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Thread: LifePO4 without BMS / Balance connectors

  1. #1

    Default LifePO4 without BMS / Balance connectors

    Hi there,

    Build a 13.2V 1400mAH LifePO4 pack today and i was wondering if i do need balance connectors for a LifePO4 pack ? During charging, the cells where all the same voltage and don't get hot at all.

    Max powerdraw i've done is 8.5A so far ( rated for 14A continuous ). Don't know how much current the nickle strips can take ( did solder it with some soldering liquid and tin ).
    Just need a small battery pack for hobby reasons

    .
    I know i need proper connectors for the pack, since it can short circuit this way.



    btw: Got a package from china with a flashlight and 2 yellow ultrafire 5000mah cells, obviously fake ones and rapper says: Sheef Life .

    Battery life is not the greatest ( about 1.5 hours before they go dim. Power draw from the flashlight is 450ma on full power. How much lumen will that be ? It very very bright.
    Proud to be dutch

  2. #2

    Default Re: LifePO4 without BMS / Balance connectors

    I'd strongly recommend that you add either the BMS or balancing tap connector as the older the LiFePO4 cells get, the more out of balance they actually will get.

    Never ever rely on the knowledge that they will auto-magically balance on their own - this is one expensive mistake you would ever make since LiFePO4 cells are considered the specialty Lithium-ion batteries and should be treated as such if you want them to last a very long time - after all, LiFePO4 cells are technically still expensive.
    ****** Custom filtered blacklight flashlight based on both Nichia NVSU233A-U365 and Convoy S2 flashlight host (Pictured in avatar icon) ******

  3. #3
    BVH's Avatar
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    Default Re: LifePO4 without BMS / Balance connectors

    If you use and charge Lithium cells in-Series, use balancing.
    WWII 60" Carbon Arc (Sold), 1.6KW NightSun, 1KW VSS-3A, .8KW TrakkaBeam, 600W M-134 Light, 500W X-500-14s, 500W Starburst, 500W A120b, 450 Watt AEG German Leopard 1 Tank Light, 300W Locators, Megaray, 150W Communicator, Maxabeam Gen3, Pichel 75W Mini-Novas

  4. #4

    Default Re: LifePO4 without BMS / Balance connectors

    It all depends on currents and capacities, lower capacity and current cells are safer in terms of ka-boom, but as already said, balancing is best solution for almost any battery chemistry (have no idea, why we never see balancing chargers for Pb batteries?)

  5. #5

    Default Re: LifePO4 without BMS / Balance connectors

    Due to their very low IR LiMPo4 including LiFePO4 are more tolerant of over-voltage than other chemistries, but it is a very good idea with any lithium chemistries. I am getting away with it with one of my 20Ah 12.8v packs but I check the individual cell voltages with a multi-meter. So far it only needed balancing once. but I check at every charge.

    LifePO4 does not need full BMS, PCM is sufficient. In fact single cell over-voltage limiters (base a comparator or a Shottky diode) and an overall over-discharge cut-off works fine.

    Some people have done fine without any management by undervolting. Because the discharge curve is so flat with a voltage peak in the last few percent capacity, you loose very little capacity with this and some heavy RV and boat users have reported they can tolerate "float" if you back off on the voltage.

    BatterySpace.com has a wealth of info on LiFePO4 control and some good products. I have no affiliation just an occasional customer.

    Mike

  6. #6

    Default Re: LifePO4 without BMS / Balance connectors

    Actually, some PCM (Protective Circuit Module) are actually full-blown BMS, as is the case with my LiFePO4 battery pack I bought from Bioenno Power, and I would recommend BMS over PCM if you are to expect that they last over 5 years which is not a big deal for the Lithium Iron Phosphate batteries, the software in BMS has to ensure the batteries are healthy enough for the load you're about to throw onto - it's not absolutely required but is better for long term battery life so long it's in service. If you want either BMS or PCM, try to find one that has extra low leakage current, so you don't have to deal with the mysterious battery pack death.

    And always check the battery pack with either PCM or BMS to see if they have sufficient charge - if not, charge it back up to 40% - I usually charge mine to 100%, as it doesn't matter to those LiFePO4 cells, they already sit at much lower voltage so electrolytes don't have a chance to fall apart like they do with certain Lithium-ion batteries.

