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Thread: How do I avoid trashing my LiPo batteries?

  1. #11
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    Re: How do I avoid trashing my LiPo batteries?

    The Tenergy board is one of many generic PCM boards available from countless sources for LiPo and LiFePO4 batteries to handle both balancing and discharge cut-off. 50uA is roughly the same order of magnitude as the self-discharge rate of an 1100mAh battery (self-discharge rates vary from 2~10%/month for rechargeable Lithium chemistries), so the PCM boards have little effect other than maybe dropping the mandatory recharge interval of stored cells from ~6 months to 3~4 months in smaller capacity batteries.

    The reason most hobby batteries ship without PCM boards is the minimal packs are intended for short-term, very high-current, direct connections to motor controllers and servos, then completely disconnection and recharging once the R/C vehicle is finished with its run. Adding a PCM in that use-case is simply a waste of battery capacity as heat in the cut-off MOSFETs and extra weight of the PCM, plus the extra cost of the PCM (higher current limits requires more low-resistance MOSFETs which increases board size/weight and cost).
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
    bleh

  2. #12

    Re: How do I avoid trashing my LiPo batteries?

    higher current limits requires more low-resistance MOSFETs which increases board size/weight and cost
    In fact, I've never seen these boards with very high current ratings. The highest I've seen is 15A continuous / 30A burst. The TO-220 P-channel power MOSFETS I'm using are rated for a lot more of that (even without additional heat sink) -- but also cost several dollars each :-)

    I wonder why they don't sell some intelligent lipo battery that protects itself.
    They do, but then you have poor power handling. You can buy Lead-Acid replacements LiFePO4 batteries that have all the protection and balancing built-in; charge with 15V; discharge to 11V or so.

    The basic cut-off is very simple. Big fat power P-channel MOSFET with a pull-up for the gate to keep it off. Separate N-channel signal MOSFET (like a BS-170) pulling the gate down. The current for that can be used to also light up a LED. To turn off, ground the gate of the N-channel, which will make it stop conduct, which will turn off the P-channel through the pull-up. Size the pull-up for the P-channel gate to give the right current through the LED when on.
    Optional things you can add include pushbutton on, pushbotton off, zener protection for the gate voltage, sensing of voltage on the board instead of leaving that to an MCU, etc.

    The AX-12 probably draw slightly less than an amp total together when idle. Probably a lot more when under significant load -- they're rated to about 1A when stalled. You can also see this as a spike of inrush current when turning on, perhaps. i'd size the protection circuitry to 10A or better, assuming I don't actually plan on stalling everything at once...

    (Also: AEG gearboxes draw several amps each. In case that's part of the plans for some reason :-)
    Last edited by jwatte; 12-23-2014 at 12:17 AM.

  3. #13
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    Re: How do I avoid trashing my LiPo batteries?

    Quote Originally Posted by jwatte View Post
    In fact, I've never seen these boards with very high current ratings. The highest I've seen is 15A continuous / 30A burst. The TO-220 P-channel power MOSFETS I'm using are rated for a lot more of that (even without additional heat sink) -- but also cost several dollars each :-)
    As Charles G. put it: the design paradigm of "CTRL-C CTRL-V".

    Most of the generic PCM boards use common drain N-MOSFETs as a low-side bidirectional switch, instead of high-side P-MOSFETs with protection components, for simple and inexpensive bidirectional power control (N-MOSFETs generally require less silicon than P-MOSFETs to achieve a given Rds, so usually less expensive and lower capacitance). Also makes it very easy to do PCB layout and heat-sinking since the tabs of DPAK, D2PAK, and several other power packages all connect to the drain, so you can make big arrays of them tab-to-tab where common source requires rotation and/or offsets to keep short and thick common traces.

    Edit: checking another source of PCM shows the MOSFETs as a high-side, common drain, bidirectional switch, so guess there is a bit more variation than I thought.
    Last edited by tician; 12-23-2014 at 01:26 AM.
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
    bleh

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