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  1. #1
    lumen junkie
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    Maxflex: V cut, V mid, V low

    I've got my Maxflex pretty much programmed how I want it but what are you guys setting your Maxflex cutoff and warning (mid/low) voltages at using a 4 protected cell 14.8v battery?

    I've also got the thermal protection set at 60˚ and it's been fine so far (7up).

  2. #2
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    Just double the default settings as they are for a 2 cell pack .7.4 v

  3. #3
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    I never use Vcutoff setting on maxflex. Or, better to say, I set it to 0V. The reason is, I always use protected cells/packs, so I see no reason to have it set on maxflex. Besides, if your protection circuit cut the battery off at 6.0V (e.g. for 2S battery) and you set the same cutoff voltage on maxflex, there are rather good chances your light will be shut off even before the battery voltage reaches 6.0V. Namely, at the end of discharge, the current from the battery will rise and so will the voltage drop on your wires. So, you can easily see voltage drop of 0.2V on your wires from battery to maxflex, which means maxflex will shut it down when the actual voltage of your battery is 6.2V.

    Of course, you can anyway set the Vcutoff on maxflex as an additional safety measure, or if for some reason you want to shut the light off before it reaches the voltage of protection circuit cutoff.

  4. #4
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    Quote Originally Posted by ortelius
    I never use Vcutoff setting on maxflex. Or, better to say, I set it to 0V. The reason is, I always use protected cells/packs, so I see no reason to have it set on maxflex. Besides, if your protection circuit cut the battery off at 6.0V (e.g. for 2S battery) and you set the same cutoff voltage on maxflex, there are rather good chances your light will be shut off even before the battery voltage reaches 6.0V. Namely, at the end of discharge, the current from the battery will rise and so will the voltage drop on your wires. So, you can easily see voltage drop of 0.2V on your wires from battery to maxflex, which means maxflex will shut it down when the actual voltage of your battery is 6.2V.

    Of course, you can anyway set the Vcutoff on maxflex as an additional safety measure, or if for some reason you want to shut the light off before it reaches the voltage of protection circuit cutoff.
    What is the true per cell cutoff voltage on your pack protection? While it could be 3V per cell, 2.5V per cell is more typical and is meant as a true last ditch safety limit. That's why you should set the cutoff on the maxflex to 3V per cell.

    If your pack protection is lower than 3V per cell and you're discharging to that level, then the weakest cell will be going to that low level. The lower you discharge a li-ion cell, the more it deteriorates. The weakest cell will get weaker more quickly than the stronger cells and pack will become unbalanced and fail sooner than with a higher cutoff voltage.

  5. #5
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    +1 3 volts is as low as you should go.

  6. #6
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    MtbMacgyver, I agree with you "technically". However, my philosophy on who decides when my light should unconditionally be shut off differs slightly. My stand is, it should either be me or the battery built-in protection, not maxflex. Let me explain.

    I'm all for it that when the battery is nearly emptied I should get a clear warning. And that's what I use maxflex LOW treshold. I usually set it to around 3.2V per cell. When it is reached, I'm aware that my battery is almost dead. But I can decide wether the remaining juice will be sufficient to bring me to the end of the trail or out of the wood or not. I might even take the risk to drain the battery to the point which is not good for it's health any more, but it is mine decision. Sometimes I might decide that buying a new battery after the ride is better option than having light being switched off in the middle of the dangerous technical section, risking far more expensive (or painful) damage to some other component. Of course, in those circumstances the light could be shut off by battery's protection circuit anyway, but that is unavoidable and I have deliberately chosen to prolong that moment for as long as possible. In short, I don't want maxflex to shut it off when not 100% necessary, I want it to be done either by me or by protection circuit. I was given a very explicit warning that my battery is almost at the end, from there on I'm the one who needs to take an appropriate action (and having battery protection circuit as last safety measure and assurance that battery itself will not harm me by catching fire or something, because of to-deep discharge).

    Of course, with unprotected battery, setting maxflex's Vcutoff is a must. But with protected pack, I want to be the one to decide whether it is better to risk deteriorating my battery or broken my bones or bike.

    YMMV, of course.
    Last edited by ortelius; 12-13-2009 at 02:37 PM.

  7. #7
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    I always use the low v cutoff, I paid for the feature so I use it. I however have to as my battery packs are unprotected so I set it to switch the light off at 3v per cell.

    If I have to continue I have a spare pack. But the maxFlex will still allow you to switch the light back on after it has shut off for 1 minute intervals to help you out of the woods.

    ortelius, I fully understand your point but I would still set the low v cutoff to just below that of the battery pack protection just incase that was to fail, which from experience I know they can, hence my unprotected packs.

  8. #8
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    Quote Originally Posted by ortelius
    MtbMacgyver, I agree with you "technically". However, my philosophy on who decides when my light should unconditionally be shut off differs slightly. My stand is, it should either be me or the battery built-in protection, not maxflex.
    I understand your logic now, but it would be good to include that explanation when giving advice for someone else programming his or her driver. I think your original post could have been interpreted, as it's a good practice to regularly run the battery pack down to the protection cutoff. In reality, it sounds like you avoid that via your usage pattern.

    I handle not being stuck in the dark by having both a bar and a helmet light. There are a lot of reasons a light can cutoff sooner than you expect or fail so I think having redundant lights is the foolproof way to handle the safety aspect.

    I also use a low battery warning and I turn the light to low if I need to stretch out the runtime. If you really need to stretch the runtime, you can also easily re-program the max current on the driver to 350ma on the fly. Even after a light has hit the 3v per cell cutoff, you can cut it back on and get quite a long time out of it driving the LEDs at a low current like 150 or 200ma in an emergency. That's because the battery voltage rebounds quite a bit at really low current. This gives a more predictable amount of additional runtime than running at a higher current until the weakest cell causes the protection circuit to shutdown the pack.

    To answer the original post. It's actually kinda hard to pick perfect warning voltages unless your driver / controller also measures the temperature of the pack. Look at the discharge curves in the following data sheet.

    http://www.panasonic.com/industrial/..._CGR18650E.pdf

    The voltage will be quite a bit lower when the temperature is down around freezing. Here's how I set my drivers for a 4 cell pack and a light that draws 700ma from the pack on high.

    Vmid = 14 V
    Vlow = 13 V
    Vcut = 12 V

    I just keep in mind that the Vmid warning will go off kinda early in cold temperature. You'll also need to set Vmid lower if you're drawing a higher current from the pack. I would set Vmid to 13.5V for example if I were drawing 2 amps from the pack.

  9. #9
    lumen junkie
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    Great info guys...thanks. I'll most likely carry a spare battery so I can be a bit conservative with the Vcut. I like the option of once hitting Vlow to just head home on Level 1-2.

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