Need help with custom DIY 8.4v power bank out of 18650B batteries

Discussion in 'Power Electronics' started by imamacuser, Mar 11, 2017.

  1. imamacuser

    Thread Starter New Member

    Mar 2, 2017
    I’d like to make a DIY power bank using 18650B batteries to simultaneously power my DSLR, audio recorder, and 7” camera monitor. I know almost nothing about wiring electronics, so I really appreciate everyones help.

    My current plan is to wire six 18650B cells in series (22.2v, 3.4 Ah) and have two 8.4v DC outputs, one 5v DC output, and one DC input for charging the cells. I guess I need to incorporate cells wired in parallel to increase the overall amperage.

    My camera's power cord outputs 9v 4.5 amps, but it then connects to a dummy battery which may or may not step down the volts and amps (the proprietary camera batteries are 7v 1.9 Ah). My monitor’s DC input can accept 7-24v, but it’s power cord is 12v 1.5 amps (specs say that the monitor draws 12 watts). My audio recorder’s power cord outputs 5v at 2 amps. Near as I can figure, I need at least 8 amps to simultaneously power all devices, and I’d like that battery pack to run for at least two hours.

    I really need help calculating the number of cells that I’ll need, as well as schematics to series & parallel wiring combination. I’m also not sure how to wire in a BMS, protection circuit, and over-current and over-temperature protection. I’m also not sure how to wire the outputs; I know that I’ll need a few buck step down converter to get the outputs at the right voltage, but I think I’d need to see some schematics to understand how it’s done. Come to think of it, I’m not sure how to wire the charging input either.

    I’d like to keep everything as small and inexpensive as possible, but I’m curious What parts you guys would recommend.

    What gauge wire should I use? The answers I got from online calculators ranged from 12-18 AWG.

    I already have six 18650B batteries (Panasonic 3500 mah) along with a multimeter and soldering iron. I have a little experience soldering, but I’m not particularly good at it.

    Is there anything I’m overlooking or anything you would recommend doing differently?
  2. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    8amps @2hrs is 16Ah batteries minimum!!
    , so whatever voltage you need put cells in series to create that voltage, and put cells in parallel to create the ampere hour rating.

    So if each cell is 3.4Ah, you're looking at 5 rows in parallel with 6 cells in series,
    That's 30 cells.

    Personally i would use Sla batteries.
    Dr.killjoy likes this.
  3. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    And use DC to DC converters for the lower Voltages.

    While using 18650 cells, recharger should be a balanced type. Should incorporate series cell's temperature monitoring methods and charging varies with battery temp.
    SLA is easier to charge and fairly easy to design too.
    Dr.killjoy likes this.
  4. ian field

    AAC Fanatic!

    Oct 27, 2012
    If nothing else - the SLA is *MUCH* safer than a novice mucking about with lithium cells. Especially series connected ones.

    Balancing is absolutely essential - any dud or failing cells can push more voltage on the others. Terminal voltage is critical - overcharged lithium cells can vent with flaming gas!!!
    Dr.killjoy likes this.
  5. imamacuser

    Thread Starter New Member

    Mar 2, 2017
    SLA batteries are a lot heavier than Li-ion batteries aren't they? I had planned to mount the battery behind my camera on a set of 15mm rails. Basically I'd like to make a scaled down version of this V-Mount battery and this power distribution plate incorporated into one unit. I Believe that V-Mount batteries are 14.8v and use 18650 batteries wired in series and parallel; here's a guy who made a DIY one. I'm going to double check the amp draw of my devices and make sure that the online specs are correct.