Li-ion charging during CC stage

Discussion in 'The Projects Forum' started by mjhilger, Feb 28, 2011.

  1. mjhilger

    Thread Starter Member

    Feb 28, 2011
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    Ok, so I have read severl blogs, threads, web pages, etc regarding how to charge Li-ion cells. I know the correct way is CC/CV. But here is the question and if it has been answered elsewhere, I appologize. I spent several minutes reading through several posts and didn't find any on this particular question.

    What if I don't charge at a constant current rate? What happens if the current is allowed to vary maybe as low as 1ma up to 500ma. It would be monitored via micro or analog to verify it stays under 4.2 during this part of the charge, thereafter held at the 4.2 until the current diminishes off.

    The premis for the question is a small solar panel charging. I know this has been addressed in the other posts, but I have seen no discussion regarding the effects on the cells with the current dynamic. I know this is not optimum but what kind of damage (if any) does it cause the cell?

    BTW I purchased a solar charger off ebay for about $15 which is about the size of an iPhone, contains an 800 mw solar panel, a 2600mah li-ion poly battery, and switch selectable output usb voltage. (it is meant as a multi phone charger - I just want the 5.0 volts out)

    Thanks,
    mitch
     
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    The OEM people(manufacturer) would have you believe any deviation from their method(and their charger) would irreversibly damage the battery.

    While the recommended method may be the quickest/safest way, it is by no means the ONLY way to charge a battery.

    Excessive current flow, and excessive temperature rise are your two real enemies when recharging a battery. Control them and you're golden.
     
  3. Audioguru

    New Member

    Dec 20, 2007
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    You must limit the charging current to avoid blowing up the battery or causing it to catch on fire.

    I made a Li-Ion battery charger from a wall-wart that had a limited output current and an LM317 voltage regulator set to exactly 4.20V.
    It didn't turn off when the charging current dropped when the cell was fully charged but I remembered to disconnect the fully charged cell. I used it every day for months.
     
  4. mjhilger

    Thread Starter Member

    Feb 28, 2011
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    16
    I appreciate your input. I have a couple of chargers and they are great for throwing a battery in to charge it. But I have a couple of items where I want to put batteries within, and it would be difficult to take them out to charge each time. So rather than spending a lot of effort designing and constructing a protection / charging circuit; I was hoping I could just limit the current to say .4C (knowing it would stay cool) and just max the voltage out as was described with the 317. So set the voltage to a fixed point of 8.45 for a 2 cell set up and place a 15 ohm resistor between it and the cells, the current would slope steeply at around 140 ma or so depending on depth of discharge, but slope 1/x style to the fixed voltage. While this would not provide a cutoff for current, the charge current at the set voltage would be in the low uA region for perhaps only a few hours.
     
  5. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    Do not forget that an overcharged(overtemp as well)battery might begin to draw excessive current due to the overcharged condition; liquid in the electrolytes begin to boil and form gas pockets which can force physical structures into each other-creating dead shorts within the cell. At this point, the battery becomes an explosion, or fire hazard. Lithium batteries have greater capacity, but also greater dangers if not carefully regulated during charging and overseen by a competent individual. Also consider all the possible failure modes of your charger circuit and what that might do to the charge it delivers
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    Charging a pair of those batteries in series is not such a good idea. As they age, their capacity will decrease, but not at an identical rate. You'll sooner or later wind up with a situation where one battery is being grossly overcharged while the other is still charging.

    I've attached a schematic of a charger I built when my father-in-law showed up without his cell phone charger, and his battery died. It worked pretty well. Charge current is pretty constant up to the terminal voltage.
     
  7. Audioguru

    New Member

    Dec 20, 2007
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    Multi-cell Lithium rechargeable batteries have an extra connector that is attached to a balancing connector on the charger that safely charges each cell independently from the other cells. Here is an example:
     
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