Lipo battery discharge project

Discussion in 'The Projects Forum' started by dawg, Nov 16, 2009.

  1. dawg

    dawg Thread Starter New Member

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    Hi everyone.

    This is my first post, I hope you don’t mind me making some mistakes along the way or ask stupid questions

    I am trying to make a battery discharger for r/c lipos.

    It’s purpose is to cycle new batteries and discharge any unused batteries for storage..

    Lipos are the biggest thing since sliced bread for radio control flying things.

    Anyways, there are times when I need to discharge the batts, and since they are high capacity no battery charger with discharge capabilities can load up more than a few amps, if that.

    So, I am making a high capacity discharger for batts from 2s (2 cells) all the way up to 8s (8 cells)

    The lipos have a min discharged voltage of 3.0 v/per/cell up to 4.2 volts/p/c. ie: min = 2s x 3 = 6v, max = 2s X 4.2 = 8.4v

    When using lipos, I discharge the lipos up to 80% capacity.

    Therefore 80% of say 5000mAh is 4000mA. This is what manufacturers say to discharge them to. More than 80% and its no good for the batt.

    The discharge load will consist of 3 x 55w halogens in series (36v) and 3 x 12v fans also in series but across 7.4v minimum and 36v max..

    Here is the control circuit

    [​IMG]


    Now comes the bit I need help with. My math is bad so please keep it simple math wise.

    Each lipo has different low voltage so I need to be able to select battery cell count.

    These I like :
    LM339 or similar
    TL431 or similar

    2s = 7.54 v ~ 8.4 v
    3s = 11.31 v ~ 12.6 v
    4s = 15.08 v ~ 16.8 v
    5s = 18.85 v ~ 21.0 v
    6s = 22.62 v ~ 25.2 v
    8s = 30.16 v ~ 33.6 v

    I am thinking Fig 2 or Fig 3 is a cell count circuit which can display an led when the voltage is within its voltage range to show the number of cells
    Both circuits are from the makers Datasheet

    Fig 2
    Using 2 x TL431

    [​IMG]

    OR

    Fig 3
    Using 2 x TL431

    [​IMG]
    Eg:
    3s lipo min discharge voltage of 11.31v up to 12.6v fully charged.

    The next lipo say 4s has minimum discharge volts set to 15.08v.

    This allows a gap of around 3v so it shouldn’t be hard to have a circuit show the number of cells in the battery.

    Simply put, the cell count circuit shows a labeled led array the correct cells for a battery.

    This concept is fine but I would need 6 of these circuits calibrated to the specific lipo cells. 2s to 8s..

    Which ever lipo is connected will only switch the relevant led automatically while the other circuits remain OFF.

    ******************* Well this is where I’m stuck ******************L

    Is there a better way to identify 6 different voltage ranges (using led display to show the cell count) ?

    ==========================

    Stage 2

    Voltage cutoff

    When I plug the battery in, an LED lights up to show the cell count, 2s ~ 8s

    So when I press a button for 2s ~ 8s, the circuit will stop discharging at :

    Button 1 2s = 7.54v
    Button 2 3s = 11.31 v
    Button 3 4s = 15.08 v
    Button 4 5s = 18.85 v
    Button 5 6s = 22.62 v
    Button 6 8s = 30.16 v

    To prevent the battery from over discharge, some sort of voltage alarm or cutoff is needed

    [When the lipo voltage reaches its minimum voltage (see above) it enables a relay and opens a n/c contact (RL3,C1) and stops discharging.]

    I have seen many voltage cutoff circuits.

    Is there also a more efficient way to monitor a number of voltages, instead of using 6 x voltage cutoff circuits ?

    =============================

    Other thoughts

    1
    A transistor circuit might have a too wide switch zone where it might cycle the alarm cutoff on and off.
    This is because when the battery load is removed the battery will increase in voltage. (not much though [0.2v –ish]

    2
    Picaxe also comes to mind ????

    3
    A common relay for the voltage cut off circuits ie: (RL3,C1) normally closed

    4
    A common buzzer or siren for the cutoff circuit

    5
    Op amps are ok I guess but I would rather stay with comparators or zener shunts if relevant


    Any thoughts greatly appreciated. J
  2. Audioguru

    Audioguru New Member

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    A Lipo battery does not need to be discharged for "conditioning". It has a limited number of charge-discharge cycles and wasting cycles just makes it go bad sooner.
  3. dawg

    dawg Thread Starter New Member

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    Hi,
    that's fine, it's that I would like to have them discharged before storage..

