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Faster discharge? and other things...

  • firethorn Monday, October 28, 2013 7:37 AM Reply
    Never mind that I get a little electric shock everytime I touch the metal chassis....

    I've just discovered that while the max discharge current really is 1A, it is further limited by a max discharge wattage of 5W.

    With my diy "24Ah" (averaged capacity guestimate) 11.1V li ion pack, this would take more than 2 days! Are there any alternatives at about the same price range? Will running the device for this long (in one go) kill it? It's already fairly warm (1 + 1.5 hrs). Haven't discovered yet if the timer is relevant to a discharge either. If it does work for discharges, will the timer pause it or stop it?

    I just want to measure the pack's capacity. If it's too far off the mark, only then will I start matching cells.

    ===== TLDR Begins (continues till the end) ===

    In other news, there's something odd with the power supply. When I first plugged it in, it read close to 0V at the plug, basically "dead" with random spurts of micro power. Turning it off then on had no effect. Removing then replugging the IEC cable seemed to fix it. The IEC was secure in the first place.

    Whenever I touch the casing I get a cute little buzz in my fingertips. Not very nice. Apparently I have moist skin since I get shocks where some others don't. But still...

    First few tries it refused to charge or discharge my pack (various errors but mostly Connection Break errors, which is supposedly caused by the protection circuit. Which is odd since I was trying to discharge a "full" pack). After I accidentally shorted my pack (while disconnected from the charger), it did manage to charge, and then discharge. Oddly, the 1st or last battery (right most) read 1.xV after the accidental short (on the charger) while the rest read at 4.1xV-ish. Only then would it charge. And the charge took only a half hour or so. Started immediately in CV mode (presumably) at about 0.5A.

    I'm using the 3 cell 18650 holders (with high resistance according to the forums), 4pin jst-xh cables and connectors for charging and connection to the individual packs, all from dx. Veroboard from a local shop with unhealthily thick tracks of lead. Banana plugs to alligator clips to one half of a 2 part terminal block (from dx), followed by 2x 22(?)AWG per terminal which is then connected to the lead tracks... for the "primary charging leads".

    Cells are an assortment of dx 18650's, mostly because I have no idea which ones are truly reliable in terms of stated capacity. But all are new (purchased very recently), bought in packs of 3 pairs, assembled as 3s2p (grouped via brand/model and batch(purchase date)).

    Although the wires that came with the jst-xh connectors are thin, I really only need about 2.65As and spread out over so many packs, this is quite safe (unless you short it). I also need it to last 12 hours which is why I bought so many different types of batteries.

    As a side note, anyone reading this who is thinking of building their own pack should try their very best to avoid protected cells. Causes a lot of problems which are physically difficult to rectify (you need to disconnect the battery, and then the individual cells, to reset the protection - this is the case for some of the cells). Build the protection (or buy it) and then add it externally, instead of per cell. It's cheaper too!

    This is a whole lot of text for such a simple question. Unrelated text I should add. But I'm just throwing it out for there for comments.. just in case. desolder? :P
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  • desolder Top 10 Forum Poster Monday, October 28, 2013 11:06 AM Reply
    Are there any alternatives at about the same price range?

    I don't know of any in the same price range, but if you have more to spend you can upgrade to the IMAX B8, which doesn't have the 5W discharge limit. AFAIK, it can discharge at 5 amps regardless of wattage.

    Removing then replugging the IEC cable seemed to fix it. The IEC was secure in the first place.

    I was also having problems with the supplied IEC power cord. It's junk. Mine was getting warm with use (scary!), and causing the power supply to randomly shut down. I checked the cable's resistance with my ohmmeter and the cable had dozens of ohms! Into the trash bin it went. I replaced it with a known good one and that solved the random power problems.

    It seems odd that they would use an IEC 3 conductor socket on the power supply, but the supplied cord is only 2 conductors. Try using a grounded cable and see if your shock problem goes away. If you're still having shock problems, then junk the power supply and use another one. There's been several complaints in other posts about the power supply being flaky, so it's probably best to toss it anyway.

    I'm using the 3 cell 18650 holders

    They're not perfect. They have high resistance, as you noted. But that can be fixed. The primary culprit seems to be the riveted construction, which tends to loosen over time. Drill/cut out the rivets and use some nuts, bolts, and washers instead.

    As a side note, anyone reading this who is thinking of building their own pack should try their very best to avoid protected cells.
    Agreed. And to this end, DX *finally* has 3S and 4S protection PCBs: sku.255211 and sku.255209. They still don't have 2S protection PCBs yet.

    What's up with the scrubbed LED driver IC markings?
    What are they trying to hide?

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  • firethorn Tuesday, October 29, 2013 5:18 AM Reply
    Thanks for replying.

    There's a boatload of good info there. Including, the... *gasp* protection circuits! Very tempted to switch out my entire pack!

