6V flashlight conversion to re-chargeable lead-alum

Discussion in 'The Projects Forum' started by djbuggybee, Jul 23, 2013.

  1. djbuggybee

    Thread Starter New Member

    Jul 23, 2013
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    Hello everybody, I am working on a new project inspired by alternative energy solutions! My idea is to take an ordinary 6v lantern and convert it to a rechargeable lead-alum and 12v LED with an integrated solar panel (9-12v, 0.05-0.1a) and 12v in for charging as well regulated 20w 12v out. This is to take camping, to use as a flashlight and backup 12v power source.

    Here is what I have come up with so far:
    6v 2aH lead-acid (Sulfuric) converted to lead-salt (Alum) 'crystalline' battery
    9-12v solar panel to charge up the battery in the sun
    A regulated buck-boost converter; ~1-12vin/vout, 20w (Preferably a charge pump)
    12v green LED bulb and base
    banana plug sockets for 12vin/vout
    adjustable brightness

    Alright, now let me go into some detail:
    First off, the battery is a lead-acid converted to lead-alum, which exhibit around 4.8v standing voltage and 3a peak current at a fresh charge. In great depth of discharge, below 2.4v, the current drops to around 0.05-0.075a. As it goes, these cells do not 'sulfate to death' during deep discharge and also re-charge quite effectively. The typical power is around 1w continuous. The battery seems to prefer charging at a rate of 12v 0.1a.

    The 12v regulated output and 12v LED bulb were chosen for extensibility of the system. It is my desire for the flashlight to double up as a portable power source. The losses should be minimized absolute. When using this unit, it may not have a consistent supply voltage and re-charge cycle. Keeping this in mind, I would like to be able to use it even when the battery cell drops below 1.5v. For this, I will need a buck-boost DC-DC power converter that can accept a ~1-12vin, 12vout and up to 20w power dissipation, even if I never end up placing it on that kind of load. Also, the power supply mode should work even when the LED bulb is off, by flipping a bypass switch or something.

    I choose 12v LED green color because of the increased visibility of surroundings at night. Red is also an option for night eye considerations. Regardless, the plan is to use an ultra-bright 12v LED auto bulb; for simplicity. Of course, I would also like adjustable dimming (and possibly strobe function?).

    I had some questions I hope some of you might be able to answer. For example:
    What is the best way to provide stable 12vout from a fluctuating 1-12vin that has 20w rated power dissipation and is efficient as possible? I am thinking charge pump, because there is minimal magnetic inductive losses, but where to find one/how to make one with the handling capability?
    How does one create a buck-boost regulated switching capacitor charge pump from scratch?
    How can I protect all the components from damage without loosing power?
    Can anyone recommend a 12v solar panel that has 5cmX10cm dimensions?
     
    Last edited: Jul 23, 2013
  2. wayneh

    Expert

    Sep 9, 2010
    12,145
    3,055
    You need to work out your math. You can't get 25W out of a 1W battery. You might need 30 of them to ensure a "guaranteed" 25W.

    A 5X10cm solar panel is nowhere near large enough to charge a system that is expected to deliver 25W, even if you have more hours of sun than hours of usage. If your camping is anything like mine, full sun cannot be taken for granted. A rule of thumb is that, for whatever power demands you have, you'll need double that at the panel. For instance, if you need 75W•hrs (3 hours of 25W) each day, your panel likely needs to deliver 150W•hrs per day. That might include a cloudy, rainy day with maybe only 2hrs of equivalent full sun.
     
  3. djbuggybee

    Thread Starter New Member

    Jul 23, 2013
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    I do understand that 1w continuous does not require a *20+ power rating, but its peak power is 4.8v*3a=~15w, and there is a higher voltage with the panel in the sun. The reason I want the system rated for 20w is because I will be hooking up other smaller voltage panels (less than 12v) to it to provide regulated 12v out during camping. In the case of the other panels, I have one right now that I built that is 1.5v 5a, and I plan to build another one once I fond a suitable buck-boost power regulation scheme.

    Oh yes, I totally agree on doubling up the watts with solar power, especially if you are charging lead acid, which loose ~30% during charging anyways. Lead-alum batteries are much more efficient when applying charge. Once graphene solar panels are 60% efficient and cheap enough, you wont have to double up anymore!

    So maybe I will rephrase my question, and ask if anybody knows; where to find/how to build an efficient regulated buck-boost capacitor charge pump that has some moderate power handling (1-12v 2-20a @ ~20w)? Does it require an IC?
    If not a capacitor charge pump, then what about an efficient regulated buck-boost circuit that can handle a wide voltage and power range? (1-12v 2-20a @ ~20w)? I am thinking along the lines of an xtra beefy joule theif/blocking oscillator to supply power from a fresh and "dead" battery, as well from a variety of different panels.
     
  4. wayneh

    Expert

    Sep 9, 2010
    12,145
    3,055
    I haven't done much searching for DC-DC converters but I think you just need to go shopping for one that matches your needs. There are many ICs available (check out TI, for instance) and many of those are used in pre-built boards you can find on e-bay. Building one yourself would be a last resort, but in that case you can shop for an IC to handle most of the work. It will require a few external components which will typically be detailed in the data sheet.
     
  5. djbuggybee

    Thread Starter New Member

    Jul 23, 2013
    5
    0
    Thanks for the responses so far!

    I have looked on multiple websites (Integrated, maxim, TI; to name a few) and have not found any DC-DC converters that can handle 20w power dissipation with the characteristics I described. I figure I will have to build my own charge pump using a power switching transistors with a timer/signal generator and voltage monitoring to achieve my end goals; one of which is a buck-boost converter suitable for adapting the varying power sources in off-grid installations. Sounds fun!

    Maybe once graphene technology develops to proliferation, the power handling of IC's will improve. Until then, it looks like its back to the drawing/dev board. I wish that companies were not so anti-trust and would release detailed schematics for circuits so somebody like me could use them as a blueprint to create a robust unit. A pipe dream, I know.

    Hopefully by the end of this I have something that will fit inside a flashlight that can be used to supply cell-phone chargers and the like with regulated 12V from "any power source". Such a regulation scheme could prove quite useful to folks down the line.

    I will keep you posted on discoveries and progress as I go along, and post schematics/pictures at the end of all this. As always, I am welcome to your feedback.
    Talk to you later!
     
    Last edited: Jul 30, 2013
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