using 80650 cells to replace a lead acid battery

Discussion in 'Power Electronics' started by albach, Jun 14, 2018.

  1. albach

    Thread Starter New Member

    Jun 14, 2018
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    Hi guys i recently found my old Energie Station (german brand name form Einhell) which was a 50 Ah sealed Pb gel battery with a charger and some cigarette lighter outlets. I want to do a little water camping trip in the summer and wanted a small reliable energy source for charging my phone and cameras. the Pb battery is dead and now the question arose to replace it witch 18650 cells. I have a few 2p4s packs and 3 of them have the exactly same size a the pb battery.
    My question is if i use the 2p4s cells and connect them in parallel i get 10v if they are completely (2,5v) per cell empty and 16,8 completely charged (4,2V per cell).
    For a cigarette lighter output both is wrong. I also want to be able to charge everything off a 12v outlet at camp stations or my car.
    My idea is to use one stepup converter so get 16,8V for charging to the bms and one boost buck converter to get 12 v to the outlets maybe also two one for each outlet.
    is there an easier option or are my thoughts correct. I will put fuses in front of the outlets so hopefully only the will blow up in case of an overload or short. Charging is no problem because the cells already have a bms fitted to them
    thanks for your help
    Albach
     
  2. ian field

    AAC Fanatic!

    Oct 27, 2012
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    It sounds risky to me - if all the battery packs you intend to parallel are identical, you might just about get away with it. each pack has charge balancing circuitry that can only handle so much current. you need to find out how much, and how to safely share the charge current.

    I'd strip the packs down and remove the original charge balancers, parallel all the sets of 18650 cells and buy a single balancer module that's rated for the whole combined capacity.

    Whichever way you go - you need to balance all the packs by connecting them with current limiting resistors till they settle at the same voltage. Just dumping them together in parallel could cause entertaining amounts of equalising current to flow.
     
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  3. albach

    Thread Starter New Member

    Jun 14, 2018
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    thanks for your reply what is the difference of 5 identical or one big BMS the batteries are all identical. from the data-sheet of the batteries i know each cell can deliver 5 A each so 2p 10 per unit and i will use 5 of them in parallel so theoretical i could draw 50 amps out of the battery pack i will at most draw 20 A, 10 A per cigarette outlet so i think i will be safe my question is about the dc dc converters or if there is an more elegant solution a phone charger might work with more or less voltage but my small air compressor does not want more than 14 volts or it will not work properly
     
  4. albach

    Thread Starter New Member

    Jun 14, 2018
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    one further question i found the cells (cgr18650CG from panasonic) original data-sheet (http://www.meircell.co.il/files/Panasonic CGR18650CG.pdf) which specifies a 430mA discharge current bus also shows the discharge carracteristics with 4.3 amps per cell on some onlinestores they specify them as 10 A cells but down in the specification max continuous discharge current is given at 430 mA
     
  5. ian field

    AAC Fanatic!

    Oct 27, 2012
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    Same idea for my 12V tyre pump - I might try a bog standard laptop pack and see what happens. As yet, no plan B if it doesn't like the lower voltage.
     
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