cells. Many of the good hobby chargers are setup with "plugs" for this type of balanced pack charging...

    I've never actually done all this myself, so I hope that's enough to get you moving along a little further. A "serious" bike light with a properly built Li-Ion pack would be the envy of most riders Best of luck with your venture into this project, it may be pricey in the end, but probably well worth it....

    Eric

    PS: use quality name brand cells if possible for a pack build, if you can get ahold of LG or Sony or Panasonic or something like that. that would be best. Cheap cells are not always worth it.

  9. #9
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    Default Re: 18650 battery packs

    Quote Originally Posted by Charlotte View Post
    Hmmm - I'm tempted to do that, I really am. That way, I can put them together in a physical configuration that is easiest to strap onto the bike's top tube. What worries me is how it's done. Do you just solder all eight of them together as 4s2p and then whack the PCB in front, between the batteries and the fly lead?

    Or is there some cleverness going on inside the commercial packs that balances each individual cell through the protection PCB?
    I bought 8 x 18650's of 2400 mah capacity with solder tabs. I arranged them into 4 pairs and soldered them together in parallel which then gave me (effectively) 4 x 3.7V, 4800mah batteries.
    The next step was to connect each of these 2-cell batteries in series. If I'd just soldered them end-to-end my pack would have been too long, so I ended up with a folded-back design. This all sounds complicated but it really isn't. I wish I'd taken photos of the various stages.
    Originally I intended to use the led driver (Maxflex) to provide the low voltage cutoff and prevent over discharging of the cells. So I wired in balance taps to a jst plug and bought a balancing charger from DX :

    http://www.dealextreme.com/details.dx/sku.12618

    It all worked fine but I wasn't really happy using the Maxflex as my over-discharge protection.
    Then I decided to change the driver to a blue Shark which doesn't have any voltage cutoff facility. So I bought a protection pcb, cut off the jst-xh plug, soldered the balancing taps to the pcb terminals and wrapped the whole lot in two layers of heatshrink, one under the pcb and one over it.
    It was all a bit of a faff to be honest, and I wouldn't recommend it just to save some money as you can buy a similar pack for £60 + postage. But it was an interesting little project and I now have a pack that will power 7 x Cree Q5's at 1A for around 2.5 hours.
    When soldering the battery tabs you need a soldering iron of reasonable power. Mine is 35w and I'd have preferred something with a bit more oomph. The reason being, you want to get the tab up to temp very quickly so you can have the iron in contact with the tab for as short a time as possible. The longer you have the iron touching the tab the more heat will travel along it into the battery. Heat + Li-ion batteries = not good.
    I basically copied this design :

    http://cgi.ebay.co.uk/Li-Ion-Battery...742.m153.l1262

    Scroll down the item page and you'll see where the pcb can be placed.
    I couldn't get any real answers from that Ebay seller when I asked about the protection pcb he was using so I bought the loose batteries from him anyway, and the pcb I bought was this one :

    http://www.all-battery.com/protectio...k12alimit.aspx

    Buying from two different US sources meant paying unnecessary postage charges but the total cost was £58 which I was happy with.
    Last edited by steve6690; 12-05-2008 at 08:10 AM.

  10. #10
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    Default Re: 18650 battery packs

    Hello Charlotte,

    Balancing the cells in a Li-Ion battery pack is the best way to insure no problems during charging...

    However, tests have been done that show that if high quality cells (Sanyo, Sony, LG, and Panasonic just to name a few) are used, if they are properly charged (at 0.7C to a maximum voltage of 4.200 volts), if the discharged is limited to 3.0 - 3.3 volts under load, if the maximum current is limited to 1C, and if you take the time to match the cells on voltage under load so all the cells in the pack are holding the same voltage, you can expect the cells in the pack to pretty much stay in balance over 250 - 300 charge/discharge cycles. After that, the pack will gradually become more and more unbalanced and eventually you will run into problems during charging.

    The RC people have noticed that balancing is not required during each charge, if you treat your battery pack properly, but it is cheap insurance and keeps houses and garages from burning down.

    Safety concerns suggest balancing during charging, however, under optimum conditions the pack should stay balanced for a couple of hundred cycles.

    I should add that this study was done when Li-Ion cells were only lasting about 3 years and then they would die even if they were not used. I am not aware of any recent studies done using current cells.

    Tom
    Behind every Great man there's always a woman rolling her eyes...

    Most batteries don't die - they are tortured to near death, then murdered...

  11. #11
    Flashaholic* Packhorse's Avatar
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    Default Re: 18650 battery packs

    Is it OK to use protected cells to make packs or would they still need a separate protection PCB?

  12. #12
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    Default Re: 18650 battery packs

    I doubt the pack would balance properly using protected cells with no balancing feature. I used unprotected cells in my pack, but added the protection back in by using the seperate pcb.
    When I was looking for 18650's I didn't come across any protected cells that had solder tabs.

  13. #13
    *Flashaholic* mdocod's Avatar
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    Default Re: 18650 battery packs

    Quote Originally Posted by Packhorse View Post
    Is it OK to use protected cells to make packs or would they still need a separate protection PCB?
    For pack building, you can get more cell for your money by using quality name brand unprotected cells, and protecting the pack as a whole with a proper external PCB. Unprotected 18650s will also have lower resistance, and are available in higher capacities.

  14. #14
    *Flashaholic* LuxLuthor's Avatar
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    Default Re: 18650 battery packs

    If you have room for the protection & balancing PCB in your intended device, as others have said, quality unprotected cells are the way to go. I have made packs with AW's protected cells for myself using my battery pack welder, nickel contact strips, and with balance tap wires coming down through middle of pack into a pigtail with JST connector that fits inside tailcap spring of D Mag...because I wanted to use smaller size cells, and only AW had the right sized ones.

    The problem with most MagD tube setups is there is not enough extra room for a PCB in the pack. Another way to "skin the cat" (just occurred to me what a grusome expression that is--never thought about why someone would want to skin a cat until just now....yuck! ) is to use a voltage display LED, like these handy little puppies.
    Last edited by LuxLuthor; 12-05-2008 at 08:51 PM.

  15. #15
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    Default Re: 18650 battery packs

    Charlotte,

    Easiest route I see is to rip appart the Makita LXT 3Ah pack.
    Don't remove ANY of their welded tabs.
    Use a pair of cutter and separate them so you'll get 2x 4S and 1x 2S pack.
    Then just parallel the each set of cells (5 wires) so that it improve the reliability.
    While you're at it install some JST ballance tap at the same solder joint.

    If you don't have the time, just ask your local RC if they can build one for you. Here in the US this would be < $70/pack.

    No need to monitor / display the voltage. Just pick up the george80's bFlex driver. It have a low voltage warning where you can configure for your situation (12V).

    With 3 Cree XRE and a little imagination and time, you can build yourself a TK 500 lumen whooping bike light for < $200. Comparable in feature with Lupine

    Good luck,
    Jonathan

  16. #16

    Default Re: 18650 battery packs

    I would like build a 20x 18650 battery pack, do you have a trick for me?
    Is more good 3.7V or 7.4V configuration (I will use Q5 and P7 LED)?


    Thanks in advance

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