    Sometimes I don't fully use up the lipo and would like it to be in storage charge to make it easy on the battery..

    Well I guess I'll have to stop cycling new batts slowly.
  4. Bernard

    Bernard Senior Member

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    Are all of the batteries 5 Ahrs??
  5. Bernard

    Bernard Senior Member

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    Last edited: Nov 20, 2009
  6. SgtWookie

    SgtWookie Expert

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    I wouldn't discharge batteries connected in series, even if they're all the same part number made by the same manufacturer.

    As batteries age, they will have different charge capacities. If you try to discharge them in series to a specific voltage level, you will wind up discharging some of the batteries more than others.

    I don't recommend connecting batteries in parallel, unless they are charged to the exact same voltage, or connected by a resistive load. If you connect two batteries together in parallel that are at a different voltage level, one battery will try to charge the other battery at a high current level. This will cause both batteries to heat up, and possibly rupture.
    MMH likes this.
  7. dawg

    dawg Thread Starter New Member

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    Hi,
    thanks for the replies,

    the batts cells are exactly the same volts when charged, and this happens always when charged up.

    I monitor the cells during discharge

    I check the imbalances every time they are used.

    On new packs the cells are < 1 ~ 2 mΩ

    Used cells after many hours are within 2 ~ 3 mΩ of each other after sitting around for a while.

    And that is what I call a 'bad' pack..

    Lipos not used for a bit require a few 'cycles' brings All the cells into perfect harmony. ie: reduce differential resistance per cell..

    that's why I need to 'cycle' them.

    I do not want to simply charge them and use them straight away as the internal cell resistance are 'fighting' the pack..especially when things are sucking over 30 amps continuous



    The lipos are from 2s to 8s and vary from 2000mAh up to 5 amps hr

    I charge my batts at no more than 0.5c

    I baby thems lipos and have been using some packs as old as 3 years, and show no sign of stopping..

    Those 'old' packs are used around 180 ~ 200 times per annum.

    Most lipos I have are all series.

    regards,
    Adrian
  8. bountyhunter

    bountyhunter Well-Known Member

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    I believe that is backwards. Storing a rechargeable battery in a discharged state usually screws them up over time.

    Ditto on the fact that it is dangerous to deeply discharge cells stacked in series: the lowest cell will go dead and reverse polarity in the string and be destroyed. You have to monitor EACH cell continuously when doing this and stop discharging when the lowest cell reaches it's end of life voltage.

    EDIT TO ADD:

    I didn't realize LiPo was "lithium polymer". I worked with Valence Technology here in San Jose some years back (mid 90's) which was the first commercial company who made LiPoly batteries. Those batteries were VERY sensitive to discharge, and in fact theirs would get hot enough to catch fire if ischarged too deeply because they would grow internal shorts and the cell would dump quickly and burn itself up. Their first major contract was batteries for the Army's night vision goggles and the batteries were burning up. AFAIK, they never found a solution to that problem.

    Be VERY careful with them as they contain enough energy to generate a ton of heat instantly if there is any kind of a problem with the cell or the load.
    Last edited: Nov 18, 2009
  9. Bernard

    Bernard Senior Member

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    Might be dooable-but messey[ lots of parts ]. Use a LM 3914N as LED display volt meter, in dot mode; outputs feed LED's and inverters, into D latches. Selected latch closes switch,4066, connecting battery via V divider's to a comparator. Comp. output disables constant current discharge ckt. when battery falls to 40% charge V. The circuit cannot determine A hr rating of battery, so set to lowest capacity to serve all batteries, or hand select discharge rate. Each LM.. step represents one cell. so that #8 step=8 cells, adjusted to represent a 50% charge V. A deeply discharged battery might give an eronious cell count. I think it would be easier to look at the battery and turn selector switch to proper cell count, set discharge current, take a long walk.
  10. dawg

    dawg Thread Starter New Member

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    quote
    I think it would be easier to look at the battery and turn selector switch to proper cell count, set discharge current, take a long walk.

    Yeah, good idea, thanks Bernard.

