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Best Method to Slow Charge NiMH Batteries

Discussion in 'Electronic Repair' started by [email protected], Dec 23, 2012.

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  1. Guest

    Since I created a NiMH battery pack using recently bought NiMH "C"
    batteries, I am in search of knowledge for the best way to slow charge these
    1500 MAH "C" cells in series. I prefer using a timer and a constant current
    charger (I made) that supplies a constant 150MA.

    There is a lot of information on the web, but one site says one thing,
    and another says something else. provided the kind of
    information I was looking for:

    (Battery Capacity in MHA/Charge Rate) X 1.4 = Time to charge (slow
    charge). Note: Batteries "fully discharged" to 1V each cell.

    In my case, 1500/150 = 10 X 1.4 = 14 hours. This seems logical
    to me, but I wonder if anyone sees a flaw.

    Thank You in advance, John
  2. mike

    mike Guest

    What does it say on the manufacturer's technical datasheet or charging
    application note?

    If they were NiCd, you'd have no problem.
    The consensus appears to be that NiMH don't tolerate long-term overcharge.
    Opinions vary on short-term overcharge. I expect you're doing about
    the best you can do with simple schemes.

    The difficulty is not in the equation. The difficulty is in determining
    full discharge. How do you determine that?
    Do you measure the voltage? under what conditions? OR do you just
    charge it when the performance of your device drops? OR at random?
    The range between 1.1V and 0.9V is huge depending on the load current
    you use to test. And the device probably quits long before that.
    I know because I have a computer hooked to a programmable power supply
    and load fixture and I've tested it on a many different cells with widely
    varying history.

    One example application is the emergency flashlight.
    You can't just charge it when it goes dark. Even in the best of conditions,
    when it's starting to dim, you know you should charge it IMMEDIATELY...
    but you've only got one more thing to you do it.
    More batteries are damaged from overdischarge than anything else.
    For it's intended use, the emergency flashlight needs to be at FULL
    charge all the time. So does your cell phone and your car battery and
    your electric drill and...
    Even low-drain applications are not immune. Do you want your alarm
    clock to go dead at 4AM on the morning of your big presentation.

    Bottom line is that you expect your device to perform for the entire
    duration of the need...even when you have no idea how long that is.
    That requirement often leads to systematic overcharge.

    I expect you're doing about the best you can do with simple schemes.

    If you've done something like solder on the cells, you can expect
    some variations in self-discharge and capacity. Some amount of overcharge
    tends to keep them equalized.

    Are we having fun yet?
  3. In the dark ages, I've charged NiCd and NiMH batteries with a timer.
    This is one of those rare cases where I have to disagree with Mr Lieberman.
    How warm is warm? "Getting warm" is an indication that charging is complete,
    or near-complete.

    Many NiMH manufacturers claim their cells need to be "slammed" to get maximum
    charge. I'm reluctant to do this. However, I've charged NiMH cells in those
    "15-minute" chargers, and though the cells got hot, they were not destroyed.
    (I no longer do this. I have about 20 cells of 2500mAh and higher capacity, so
    I never need a quick charge.)

    The best way to charge batteries is with a charger that lets you set the
    charge rate, and watches for the signs that indicate the cell is charged, such
    as the MAHA C9000. I generally charge at 0.3C or 0.4C, which is considered on
    the low side for NiMH cells. Contrary to what Battery University claims, I've
    never had problems with the cells overheating, or the charger failing to halt
    at around 1.42 to 1.45 volts. (Note that his gripes are principally directed
    at "consumer" chargers.)

    The C9000 displays the battery voltage, so I can choose to stop charger at
    whatever voltage I feel comfortable with. I'm not stupid enough to let any
    charger operate without checking it occasionally. If you like, stick a timer
    on the charger to shut it off.
  4. <>
    shows that the first charge cycle results in a 1180mA-hr capacity,
    while the third charge cycle increased it to 1360mA-hr or about 15%
    increase. Is that what you mean by "slammed"?

    No. It means hitting it hard with a high charge current.

    I've never had that problem -- as far as I can tell. I've had cells that sat
    around for the better part of year deliver 20 or 30 flashes, with rapid

    Most of my cells have been close to rated capacity. In one case, a cell was
    about 20% low, and MAHA replaced it.

    Nice charger/analyzer. I dunno the break-in and battery-forming
    features. Stressing the battery doesn't seem like a good way to
    increase battery life though it might produce rated capacity earlier.
    However, it's a much better charger than the common consumer junk
    chargers, and would probably be a good charger for the OP.

    Agreed. Thomas sometimes sold the C9000 for ~ $40, but those days seem over. I
    have two, so I don't have to wait if a lot of cells need charging.
  5. "Jeff Liebermann" wrote in message
    Yuck. Why would I want to do that? If there were dendrites in NiMH
    batteries like in NiCd, then perhaps it would make sense. Besides if
    such a high current blast was necessary to produce a proper battery,
    or to produce bigger numbers on the data sheet, the manufacturers
    would already be doing it.

