LiFePO4 battery charging

Discussion in 'General Electronics Chat' started by ivars211, Oct 15, 2014.

  1. ivars211

    Thread Starter New Member

    May 5, 2013
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    Hi, after several hour of googling, I did not find anything suitable (most likely wrong keywords or I am just a noob). There are many charging circuits for Lipo batteries, but almost nothing for LiFePO4 .

    Basically, I have this 3 cell battery that I need charger for. If I understand correctly then I have to charge each cell individually with constant current until it reaches 3.65V and then continue with constant voltage of 3.65V.
    I found several circuits using LM317, but all of them where designed to charge single cell. Where as some ICs are designed for all 3 cells in series.

    So do I have to charge each individual cell or can I just charge them in series? Is there any simple charging circuit that I have not found yet? Btw I would like to use general AC-DC wall plug converter to power up the charging circuit.

    Thank a lot.
     
    Last edited: Oct 15, 2014
  2. Papabravo

    Expert

    Feb 24, 2006
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    Does the manufacturer of the battery have any helpful information on charging their products?
     
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
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    You maybe want to look at one of the RC type Imax multi cell, multi composition type of battery charger, it will also charge (or discharge) to proper storage level.
    Max.
     
  4. ivars211

    Thread Starter New Member

    May 5, 2013
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    Thanks for your replies.
    They do offer some multi-purpose chargers, but I would like to have the charging circuitry inside the enclosure and 12V DC plug coming in to power it, if possible.
     
  5. Papabravo

    Expert

    Feb 24, 2006
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    I don't know of any a priori reason why the cells need to be charged individually , assuming you can even get them apart. It might come down to getting one of their chargers so you can reverse engineer it. There is all kinds of information on how to build chargers out there, you just have to look for it.
     
    Last edited: Oct 15, 2014
  6. ddurgin

    New Member

    Mar 2, 2011
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    From 12v you can regulate to 10.95V (3.65 x 3-series) with a linear regulator (an LM317 circuit should work). It looks like a current limit of about 0.5 to 1-amp should be OK for that cell pack. So your input will be the desired 12v and output should be 10.95v limited to ~1amp. The pack pictured almost certainly includes a protection circuit to keep the cells from getting to far out of balance (relative to each other), so I don't think you'll need to worry about that.
    Design Notes:
    (1) Make sure the final voltage is accurate; most Li batteries are specified for +/-0.05V top off, so 3.70 x 3 (11.1v) should be your absolute maximum charge voltage. (2)The charge will start at the current limit (constant current phase), then the voltage will slowly rise to the top off (constant voltage phase). (3) The battery is charged when the current tapers off to about 50mA. (4) You should either disconnect then or design in a termination threshold to cut off the charge; I don't think any of the Li chemistry cells are rated for trickle charge after full capacity is reached (cutoff is not a huge concern but it will shorten the battery life if left to trickle charge for long periods).
    And finally... be careful, don't let a new charger design run unattended, an off the shelf Li pack should have built in protection but there is always a small chance of fireworks with batteries!
     
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  7. ivars211

    Thread Starter New Member

    May 5, 2013
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    It seems that 0.5A constant current @ ~10.95V should work. Any tips for designing charging cut-off when the current tapers off to ~50mA?
     
  8. ronv

    AAC Fanatic!

    Nov 12, 2008
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  9. MaxHeadRoom

    Expert

    Jul 18, 2013
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    ...And ebay 251640755087
    Max.
     
  10. ivars211

    Thread Starter New Member

    May 5, 2013
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    Yes, I could just buy all in one unit, but it's not what I need. I need to fit all the charging circuitry inside my enclosure and those two posted just won't do. Ronv, that charger will charge only Lipo, not LiFePo4 batteries.
     
    Last edited: Oct 16, 2014
  11. ian field

    Distinguished Member

    Oct 27, 2012
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    Charge balancing is critical for safety - a series stacked battery should have a strip of PCB with various chips that take care of this.

    Without charge balancing, splitting the battery into individual cells and charging them separately, is some what less risk of a cell venting with flaming gas.
     
  12. ronv

    AAC Fanatic!

    Nov 12, 2008
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    1. Conventional charging
    During the conventional lithium ion charging process, a conventional Li-ion Battery containing lithium iron phosphate (LiFePO4) needs two steps to be fully charged: step 1 uses constant current (CC) to reach about 60% State of Charge (SOC); step 2 takes place when charge voltage reaches 3.65V per cell, which is the upper limit of effective charging voltage. Turning from constant current (CC) to constant voltage (CV) means that the charge current is limited by what the battery will accept at that voltage, so the charging current tapers down asymptotically, just as a capacitor charged through a resistor will reach the final voltage asymptotically.
    To put a clock to the process, step 1 (60%SOC) needs about one hour and the step 2 (40%SOC) needs another two hours.

    3. Self balance
    Unlike the lead-acid battery, a number of LiFePO4 cells in a battery pack in series connection cannot balance each other during charging process. This is because the charge current stops flowing when the cell is full. This is why the LiFEPO4 packs need management boards.


    Based on that lets try a spec out on you.
    Fast charge - 0.5 amps (constant current)
    Constant voltage 3.65 volts
    Maximum imbalance 10%
    No turn off timer.
     
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  13. boatsman

    Senior Member

    Jan 17, 2008
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    I have four 6v 6ah sealed batteries. They are over 40 years old and have never been used or charged. I connected two in series to make 12v and connected them in place of the 12v battery in a UPS and plugged in the UPS. The voltmeter reading across the battery terminals was 13.2v. I left the setup connected overnight (more than 12 hours) and the following day I disconnected the batteries and checked the voltage across the terminals. Zero volts. Is this because the charging is incorrect or because the batteries have just deteriorated over this period of time?
     
  14. boatsman

    Senior Member

    Jan 17, 2008
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    I forgot to mention that the batteries are sealed lead .
     
  15. ian field

    Distinguished Member

    Oct 27, 2012
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    40 yr old lead-acid batteries that still worked would be a miracle, regardless what state they were stored in.

    Discharged sealed lead-acid batteries need recharging within a week or two to avoid irreversible sulphation of the plates.

    The definition of discharged can vary depending who you ask, some authorities have it that discharging below about 11V will damage the battery.
     
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