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  1. #1
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    Lipo's....to protect or not,

    This both a simple but complex conundrum. I do a lot of rc helicopter flying so I have many 4s, 5s and 6s unprotected lipo's. They are unprotected as the speed controllers we use sense the input voltage and cut/reduce power at a preset voltage. This way the lipo's are protected and any imbalance in the cells discharge is equalized when next charged with the balance charger.

    So, if i use a bflex or the like and set the battery warning and cut-off voltages do i really need a protected lipo. Obviously if a buckpuck is used then a lipo with pcm is a must as that driver cannot sense voltage and can therefore cause potential problems (campfire on the handlebars ).

    Look forward to hearing what other have done or are doing.

  2. #2
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    Quote Originally Posted by brad72
    This both a simple but complex conundrum. I do a lot of rc helicopter flying so I have many 4s, 5s and 6s unprotected lipo's. They are unprotected as the speed controllers we use sense the input voltage and cut/reduce power at a preset voltage. This way the lipo's are protected and any imbalance in the cells discharge is equalized when next charged with the balance charger.

    So, if i use a bflex or the like and set the battery warning and cut-off voltages do i really need a protected lipo. Obviously if a buckpuck is used then a lipo with pcm is a must as that driver cannot sense voltage and can therefore cause potential problems (campfire on the handlebars ).

    Look forward to hearing what other have done or are doing.
    For my 7up lights I went with 4s lipos and run them unprotected. I figure since I'm using MaxFlex drivers in these lights and set the low voltage warnings and cutoffs appropriately there is really no chance to over-discharge a battery pack. On the charging side I'm using an RC type balance charger and double-checking the charge termination voltage and balance with a BVD-Pro tester so I'm pretty confident of not over charging any cells.

    So far, nothing has blown up... yet...

  3. #3
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    I have done exactly the same as kwarwick for the last 2 years and have not had a problem.

    Just remember that the max / b Flex waits a whole minute from detecting the cutoff voltage to actually cutting it and it is only measuring the pack voltage and not individual cells.

  4. #4
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    Hi Brad, likewise.

    Only thing is I'd check that the shutoff settings in the maxflex are correct using a variable power supply as the cost of getting it wrong is just too high.

    Also be aware that the maxflex takes a full minute to shutoff & during this time the battery voltage may be plummeting - be a bit conservative with the threshold.

  5. #5
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    Thanks for that guys. I hadn't thought about the 1 minute shutoff time so I might tweak my cutoff voltage up a bit.

    Well I'm a happy boy now as i really didn't want to buy new packs and a charger. Rather spend my money on making lights.

  6. #6
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    its short circuit protection that is most important, if you short out a lipo pack you have a great bomb, IMHO you should allways run short circuit protection.

    So you don't need a bms if you run a maxflex/bflex but great idea to run a fuse on
    main power out

    you then have under volt protection with driver, over from your charger, and short from you fuse,

    on all my stuff I run a bms on board the battery, even when I use bflex

    better to be safe than sorry

    k

  7. #7
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    Thanks for the great advice Ktronic. So would a 1 amp fast blow fuse be ok since the battery will never deliver that much power unless the voltage drops all the way down to the vf of the leds, at which point the battery would be stuffed anyway.

  8. #8
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    Quote Originally Posted by brad72
    Thanks for the great advice Ktronic. So would a 1 amp fast blow fuse be ok since the battery will never deliver that much power unless the voltage drops all the way down to the vf of the leds, at which point the battery would be stuffed anyway.
    sure that would work, also 5A or even 10A would also be fine, as if I shorted, my packs would pump 30-40A easy
    hope this helps

    K

  9. #9
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    I wouldn't go for less than 5A because they are just too fragile (fail due to corrosion or vibration). 5A LittleFuse great as they are leaded just like a resistor (hot melt glue makes great cable/fuse support).

    You can also get 5A ~85 degree Celsius thermal breakers that auto reset. They are probably less reliable but the reset feature could be useful.

    HTH

  10. #10
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    I managed to short the very small 500mah 11.1V lipo I use in the day for my front flasher. The wiring from the main battery cable to my socket lasted about 2 seconds before burning out in a rather spectacular manor. I am a lot more carfull with lipos after this experiance.

  11. #11
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    Should of asked this question a few days ago as I just placed an order with Farnell and I could of put the Littelfuse on it.

    Thanks for the great advice guys

  12. #12
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    The protection circuit also acts as a back up should you're driver or charger fail.

    I would never build or sell a Li-Ion or Li-Po battery without a protection circuit.

    Brad, if you're in the UK I can post you one out.

  13. #13
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    In RC, it's impractical to run a protection circuit because of the high currents involved. And there is already a fair amount of risk of bad things happening in RC (crashes and other mishaps) so this doesn't increase the risk all that much. I run unprotected lipo packs in my RC planes. But, I don't spend hours with my RC plane battery in a jersey pocket.

    For bike lights, I personally always run both a voltage cutoff in the driver and an in-pack protection PCB for either a li-ion or lipo pack. yeah, you'll get away with not running one 99% of the time, but the protection PCB is for that 1% case when something goes wrong. The way I look at it, why wouldn't you run a protection PCB. They're cheap, less than 10 bucks, and they're readily available in the current ranges used by bike lights. I certainly would never sell or give a light with an unprotected pack to someone. Even ignoring the legal liability, I wouldn't want to live with the guilt of having someone's house burn down or worse.

  14. #14
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    MtbMacgyver, your right about the 1% rule with the batteries. Lipo's are certainly not forgiving when they fail, \and hey, if i crash my helicopters they are usually 20 of so meters away. That's why all my lipo's live and get charged in the back shed

    Is it practical however to fit a pcm to the -ve balance leads plus B+/B- leads to a ready made RC pack while still leaving the balance plug for charging or would building a new pack would be far simpler ?. (i'm trying to picture the pcm circuit)

    thanks
    Last edited by brad72; 06-04-2010 at 06:36 PM.

  15. #15
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    Quote Originally Posted by MtbMacgyver
    The way I look at it, why wouldn't you run a protection PCB. They're cheap, less than 10 bucks, and they're readily available in the current ranges used by bike lights.
    Hi! Do you buy them from Batteryspace? The problem is that such PCBs are not easily available outside of US.
    CNC LED light housing for DIY projects

  16. #16
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    Quote Originally Posted by brad72
    Should of asked this question a few days ago as I just placed an order with Farnell and I could of put the Littelfuse on it.

    Thanks for the great advice guys
    Auto fuses are an option as they are reasonably small and flat. K-tronik put me onto that one a little while ago.

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