    Still, building the battery pack has to do with your choice of hardware. You're actually free to decide what you want to do with it.
    ****** Custom filtered blacklight flashlight based on both Nichia NVSU233A-U365 and Convoy S2 flashlight host (Pictured in avatar icon) ******

  7. #7

    Default Re: LifePO4 without BMS / Balance connectors

    Quote Originally Posted by Dr. Mario View Post
    Actually, some PCM (Protective Circuit Module) are actually full-blown BMS,
    Agreed. It is available.

    Mike
    Last edited by Arizona_Mike; 05-15-2017 at 03:17 PM.

  8. #8

    Default Re: LifePO4 without BMS / Balance connectors

    So far, there are two type of PCMs; dumb and intelligent. Intelligent PCM actually do top balancing and circuit breaker function in the software, or in conjunction with the hardware Lithium-ion battery multi-function protection chips via I2C bus, all managed by slightly larger chip (or small packaged chip) which is the brain of the operation, a microcontroller. ARM ones tend to be even more efficient than both PIC1x and AVR families used to be common in the battery packs, as they use 130 - 45 nanometer node, quite beneficial for the battery pack. Dumb PCM doesn't have CPU of any type, rather the finite state machine manage all the battery charging process, and balancing is done totally passive with Zener diodes, resistors and transistors.

    Building your own PCM is very tricky as even a component make a big difference between properly functioning LiFePO4 battery pack which lasts 20 years or so, and the one that catches on fire upon first recharging cycle. Not to mention tricky software development (FreeRTOS is a popular choice of the embedded OS for the DIY intelligent PCM), which safety is the priority.
    ****** Custom filtered blacklight flashlight based on both Nichia NVSU233A-U365 and Convoy S2 flashlight host (Pictured in avatar icon) ******

  9. #9

    Default Re: LifePO4 without BMS / Balance connectors

    Never do things in software, which can be done in hardware.

  10. #10

    Default Re: LifePO4 without BMS / Balance connectors

    Quote Originally Posted by CuriousOne View Post
    Never do things in software, which can be done in hardware.
    I'd politely disagree, however true to a certain degree. How do you keep note of battery age then? Many laptop battery pack do all the functionality in software nowadays, with hardware as a last resort. Smartphone also do all that in software too, so it doesn't make it less safe, you just gotta do it right.
    ****** Custom filtered blacklight flashlight based on both Nichia NVSU233A-U365 and Convoy S2 flashlight host (Pictured in avatar icon) ******

  11. #11

    Default Re: LifePO4 without BMS / Balance connectors

    Quote Originally Posted by Arizona_Mike View Post
    ...Some people have done fine without any management by undervolting. Because the discharge curve is so flat with a voltage peak in the last few percent capacity, you loose very little capacity with this and some heavy RV and boat users have reported they can tolerate "float" if you back off on the voltage.
    Mike
    Mike, can you elaborate on undervolting and how it's done to keep a lifepo4 pack balanced?
    Last edited by filibuster; 05-16-2017 at 11:26 PM.
    Lantern Light Output Comparison Review:
    http://getpreparedstuff.com/led-lantern-review

  12. #12

    Default Re: LifePO4 without BMS / Balance connectors

    Is there balance leads to buy from China to solder ? Average capacity on a cycle is about 1250mah. I charge it with 3 amps. Gets it full in about 25~30 minutes from empty ( about 10~11v ).


    Edit: Just bought balanceleads for 4S Config from GearBest.
    http://www.gearbest.com/rc-parts/pp_26868.html
    Last edited by daan_deurloo; 05-21-2017 at 07:38 AM.
    Proud to be dutch

  13. #13

    Default Re: LifePO4 without BMS / Balance connectors

    Quote Originally Posted by filibuster View Post
    Mike, can you elaborate on undervolting and how it's done to keep a lifepo4 pack balanced?
    Basically undervolting in a battery can allow minor unbalancing to occur without overdriving the higher voltage cells--until the next manual check and manual re-balancing. There have been several writeups by RV and boat folks on the web, mostly using alternator/regulator setups that are not optimized for LiFePO4. Personally I recommend a minimum of a cell-parallel PCM arrangement.