    [I will make your circuit anyways, for learning something new..] : )

    quote
    Be VERY careful with them as they contain enough energy to generate a ton of heat instantly if there is any kind of a problem with the cell or the load.

    I accidentally shorted a screwdriver with the terminals once across 25v, no more screwdriver. These lipos can produce over 200 Amps burst for 30 sec.

    The current is spooky.

    Maybe I'm over designing this ???
  11. Duane P Wetick

    Duane P Wetick Active Member

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    Its probably a good idea to get yourself a good regulated laboratory bench power supply and adjust things yourself based on your experience. Keep a chart of your settings nearby and reference it regularly (memory fails) as there is a real danger of explosion or fire! And don't forget to get your test equipment calibrated on a regular schedule.
  12. dawg

    dawg Thread Starter New Member

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    Yep, sure thing..
  13. jpanhalt

    jpanhalt E-book Developer

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    Don't your packs have a "balancing and charging" plug? If so, your charger can tell how many cells are there by which leads have power to them.

    BTW, there is a small typo in the link given by Bernard. The "www" needs to be added. I second the comments against discharging.

    More than half of our club members fly electrics, and of that that number a large proportion fly only electrics. Not a single member uses a homemade charger. LiPo's are great batteries, but they have to be treated with respect. Ruin one 3S, 22C, 2200 mAH pack (bought in the US) and you have pretty much paid for a decent charger-balancer. Prices are coming down sharply on the batteries, particularly from Hong Kong, but I think trying to design and build something like this from scratch is false economy.

    John
  14. dawg

    dawg Thread Starter New Member

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    false economy ??

    Huh ?

    Why so ?
  15. thatoneguy

    thatoneguy AAC Fanatic!

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    You don't want to do this at all if you do not have a pack wired for balanced cells, and even then, i'd strongly suggest a battery analyzer for the purpose.

    Even a Triton for $100 will do what you are looking to do, though you should be doing it balanced, not by selecting how many cells and simply doing a charge dump. That will unbalance the cells faster than either standard use or normal charging.

    If you do not have a balance-connector cell set, the current draw should be extremely minimal. Attempting to rapidly discharge the battery "just to make it dead" has sold more rechargeable batteries of every chemistry than simply using them ever would.
  16. Audioguru

    Audioguru New Member

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    I salvaged Li-Ion cells from a laptop pc, a portable DVD player and a modem. I matched them and use them to power my electric RC model airplane. I charge two cells in series from a 9V/500mA wall-wart that is about 12V when it has a low current load and it feeds an LM317 adjustable voltage regulator that is set to exactly 8.4V so each cell charges to 4.2V. I charged them many times and their balance remains perfect.
  17. thatoneguy

    thatoneguy AAC Fanatic!

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    Have you performed high current drains to run them dead after each use?

    That is the aspect I am referring to, especially when 3 or more cells are involved. If one is healthier, it will hurt the weakest cell in the pack. Normal current drains, using them until low, but not to "dead" last much longer than a pack run dead quickly after each use for the sake of running them dead.
  18. Bernard

    Bernard Senior Member

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    Since i've had a pair of LM3914's for 30 years, thought it tine to bread-board one[ $1.96, A20315 The Electronic Goldmine ] The circuit is suposed to discharge @ 1A, Li batteries, automatically selecting no. of cells & stopping discharge @ 40% charge. Problems: sometimes possible to have two cells selected if exactly on an input transition V; best to have input rivider set to slightly lower than 10 cell 40%V to insure that cell selected stays on after discharge is reached or a new cell will be activated causing further discharge. The red LED V ref. for constant current will go off when discharge is reached, & battery drain will drop to about 100μA. Any questions-just ask.
    PS: Scaling resistors are approx. & consist of 100k pots and a resistor. A well regulated V supply is necessary for accuracy. Pots about $1, for 20 turn , also TEGM.

    Attached Files:

  19. Audioguru

    Audioguru New Member

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    My airplane disconnects the 6A motor when the two cells drop to about 6.8V (about 15 minutes) then the battery still has enough power to activate the servos and power the radio receiver. The motor is very hot and the battery is hot. After they cool the battery recovers and the airplane flys a few more times with the same battery for many more minutes. Each flight from the same battery is shorter. When the airplane can no longer make one circle around the field then I put that battery away for charging and use another freshly charged battery.
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