    Will try out all the suggestions. If anyone else is reading this, if I don't reply, assume it worked.
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  • NightRider Tuesday, November 5, 2013 7:38 PM Reply
    I think you'd be much better off with batteries with solder tags instead of using battery holders. A good (and even free) source would be used laptop batteries. Just filter out the bad cells with this charger's discharge function. I'd recommend using an external protection circuit such as mentioned above (check that it meets your current requirements).
    Of course, be careful with shorts because shorting parallel Li-Ions can give very large currents. Also, Li-Ions are known to catch fire if badly abused, so just use common sense and safety measures.
    Good luck.
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    post edited by NightRider on 11/5/2013 at 8:40 PM
  • firethorn Monday, December 2, 2013 11:33 PM Reply
    Thanks NightRider.

    However, at the time, at even now, with past difficulty in soldering tabs on cells, and the ability to "fix a cell" easily, creating a "proper pack" is not a real option for me. Still on the table though, just at the far edge.

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  • desolder Top 10 Forum Poster Tuesday, December 3, 2013 11:49 AM Reply
    I think NightRider meant to suggest using "tabbed" cells, cells which have small metal strips spot-welded onto them. You can much more easily solder onto the spot-welded tab, rather than soldering directly to the cell itself. Laptop cells are all spot-welded with tabs/strips, so that is why he suggested that source. DX also sells one set of tabbed cells: sku.91269. However, the reviews for these are quite poor, with reports of high self discharge and low capacity, so stay away from them. Another option is to go with another vendor (sorry can't link here due to rules) who will add spot-welded tabs for you, for a small fee (about USD $0.25 per cell).
    What's up with the scrubbed LED driver IC markings?
    What are they trying to hide?

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  • firethorn Tuesday, December 3, 2013 6:27 PM Reply
    ahh.. pretabbed... possible..
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  • firethorn Tuesday, December 3, 2013 7:04 PM Reply
    I wonder if I can find pretabbed LiFePO4 cells on dx.. hmm..
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  • NightRider Tuesday, December 3, 2013 10:01 PM Reply
    Indeed, I meant pre-tabbed cells and it's very easy to solder to the tabs away from the heat-sinking parts of the cells. Actually I use the cells in pairs or triples as they come out of the pack connected with thin strips of metal, and cut off any tab protruding out of the cell's circumference for safety reasons (so they don't short out to another cell while lying in the box waiting to be tested and assembled somewhere) as well as for assembly practicality - the tabs interfere and might short out when I group cells/packs together to build a pack. I solder wires to the middle of the metal strip that connects between adjacent cells which is floating in the air and thus doesn't cool the soldering iron when I solder to it as would soldering on the cell itself near the spot welding, and it's also safer since I don't heat the cell at all.
    Also, bear in mind that Lithium cells from laptops are usually very high quality cells (Sony, Panasonic) and if they're not dead (most of them aren't) have very good capacity. Actually I use them in parallel so need no complex protection circuits and don't even care about capacity matching as necessary with series connected cells - in parallel connection, all the cells "march together" (cover up for each other) during discharge since they're all at the same voltage so even if there's a weaker cell, it's weakness is covered up by the stronger cells, though you should also filter out the really weak cells with your B6 charger in the first place. The 5W limit is no problem since 4.2V x 1A = 4.2W max (as the cells discharge the power goes even lower) is less than the limit. Sure it takes time but it's the B6 that's working, not you...:D (you could be doing other stuff in the meantime, such as going out to breath fresh air or jogging...when you come back, the results are waiting on the B6 display)
    Again, reminding you to be careful not to short out the cells without protection...

    For short higher discharge currents (for testing cell's instantaneous high current capabilities) I use my burnt-out car headlamp bulbs (usually just the dipped filament breaks while the high beam filament is left intact and consumes 60W @ 12V = 5A per lamp, for higher currents I parallel them) while carefully monitoring the voltage during discharge (batteries with voltmeter attached within constant view range). This works usually only for unprotected cells since the initial surge current for the filament is very high but stabilizes quickly to the stated value. A more stable current option would be any high power 12V resistive load such as a 12 heater element that has more stable resistance (for those cases I have a 12V 150W "electric spoon" made to boil water in a cup which I found in a car accessories shop, used inside a bucket to avoid boiling the water).
    You could adapt this for capacity measurement by using a Li-Ion monitoring circuit such as sku 35609 which would warn you (VERY LOUDLY I might add...:D) when your cells are reaching full discharge so you would disconnect them to avoid damaging them. It's even perfect for your 3S configuration. Since this sku is out of stock, search for a similar device here or elsewhere.
    Good luck! ( and watch out from shorts...)
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    post edited by NightRider on 12/3/2013 at 10:45 PM
  • firethorn Tuesday, December 3, 2013 10:53 PM Reply
    you could be doing other stuff in the meantime, such as going out to breath fresh air or jogging

    and... Thank you for your post. Very informative. :D
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