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  1. #1
    Reputation: tigris99's Avatar
    Join Date
    Aug 2012

    Why protection/balance on 18650 cells?

    I'm setting up to build and 8cell pack (set so double capacity over 4 cell, not for higher voltage is the plan)
    But what I don't get is why all the extra bs in building a pack. Balancing makes sense but not a need. Circuit protection, do u plan on stabbing or submerging the packs???

    I understand all this for lithium polymer batteries (used to be huge into brushless rc) I have charging pouches, smart charger, u name it cause I've blown one up before, they are nasty. And lithium ion battery I've tried to blow up and ffs takes a lot to get one to go up. 18650 is the same as a-123 Li-ion cells used for lithium cordless power tools. 6cells in my dewalts no balance and just the charging safeties. Lasts plenty long, just a bigger version of 18650 batteries. If it wasn't for the fact of cost I would use 4 A123 cells instead.

    So what is the huge deal with all this stuff on bike lights??? As long as u have a good rc smart charger and not building ur own packs if u have a lack of patience and common sense I don't understand the big deal.

    Not being a jerk or anything, really want good factual information as to the need for all that so if I see it is all needs I can assemble my pack in that fashion.


    Sent from my HTC Sensation Z710e using Tapatalk 2

  2. #2
    mtbr member
    Join Date
    Sep 2007
    No sure where you got that info, but 18650 LiIon is definitely not the same as A123 LiFePO4. In any case, both will benefit from balancing and at least the li batteries of my 18V Bosch tools have balancing contacts and balancing charger.

    Balancing is needed in case the cells in the battery are not drained at same rate. The difference can be small but it will get worse over time and in the end will lead to failed cell(s). If you balance the cells (i.e. make certain that the voltage of each cell is exactly the same when charged) you can avoid the issue almost entirely. This will extend the life of battery pack and the added cost is relatively small as balancing leads cost practically nothing and these days even the most basic/cheap "smart" chargers have balancing functionality.

    Edit: I don't personally see the need for protection on the pack itself as I'm using TaskLed drivers which have voltage monitoring.

  3. #3
    mtbr member
    Join Date
    May 2011
    We all have our own levels of comfort. A pcb is intended as a backup that, assuming all goes to plan, should never be needed. In other words a part of a well designed system, but as you say not the only part. Quality cells that are matched, good charging, discharging, physical protection & waterproofing are all part as well.

    There are documented instances of bike packs venting with flames, which appear to have been the result of a number of shortcomings, soft shorts, poor charging, faulty pcb & water ingress.

    Balance is a different issue. Out of balance is a symptom of a problem, that balancing will not cure. Eg if due to age or other fault the cells no longer have the same capacity, they will be out of balance - either at the top of the charge or on discharge & no amount of balancing will change that. Being able to check that your cells are still in balance is a good thing though.

    You could take a chance on any of these things, but why would you?

  4. #4
    Reputation: tigris99's Avatar
    Join Date
    Aug 2012
    ok ya I was wrong on cell types, my bad. but i guess for the little extra cost for peace of mind would be worth it.

    That being said the light I plan on purchasing (for the housing) is the chinese knock off dual XML u2 thing on ebay for $50 then going to redo all the internals. LEDs themselves probably fine pending on how they look during use for how much they light up my path, but new controller (forget what its called), self built pack, etc. I have the $25 knockoff from amazon single led thing just for commuting and such atm that will be also redone and used as a helmet light since im seeing the point behind having one for the trails.

    Thanks for the help, seems I misread some things now that info has been corrected ill plan accordingly.

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