    Where did you see this recommendation? I couldn't find anything with
    Google under "NiMH slamming" variations, except under National
    Institute of Mental Health. I did find this blurb that recommends
    against the practice:

    "Slam" is my choice of words.

    I can't give you any references, but I've repeatedly seen in the documentation
    for NiMH cells, that hitting them hard is necessary to get a "full" charge.
    You are repeatedly warned NOT to charge them below 0.3C, and higher values are

    Most of my flashes are 500-series Canons, which are hardly wimpy li'l

    Scroll down to the two pink graphs near the bottom of the article. The
    conventional battery is down to half capacity in 75 days, and 1/4
    capacity in 150 days.

    That's not what I would consider "rapid" self-discharge -- other than,
    perhaps, compared to a lead-acid battery.

    Incidentally, this article:
    covers quite a bit of ground on dealing with NiMH batteries, including
    trickle charging, and a home made NiMH computah controlled charger:
    and a simplified USB powered charger:
    I don't know if they make them, but they definitely sell them under the
    PowerEx brand. Thomas sells four for about $12. Considering that a charged
    2500mAh cell gives about as much runtime as an alkaline cell, it doesn't take
    long to amortize the cost.

    I'm finding prices from $50 to $140 (including tax and shipping).
    < mh-c9000&_sop=15>

    The eBay prices are nuts.

    I was wrong. Thomas's holiday deal is the charger, a generic carrying case,
    four Immedion AA cells and a plastic case for the latter, for about $52. Given
    the cost of the "accessories", the charger nets at $40. This would be a very
    good time to grab a C9000 from Thomas Distributing.
  6. mike

    mike Guest

    +1 on the c9000

    I've had good results with the older
    Enloop charger...the one with the single led that can
    charge an odd number of cells.

    There are two categories of dendrites.
    Dendrites that have already shorted need to be vaporized with
    high-current. That clears the short and blasts a bigger hole in the
    Causes higher self-discharge and faster dendrite regrowth.
    Back in the day when NiCds were expensive and shorted ones were free,
    it made a lot of sense for hobby projects.

    The second class is those that have not yet pierced the separator.
    For those, I'm a fan of burp charging. Same process you'd use
    for plating when you want a smooth surface.
    I did some experiments, but I had nowhere enough data to draw a
    conclusion. And I concluded that the cost of powering the computer
    and the programmable power supply and load probably exceeded the benefit.

    The biggest problem with charging batteries is determining when to

    I had a friend who did radio repair for the city.
    Back in the day, police radios were terminated by a thermal switch.
    That worked surprisingly well, but only because the battery was
    predictably depleted during the shift.
    If you stuck a charged radio into the charger, you did some damage.
    He had a lot of failed batteries from people who left the radio off
    all day and stuck it back in the charger.

    There's so much variability in temperature, thermal time constant,
    voltage gradient, etc. that it's best to measure change than
    absolute value.
    I'm a fan of 0deltaV termination with timer and temperature fail-safe
    for NiMH cells.
  7. Most of my flashes are 500-series Canons, which are hardly
    Most DSLRs use lithium-ion batteries. I've never measured the current drain,
    but I doubt it approaches what's required to charge an electronic flash from a
    dead start. And unless you're using the focus-track feature, auto-focus
    operation is intermittent.

    --> discussion of rechargeable versus disposable batteries snipped <--

    I use rechargeables on any device where I can easily swap the batteries.
    Disposables are okay for certain kinds of toys and LED flashlights. (Most take
    AAA cells, and I have a huge box of Polaroid AAA alkalines I got cheap.)
  8. MAHA has told me it will never provide a C or D adapter for the C9000. Not
    wanting to figure out how to reliably attach wires to the charger's terminals,
    I charge my C NiMH cells * in an Eveready charger with a NiCd / NiMH switch on
    it. I monitor the charge with my hand. When the cell gets a bit warm, I pull

    * I use C cells in a potato-masher flash and a classic Sony FM radio.

    This principle isn't new. The basic system dates back to the late 60s.
    Motorola made a "pulse-discharge" charger for Honeywell flashes. The claim was
    not only that it could quickly recharge the power pack, but that it would
    often "heal" a badly performing or even "dead" pack.

    "Modern Photography" gave the system a rave review, confirming that it worked
    as claimed. Several years later I asked Burt Keppler what happened to the
    system. He said Honeywell pulled it because there were too many instances of
    the packs exploding. When I asked him why he never reported this in "Modern",
    he had no answer.
  9. mike

    mike Guest

    You need better control than that. I've had better luck discharging
    a big cap into the cell.
    Agreed. Trickle charging is for sissies. ;-) And only makes sense when
    you have no reasonable termination system...and then, it still makes no

    That's also my complaint about the C9000.
    If your cells have slightly high internal resistance, it refuses
    to charge them at rated current. If you crank the current down till
    they charge, you risk failure of charge termination.

    It's in the low
  10. [...]
    Where does the excess energy go if it doesn't finish up as heat in the
  11. mike

    mike Guest

    Cool to the touch is not the same as exactly at ambient.
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