Li-ion power pack

Discussion in 'The Projects Forum' started by HKidd, Aug 2, 2014.

  1. HKidd

    Thread Starter New Member

    Aug 2, 2014
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    I want to make a li-ion battery pack, to create a 12v 10A power source, with a PCB to control charging and discharge of the batteries. I intend to charge the batteries while still in the device, with a 12v DC external power source if possible. I am using eight 3.7v 18650 batteries, with two battery packs of 4 batteries each. The two battery packs will be running in parellel, but the batteries in each pack will run in series. This makes 14.8v, which when fully charged would be about 16.8v. What is the best way to transform input (charging) voltage from 12v to the necessary ~17v, and then to transform the output voltage from the batteries down to 12v, for use for my 12v device?

    Can anyone help me? I hope this makes sense, please ask if you need more information.
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Are you aware that lithium batteries need a special charging profile to prevent the risk of fire/explosion? Check out www.batteryuniversity.com
    What is your 'device'? Would it run ok from 11.1V (3 cells in series)?
     
  3. MrAl

    Well-Known Member

    Jun 17, 2014
    2,439
    492
    Hi,

    As Alec has already nicely noted, Li-ion battery cells require a careful charging technique or they can explode and cause fire. These things are much more dangerous than alkalines so much care and planning goes into a charger design. In fact, a good charger will have a back up circuit to check that the primary circuit is not overcharging the cells.

    For the charger you would need a boost circuit followed by (or incorporated into) a current limited voltage regulator to charge ONE cell at a time. If you had three or four wired in series, you'd have to build a charger that can monitor all three or four cells and be able to adjust the charging current if necessary. This is no easy feat if you have never built a charger before.
    Charging the cells in series without separate monitoring is a bad idea because one cell could go much above the max voltage and seriously damage the cell which could damage the whole pack.

    For the driver, you would use a high efficiency buck circuit. This is a step down regulator that will take 14 to 18v or so and step it down to 12v constant with a little loss in efficiency.

    Maybe you can find a charger on the web that does this because i know they make these particular kinds of battery packs, and people must need a way to charge them without building their own chargers.

    You could also find an adjustable buck regulator where you could set the output to 12v with a pot setting.

    All the charging gets a little easier if you can build the pack in modules, where each module is only 3.7v, so that way you can build simple chargers and then reconnect the modules to get the required voltage. Maybe use heavy duty connectors.

    Li-ion cells also have a lower voltage cutout minimum. When they reach a certain voltage like 2.5 to 3.0 volts (depending on the documentation) they should not be used anymore until they are recharged again. Supposedly internal damage can result which makes them more susceptible to internal short which could cause an intense heating problem.
    When used in series, this means if one cell goes lower than the others it may go unnoticed so some means to check for this condition is a good idea too while the pack is being used to power some device.
     
    Last edited: Aug 2, 2014
  4. HKidd

    Thread Starter New Member

    Aug 2, 2014
    3
    0
    I have read the very informative article at:
    http://batteryuniversity.com/learn/article/charging_lithium_ion_batteries
    However, I still do not profess to know a lot about Li-ion batteries.
    I am hoping to use a PCB such as this:
    http://www.ebay.com.au/itm/170680337237?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2649
    to monitor charging and discharging of the cells. This will monitor each cell individually, and will prevent discharge and overcharge, and over current draw. What I need to know. I think this module will cover that worry...?
    The next thing is to find a way to make the output current closer to 12v, and the input current closer to the rated 14.8v or as needed; hopefully from a 12v input.
    The project I have in mind, is to create a fully self-contained li-ion charging unit, that charges from it's own batteries, to 4 usb ports, and a 12v cigarette lighter plug. I have a unit that uses 12v power to power four USB ports. From this, I can then charge phones, cameras, camera battery chargers, laptop chargers, etc.
    I want to put a female power jack on the outside of the box, to plug in either a 12v power source from a car, or a 240v transformer.
     
  5. HKidd

    Thread Starter New Member

    Aug 2, 2014
    3
    0
    I really need help as to what is the best and most compact way to convert voltage up and down, between 16.8v for the Li-ion circuit, and 12v for the input/outputs. The circuit board should cover the protection & maintenance of the batteries.
     
  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,804
    1,105
    There are loads of DC-DC voltage converters advertised. You should be able to find one that can be adjusted to suit.
     
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