Can I use voltage regulator for simple solar charger?

Discussion in 'The Projects Forum' started by fireofenergy, Dec 17, 2011.

  1. fireofenergy

    Thread Starter New Member

    Dec 17, 2011
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    Hi, I'm new here (and kinda new to electronics).
    I want to take an ordinary solar panel, say, between 5 and 50 watts, and be able to charge an ordinary sla. However, I want to charge the LiFePO4 batteries as well (not li-ion). They need 3.65v, thus, four in series is 14.6v, almost a perfect match with what the lead acid needs.
    Now, the panel should put out about 17v.
    Can I "just" put a zener or an adjustable voltage regulator somehow in the (very simple) design to stop voltage from ever going past 14.6 volts?
    I understand that at very small currents, this may not work, and that these may not be precise, but am not sure. Especially since when the batteries are almost charged completely, the current will be almost nothing, which may allow the voltage to creep up past 18v or so. Which is OK for the lead acid (as long as its capacity is more than ten times the amps of the panel), but is NOT OK for any lithium based battery.
    I want to use the lifepo4 because of its superior thermal handling and its many thousands of cycles. I also want to use it because it is a perfect match for the load, the Cree XML.
    Also, I wonder if I could use a switching regulator to step down the voltage to that perfect 3.65v (thus avoiding the expense of having to use 4 of "everything).

    The light I'm building would still be bright enough using just one XML...

    In the case of the lead acid, I would need a charge controller because the leds allow the voltage to drop just slightly below what it "likes". (And that is WAY expensive for a light!) However, another reason for the lifepo4 is the fact that the leds will never dim to such a point as to discharge the lifepo4 below 2.2v (per cell) perfectly safe and acceptable... Unless that switching or zener is a "phantom"...

    Thanks
     
  2. fireofenergy

    Thread Starter New Member

    Dec 17, 2011
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    After thinking about it for a while, I would really want to find a switching regulator "deally" that would convert the 17 or so volts from a solar panel into a perfect 3.65 volts, one that could handle like 10A output, to charge a single 10Ah LiFePO4 battery!
    Tell me if this is possible...
    Thanks in advance!
     
  3. Dimitris76

    Member

    Jul 17, 2011
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  4. fireofenergy

    Thread Starter New Member

    Dec 17, 2011
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    Thanks.
    Is it hard to make something like that which could output like 10 amps (I assume it can output a steady voltage even when input is fluctuating)?

    I have a 30 watt solar panel which is like 1.7A max, which when converted to 3.6v is like 7A. I understand ohms law, resisters, a little about transistors, but don't yet know how to make PCB's... I assume it would be much cheaper (and hopefully, enjoyable) to make...
     
    Last edited: Dec 17, 2011
  5. Dimitris76

    Member

    Jul 17, 2011
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    Fireofenergy,

    even if you had a switching power supply able to deliver 10 amps... would you charge your LiFePO4 battery at 1C? How much voltage do you have to apply at the battery terminals in order to maintain a current of 10amps through it? Is it safe?

    I suggest you look at the cell's datasheet.

    Dimitrios

    Dimitrios
     
  6. fireofenergy

    Thread Starter New Member

    Dec 17, 2011
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    They can handle 1C but in this case, will get about 2/3rds of that. I would NOT use any panel larger than 30 watts.
    As for the lifepo4 battery, it's CC/CV. I guess that means at up to 1C, and then make sure that the voltage isn't any higher than 3.65. However, when I look for such protective circuit boards, they usually state "overcharge protection at 3.9v (+ or - a little bit)", even though the same sellers sell the lifepo4 and state "can take charge up to 3.8v"... go figure :rolleyes:
    Anyways, I overvolted a 1,100mAh 18650 up to 4.5v accidentally (using a 6v, .7A solar panel) and it still works (but assume I limited its long term life).
    It seems that they would stop charging (anyways) at 3.6v if that's all the charging current was... Perhaps, I'll set it at 3.5v... still like 95% charged!

    I may chance it, but NEVER with the li-ion because they are not thermally stable!

    Another thought is... NOBODY can tell me if it is safe for the lifepo4 (lithium iron phosphate) to continuously have to be subject to 3.6v at close to 1C, or even C/5 or less for that matter. I assume that the smart charger only applies a very small current, like a float charge, at the end.

    Going with that thought, it should be OK to charge five of 'em in series, since the panel will not put out hardly any current once the batteries are near fully charged. That would be a "perfect" 3.6v if the panel puts out 18v... But then again, too expensive. Could use the small 18650's but then need to use a smaller panel.

    Another thought... I charged a 1,100mAh 18650 with just a (tiny) 4v, 80mA plastic panel. Eventually it does get up to 3.9v even though I have a Shotty diode. I guess the diode doesn't drop hardly anything at all when there's very little current... which brings me to another problem... Perhaps, if left unchecked, any panel would slowly creep up the charge to its stated Open Circuit Voltage of 21.8v... 4.36v per cell... Way too much!

    In this case, would i have to use an adj voltage regulator?
     
  7. Dimitris76

    Member

    Jul 17, 2011
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    1
  8. fireofenergy

    Thread Starter New Member

    Dec 17, 2011
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    The whole idea was to use solar... So I will take a chance on this...
    http://www.ebay.com/itm/DC-DC-Conve...ltDomain_0&hash=item20bf1a5e41#ht_2832wt_1082

    Edit:
    Or better yet, just charge up 5 little lifepo4 18650's using a 30watt panel. Current will "never" get up to the 1.7A max... because of shadows and non perfect conditions. Besides, they say we can fast charge, em.
    When voltage reaches 18.25, there will be "no" current anyways which is 3.65v per cell.

    Here's a little more on the battery
    http://www.ebay.com/itm/6-piece-A12..._Batteries&hash=item2eb496e9d8#ht_5592wt_1082
     
    Last edited: Dec 18, 2011
  9. Dimitris76

    Member

    Jul 17, 2011
    45
    1
    I don't understand what are your going to do with this Step DOWN only DC-DC converter.

    The reason I showed you the other one is because it will deliver 12VDC to your battery charger NO MATTER what voltage your solar panel outputs. More voltage will be stepped down - less will be boosted up!

    Then a commercial charger like this:
    http://hobbyking.com/hobbyking/store/__15379__IMAX_B6_Charger_Discharger_1_6_Cells_COPY_.html

    that can be found in any hobby shop will take care of the rest.

    Dimitrios
     
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