    Mike

  14. #14
    Flashaholic
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    Default Re: LifePO4 without BMS / Balance connectors

    Just remember that the oft-repeated saying that LFP is tolerant of over-voltage ONLY means that they don't immediately go kaboom like other lithium chemistries, but you ARE damaging the heck out of it if the over-voltage is reasonable. Example: if you forget that you don't take LFP above 3.6v per cell, and charge it accidentally to 4.2v - no drama, but rapid cell damage / aging occurs.

    A simple 4S battery can be used, with the proviso that you aren't using CRAP. Each cell must be measured and vetted to be close to each other in both capacity and internal resistance. Wiring infrastructure must also be sound and not contribute to the resistance. (Ie, no steel washers, magnets, or other ho-key diy setups).

    This matching procedure, and initial balance, is part of the routine for LFP batteries such as motorcycle starting like Shorai, Antigravity (starter only, not the other stuff like jumpstarters which DON'T use LFP!). Yes, some manufacturers use internal balancers etc, but that is to protect itself from the non battery-savvy average consumer - which we aren't.

    LFP cells will charge to full if a cc/cv routine is used with a CV value of anywhere from 3.45 to 3.6v per cell. The amount of "absorb" time to full takes longer at 3.45v than it does 3.6v.

    Ideally, start out with closely matched cells, charge each one to full. An initial individual charge of 3.6v, at less than 0.3C (to avoid phase delay) until current naturally drops to about 0.05C to perhaps 0.025C would be considered fully charged. Assemble into the 4S configuration with GOOD wiring.

    Under NORMAL use, drop the overall voltage to no more than about 14.1v. If you have the luxury of time, then 13.8v will do the job - it will only take longer to absorb to full. STILL, keep an eye on it, and make sure no cell exceeds 3.6v at the end of charge. If it does, then you have problems - bad wiring, CRAP cells, etc.

    No need to charge to full every time! Here, the simplest measure is to just stop charging once it reaches the CV voltage point, or perhaps pull it before the absorb stage finishes.

    This is the issue with RV'ers who rely upon basically Pb chargers - those chargers typically want to finish an absorb cycle, which is not necessary if you don't want to be charging to full each and every time. This is also what many don't understand - that ANY CV voltage from 13.8 to 14.4v for a 4S pack will charge to full if absorb is given enough *time*.

    From a smart battery enthusiast standpoint -if you are safe about it - well matched quality cells, individually charged to full at the outset, can be charged as a "two terminal" battery anywhere from 13.8 to 14.4v, preferable the lower end of the scale of no more than 14.0v with an attendant longer period of absorb if you really need to charge to full.

    OF COURSE use some sort of low-voltage disconnect! 12.5v under a reasonable load (like less than 0.2C) can suffice, and still leave you with about 20% capacity remaining. Actual capacities depend upon the load current, but at reasonable draws like 0.2C or less, this is in the ballpark.

    Don't use CRAP cells, or cells with an unknown history of storage / use!

    If you research LFP threads going back to the origins of time (circa 2007 or so), many LFP projects go south due to this. They bought "used" cells. Or they did something stupid - let's bring it to our level:

    Dumb project seen #1:
    Assemble a 4S pack of LFP consisting of three 26650's, and one 14400 sized cells. Can you top-balance this with all the custom circuitry? You sure can. Will it be useful? Hardly - that 14400 will get hammered. This is the extreme end of why one needs to use closely matched cells in both capacity and internal resistance to be able to pull off a simple 4S setup.

    TIME IS A KILLER:
    Reasonable balance (no more than 0.070v [70 millivolt] per-cell difference) is fine. But some go nuts with bleeder/balance circuits that add 5-10 minutes each and every cycle trying to get the cell voltages to 0.00001v as if that actually meant something in overall capacity. Over time, each 5 minute balance cycle holding one or more cells at high voltages takes it's toll. Oh no, not right now - but down the road. Might as well have stayed with lead-acid's cycle life. Gotcha!

    Note that I am NOT anti-balance under the right conditions. Next door neighbor / grandma / kids etc prone to abusing batteries? By all means.

    Just be on the lookout for unscrupulous vendors who use the bleeder/balance circuits as a way to stuff CRAP cells inside their boxes, rather than use quality.
    Last edited by IonicBond; 05-23-2017 at 04:06 PM.

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