18650 charging question- Mtbr.com
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  1. #1
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    18650 charging question

    I've been doing some reading, and have come to the realisation that the new Samsung 30A 18650 cells I've bought recently are not being fully charged by the chargers I'm using. These are all geared towards max voltage of 4.2v per cell (8.4v), but a full charge on these new cells requires a voltage of 4.35v per cell (8.7v). This is according to the Samsung data:

    http://www.samsungsdi.com/battery/cy...185650-30A.jsp

    From my on-line reading, these lithium chargers are all alleged to have some sort of charging "algorythms". My question is if I have a properly regulated power supply set to 800-1000 mah, and 8.7v, and I charge the batteries to a max of 8.7v, will this offer a proper charge, or does the charge voltage/rate need to otherwise be altered during the charging process?

  2. #2
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    Yes, there is a new crop of higher capacity 18650 cells from a couple of manufacturers that achieve that capacity by a higher upper voltage limit. Yes, you can charge these cell from a regulated power supply as long as the supply has both current and voltage regulation and the settings you listed look good.

    I noticed in the datasheet you posted that the discharge current is limited to 0.2C which is 600ma. That's pretty low for most bike light applications. Discharge current limit is one of the specs that isn't good to push.

    What's also difficult with these cells is getting a protection PCB for them. You need one with the different upper voltage protection limit and discharge current limit. Otherwise, the protection pcb will keep you from getting to full charge.

  3. #3
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    Quote Originally Posted by MtbMacgyver
    Yes, there is a new crop of higher capacity 18650 cells from a couple of manufacturers that achieve that capacity by a higher upper voltage limit. Yes, you can charge these cell from a regulated power supply as long as the supply has both current and voltage regulation and the settings you listed look good.

    I noticed in the datasheet you posted that the discharge current is limited to 0.2C which is 600ma. That's pretty low for most bike light applications. Discharge current limit is one of the specs that isn't good to push.

    What's also difficult with these cells is getting a protection PCB for them. You need one with the different upper voltage protection limit and discharge current limit. Otherwise, the protection pcb will keep you from getting to full charge.
    Actually, looking at the datasheet a little more, I don't think that's max discharge current. I think that's the discharge current used to get a 3000mah capacity. They don't seem to state max discharge current. You'll get less capacity at higher discharge currents.

  4. #4
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    Thanks for the input. The PCB I'm using with these packs is this one:

    http://www.all-battery.com/pcbfor74v...b74v32004.aspx

    I didn't realise till early today that it actually has an overcharge cut off voltage of 4.35v per cell.

    The only issue with that PCB is the discharge voltage cut off. It's a little on the low side at 2.3v.

    Now you have me a bit worried about the discharge current. I don't really understand why the max charge current is 2 1/2 times higher than the discharge current. Maybe there's another number missing from the data?

    Here's an older data sheet that looks like the initial (and maybe unofficial) release of data. It shows a max discharge current of 2C (6,000mAh):

    http://www.euroled.it/files/samsung_...0-30a_spec.pdf

  5. #5
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    That protection PCB probably isn't going to work well for an 8.7v voltage setting on the charger. The PCB upper limit typically has to be at least 0.1v higher than the charger upper limit. That's because you can't assume the cell balance will be exactly perfect. When the charge cycle gets to the constant voltage part, the PCB will probably stop the charge prematurely. That's because even if the pack is nearly perfectly balanced, which is not possible, one cell may be 4.34v and the other 4.36v and the PCB shuts it down...and that's a pretty ideal example as far as balance. If you're going to use that PCB you'll probably have to limit your charge voltage to 8.5 or 8.6v. You should monitor the voltage of each cell and the current during the first couple of charges to see what's happening. If the current during the charge suddenly drops to zero, then the protection PCB has tripped. The current should go towards zero exponentially, but never actually get to absolute zero in a proper charge cycle.

    Yes, the those datasheets are pretty sketchy. They seems to be missing a lot of key pieces of data such as the different between the per cell charger upper voltage limit vs the protection PCB upper limit.

    What kind of light are you driving with the pack and what kind of run time are you getting from it?

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