Constant NiMH trickle Charger

Discussion in 'The Projects Forum' started by iONic, Sep 26, 2008.

  1. iONic

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    Nov 16, 2007
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    Anyone see anything wrong with this circuit. I have replaced a pair of dead NiCads with NiMH AA batteries in my mustache/beard trimmer and want to have it permanently trickle charge without damage the two NiMH 2600mAH batteries.
     
    Last edited: Sep 26, 2008
  2. hgmjr

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    One thing you probably don't want to do is ground the adj pin on the first stage.

    hgmjr
     
  3. iONic

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    Yes, you are quite right there. Circuit fixed. Can the TO-92 LM78L05 be regulated for a lower voltage?
     
  4. SgtWookie

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    Have a look at the attached schematic. You can use LM317's instead of the TLV1117 (adjustable) regulators if you'd like, but you'll get about a 4.7v dropout across the two regulators.

    U1 is set up as a constant current regulator circuit; it feeds about 258.8mA to U2. In this simulation, the Vref is 1.294v; most regulators have a Vref of 1.22v to 1.28v. So, 1.294v/5 Ohms = 258.8mA

    U2 is set up as a voltage regulator. R4 takes roughly 10mA current from the charging circuit; this is the required minimum load for an LM317 to maintain guaranteed regulation on the output. R5 is the fine Vadjust.

    C1 could be anywhere from 0.1uF to 100uF; it's just there to ensure that the regulator doesn't oscillate.

    The battery is simulated by C2; just a large capacitor.

    The green plot is the current; yellow plot is the voltage. You can see that the current supply is very linear until the "battery" is nearly charged, where the current drops off rapidly.

    The big drawbacks to this circuit are:
    1) It's linear, which means that it's not very efficient.
    2) No monitoring of battery temperature.

    However, it should completely recharge your batteries in about 11 hours.

    As far as the 78L05's - you can make them regulate at a higher voltage, but it's impractical to try to regulate them at a lower voltage. If you want current at <100mA, you could use this circuit with LM317L's; replace R1 and R2 with 25 Ohm resistors, and replace R3 with a 240 Ohm resistor. That would give a maximum of 95mA out.
     
    Last edited: Sep 26, 2008
  5. iONic

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    The typical commercial chargers that I have charge a 1.2V NiCAD or NiMH battery to aprox. 1.42V, thus the 2.85V requirement for two batteries. The device I am intending to charge gets used every day and sometimes every other day, but never anywhere near complete discharge, thus the charging current of c/40 for the 2600mAh batteries (65mA). Since it is being constantly charges with such low current I would think that overcharging or over heating would not be an issue.

    Since the 78L05 is out with respect to regulating to 2.85V I may opt to go to the low current 317 regulator. Or maybe the L200 could do this with one IC.
     
  6. Audioguru

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    Dec 20, 2007
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    The battery limits the voltage, not the charger.
    The charging circuit needs a voltage that is higher than the battery voltage for it to function.
    You could trickle charge your two AA cells from 1000V and they would work the same (if the current is limited). But if the input voltage to the charger is much too high then the charger gets hot.
     
  7. iONic

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    I'm not sure I get the point. By the time the battery voltage reaches the preset voltage, the current will not flow. Of course there is a possibility that the battery voltage could get higher than 1.42V/cell, but this is a cutoff I will set, thus in my hopeful thinking, the battery or charger should not get hot.

    Yes? - No?
     
    Last edited: Sep 26, 2008
  8. Audioguru

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    My point is that you do not need a voltage regulator. You just need a current regulator that is set to the trickle charge current.

    If you use only a voltage regulator then you must adjust it to match your batteries and every cell has a different charging voltage. The battery cell's voltage changes when the temperature changes.
    A current regulator is much better.
     
  9. SgtWookie

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    I disagree. If you continually feed batteries more current than they require to maintain their rated charge, they will rapidly degrade.
    That's why you need a voltage regulator AND a current regulator. Give the batteries the current they need to charge up. Once they're near rated voltage, taper off the current.

    If you keep feeding them maximum current, you'll have lots of heat and a potentially nasty blow-up on your hands. Whatever happens, it won't be good for the batteries.
     
  10. iONic

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    This is not a major undertaking here. I could easily just remove the batteries from the trimmer and recharge them with a commercial charger, but I thought I could save this effort, little as it may be and devise a way to keep them as close to max as possible without lifting a finger.

    Here is the big question though, as the batteries get older, there may be reductions in the battery capacity. I'm not sure if this is going to translate into decreased maximum voltage or just a decrease in the storage capacity. If it is the latter than everything is OK, but if it is a reduced max voltage of one or both cells then the preset voltage will keep the 65mA on continuously and may create some heat either in the batteries or the charger.

    should I consider solving this issue in the circuitry somehow or not? I am not an expert when it comes to "the days in the life" of batteries and how they respond to the aging process. It would be possible to do some testing on how long it would take to charge the batteries after a good workout, then determine the time it would take to trickle charge them at 65mA within 24 hours. It may be possible to reduce the current spec even more. Bottom line, even if the charger doesn't quite fully charge the batteries, it would be a long long time before I had to take them out and charge them conventionally.
    At the c/40 charge rate they could be left attached to the circuitry without damage.
    At c/50 there might not be any harm done if one of the cells fails completely. Of course I would notice this upon the next usage.
     
  11. SgtWookie

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    If you use the circuit I posted and set the output voltage properly, you won't have to worry about overcharging; as once the voltage limit is reached the current is drastically reduced. But if you want to ensure the batteries don't get a "memory" due to charging after a partial discharge, then set up a circuit to first completely discharge the batteries, and then recharge them.
     
  12. hgmjr

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    Jan 28, 2005
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    iONic,

    It may be worth your while to read the information on battery technologies contained at www.batteryuniversity.com.

    hgmjr
     
  13. iONic

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    since the batteries will be NiMH I wouldn't expect memory effects...
     
  14. iONic

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    Yes, I was hoping to get to that before committing to building the project.

    Thanks for the link.
     
  15. hgmjr

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    The article to which I linked seems to indicate that nickel-based batteries are susceptible to "memory" effects.

    Do you have information to the contrary that lead you to your conclusion that nickel-based batteries are immune to "memory" effects?

    hgmjr
     
  16. Audioguru

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    The article from The Battery University is old.
    Energizer Battery Company has newer very detailed Applications Manuals for their obsolete Ni-Cad and newer Ni-mH battery cells. www.energizer.com Click on Technical Info.
     
  17. hgmjr

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    Very good audioguru. I looked at the information to which you link and sure enough the document states that memory is not a concern in NiMH.

    Thanks for that update.

    hgmjr
     
  18. iONic

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    Using NiMH batteries with a constant less-than-trickle charge method may not be optimal for these batteries as Nickel-metal-hydride should be rapid charged rather than slow charged. The NiCD's would be even worse in this charging circuitry as memory effects would be even more pronounced. Automatically discharging and then charging them after each use would be overkill and additional circuitry. It may, after all, be simpler to just charge them with the conventional charger when needed.

    Bummer!
     
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