Lithium Ion Series Float Charging

Discussion in 'The Projects Forum' started by fusionstream, Aug 12, 2013.

  1. fusionstream

    Thread Starter New Member

    Aug 12, 2013
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    I'm having great difficulty finding resources on the web and thought I'd post here asking for help. (After checking out so many circuits on this forum for other projects)

    I intend to build a 11.1V lithium ion battery pack as a drop in replacement for a lead acid battery.

    The intent is to builld a 3SxP battery pack where x means I may add on more packs in parallel at a later date.

    There isn't much on float charging save for 1 small snippet that float charging a li ion series pack will require balancing.

    I intend to float charge them at 4V for simplicity and safety and for ease of maintenance. Batteries probably won't be connected to a "charger" indefinitely but between half a day and maybe 1 week. Load may or may not be present.

    Power source could be from mains (AC->12VDC) or from a solar panel that is voltage limited to 12V with say a simple shunt.

    Will the following circuit work?
     
  2. biscuitownz

    New Member

    Aug 9, 2013
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    I would add a resistor to make it safer but it should be fine with that setup
     
  3. fusionstream

    Thread Starter New Member

    Aug 12, 2013
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    A resistor may adversely affect voltage regulation.

    I was under the impression that when float charging, the battery would self regulate how much current it takes? Not unlike the saturation stage in proper li ion chargers.
     
  4. biscuitownz

    New Member

    Aug 9, 2013
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    As it gets closer to 12v it will use less current, but when its lower on voltage it will try to take as much current as the charger has.
     
  5. fusionstream

    Thread Starter New Member

    Aug 12, 2013
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    Ah. So in this case I'm guessing the resistor will put the voltage/current slightly below what the battery would normally take? But this will probably affect my load both during charging and when not charging? Resistor wattages will have to be quite large.

    Will this circuit (with or without those resistors) allow me to continually float 12V without having to terminate charge (this is really key to me)?
     
  6. biscuitownz

    New Member

    Aug 9, 2013
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    Yeah with or without resistors you will be able to float them at 12v. It's just that adding resistors when you are lower on voltage (10v or so) you would want to add something to limit the current to protect the batteries.
     
  7. Externet

    AAC Fanatic!

    Nov 29, 2005
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    If I understand the circuit as applying 4V to the lowermost cell + and 8V to the second +, and 12V to the third +; I disagree with the method.

    Instead, a single 12.0 V to the top + would be better, perhaps trough a ~100 ohm resistor.

    The 'correct' way is using 3 isolated sources of 4.0V, not sharing a common negative.

    Li-ion cells have a very low self-discharge rate, and a float charging would not be convenient, almost unnecesary. I would instead, charge the battery a few minutes once every couple of months... or longer. And/or after they get some use.
     
  8. Externet

    AAC Fanatic!

    Nov 29, 2005
    762
    57
    If I understand the circuit as applying 4V to the lowermost cell + and 8V to the second +, and 12V to the third +; I disagree with the method.

    Instead, a single 12.0 V to the top + would be better, perhaps trough a ~100 ohm resistor.

    The 'correct' way is using 3 isolated sources of 4.0V, not sharing a common negative.

    Li-ion cells have a very low self-discharge rate, and a float charging would not be convenient, almost unnecesary. I would instead, charge the battery a few minutes once every couple of months... or longer. And/or after they get some use.
     
  9. fusionstream

    Thread Starter New Member

    Aug 12, 2013
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    Yes but the article I read had a passing remark that floating a series pack would require balancing. So this is kinda my attempt.

    Also, if I put 4V to each battery, I'll have to interrupt the series to make that work?

    Purpose of this is not to keep it continually charged, but to provide a reservoir of power of sorts.

    Would like to charge and use them at the same time. Like a drop in 12V lead acid replacement.
     
  10. wayneh

    Expert

    Sep 9, 2010
    12,153
    3,059
    I don't see the difference. Consider the cell in the middle, which sees a voltage of 4v at the bottom and 8V at the top. ∆V = 4V and that cells knows nothing else.

    Now, I agree that having 3 regulators fighting with each other could be a problem and perhaps that's what you're getting at. But "on paper", the 4,8,12 strategy should work, if it can be implemented.
     
  11. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    "It is importent to note that trickle charging is not acceptable for Li-ion batteries. The Li- ion chemestry cannot accept an over charge without causing damage to the cell, possibly plating out Li metal and becoming hazardous" from PowerStream, http://www.powerstream.com/li.htm
    I've been reluctant to use Li batteries with home brew chargers; but relented and bought a LiFePO4 battery to play with; now even used in garden lights.
     
  12. fusionstream

    Thread Starter New Member

    Aug 12, 2013
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    That's why I chose 4V. Which I assumed would stop trickle charging or would trickle at a low enough rate to not cause damage. Hmm.

    Would LiFePOs allow me to just apply a constant potential without having to worry about it?
     
  13. fusionstream

    Thread Starter New Member

    Aug 12, 2013
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    Actually, why would the 3 regulators be fighting with each other? If the common reference point is relative to ground, 12V +/- 0.1 will always be 12V +/- 0.1 (with respect to ground).

    If you tie each regulator's ground to the battery's negative terminal, then that would force them to fight each other because the stable point of ground would be fluctuating in a cascading effect.
     
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