Battery charger

Discussion in 'The Projects Forum' started by crapola, Nov 3, 2010.

  1. crapola

    Thread Starter New Member

    Nov 3, 2010
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    I want to build a wind turbine capeable of charging a 12v car battery. I know a storebought charger I have runs on standard household 110 and uses 2 amps to charge a 12v battery overnight, but I can't figure out what voltage it runs at those 2 amps...110v X 2amps or 12v X 2amps or something else?
     
  2. jatinah

    New Member

    Jul 26, 2010
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    Hi There, The 2-Amps you are talking about sounds like a normal charge-rate for a long-charge. This 2-Amps is the current running thru the battery.

    Most chargers will use a charge factor to get your +12v battery charged up to +13.5 or so, and a car alternator can charge as high as +14.5v.
    Doing the math, your output voltage "x" your output current = Output Power. This equates to ~ 27Watts
    We know there is always a loss from power_in to power_out so we know that the 110vac in will consume more than 27W plus losses, so you'd be drawing at least .3 Amps from the wall for the charging.
    You would want to maintain a correct charging level for your battery, so even tho you take generated voltage from the wind turbine, you will still need to regulate the charging output to +13.5 and limited to 2 Amps:)
     
  3. wayneh

    Expert

    Sep 9, 2010
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    Wind turbines generate an AC voltage. To charge a battery you'll want to rectify that to DC using a full-wave bridge using 4 diodes. Six diodes if you're using 3-phase. Depending on current load, temperature and diode choice, you'll lose about 2 volts across the rectifier. So 16v AC becomes 14v DC. And a lot of the power you capture will be burnt away as heat in the rectifier. Sadly, you'll get no charging whatsoever if the wind drops your output below ~14 volts. And believe it or not, you'll need a fairly large turbine to generate the ~30 watts you need from typical wind speeds.
     
  4. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    You can get up into the hundreds of watts with a 2 to 3 meter blade on a wind turbine. But you need a REALLY windy day. 10-15 mph winds will get you 20-30 watts pretty easy with a 3 meter turbine blade. It would take some first class, high tech, wind generator to get 30 watts or more with under 2 meters span, with a 10 mph wind. How many blades on the turbine and how long are they? What kind of generator are you taking the electricity from? How far is it to the battery you are charging? How do you want to run it?(manual, or automatic charging) Meaning do you want it to taper off current and switch to a float voltage by itself, or with you there to make a switch from mode to mode?

    Detail time.
     
  5. wayneh

    Expert

    Sep 9, 2010
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    Unfortunately, learning the details tends to suck the wind out of newcomers to this topic. (Pun intended, sorry.) There are huge economies of scale with wind turbines, and even a "small" one to charge a battery is MUCH larger than people expect. Folks expect one of those little pinwheel toys will charge their car battery, or more. Newcomers to solar cells are usually disappointed as well, when they find out how much area (and $$) you need to get a relevant amount of power.
     
  6. Kermit2

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    Feb 5, 2010
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    Yeah, Got me a few acres of panels. About 1500 watts worth. The price of the real estate they cover is about equal to what I paid for them as well. ;) But it cuts down on the amount of 'spin' I put on the meter. for a few weeks in spring and fall when the central heat and AC are not running everyday and everyone is away from home at work, the meter goes slowly backwards for a few hours.


    Couldn't tell it from looking at my electric bill though :)
     
  7. Kermit2

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  8. crapola

    Thread Starter New Member

    Nov 3, 2010
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    I'm using a windprop with 4 blades, a 5:1 gear ratio, a permanent magnet alternator that produces 12v DC at 2amps at ~150rpm, and a voltage regulator. I just needed to know the correct voltage for charging the battery, someone told me that just because it's a 12v battery doesn't mean you use 12 volts to charge it. I'm only trying to trickle charge the battery at 2amps as a science experiment. At 5:1 I'll need to get the prop up to 30rpm to get the alternator up to 150rpm. Tips, Suggestions?
     
  9. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    Use a power drill and spin up the generator by hand while metering the voltage output. It should show a significantly higher voltage than 12 volts. The generator will have to output 13.8 volts while under a very low resistance load(a depleted battery). This voltage range (13.4 - 13.8) is a minimum required to charge a 12 volt lead acid. Preferably you want 14 to 15, but with only two amps that won't be needed here. It HAS to put out more than 12 volts loaded. The fully charged 12 volt battery will have 12.7 volts on it un connected. If it puts out even 13.2 volts under a load then it will work as a trickle charge.

    ?
     
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  10. wayneh

    Expert

    Sep 9, 2010
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    I have a boat in my garage, and trickle charge the battery over the winter. The manufacturer told me that a constant voltage regulator - like you might build around a LM317 - should be set to 12.8 volts for this scenario.

    You referred to 2A as a trickle. I guess "trickle" isn't defined but in my thinking I wouldn't use the term above 100mA or so. My charger will go up to maybe 800mA when first connected to a slightly drained battery but usually drops well under 100mA in an hour or so.
     
  11. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    I think you are confusing trickle charge with a battery "maintenance" input.

    Trickle charge is done in a few hours or days, then, if continued would destroy the battery by overcharging.

    What you mentioned, would first trickle charge then switch to a maintenance mode that would not overcharge a battery. One you could leave connected for months at a time. I don't think that is what he has in mind, but he hasn't said one way or the other
     
  12. SgtWookie

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    Jul 17, 2007
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    There are several different phases to properly charging and maintaining lead-acid batteries.

    1) "Bulk charge" phase, where the battery is charged at a constant current until a threshold voltage is reached. For a 12v SLA battery, this would be around 14.7v @ 25°C; automotive batteries generally would be lower. This is frequently stated in datasheets as the "cycle voltage".

    2) "Absorption charge" phase, where the battery is charged at a constant voltage until charge current decreases to another threshold.

    3) "Float charge" phase, where the battery can be maintained indefinitely. For a 12v SLA battery, this would be around 13.7v @ 25°C.

    4) "Equalization". In order to help remove plate sulfation and to stir up the electrolyte, a battery should be periodically charged at the "cycle voltage" for a period of 10 minutes or so, depending on charger current and battery AH rating.

    Many inexpensive chargers/maintaners just use #3; and are not temperature corrected. It's better than nothing, but if the battery core temperature deviates excessively from 25°C, the battery will be undercharged (if colder) or overcharged (if hotter), both of which will affect battery life. Overcharging/high temps are big killers of batteries. So is plate sulfation; many batteries are junked long before their expected service life simply because they were allowed to self-discharge.

    A typical automotive battery will be fully charged at ~12.7v @ 25°C. Plate sulpation begins when the battery charge drops to ~75%, or around 12.5v.

    You can "float charge" an automotive battery at ~13.6v indefinitely if the temp remains constant at 25°C, but it's much better to perform the equalization charge periodically.

    Temperature compensation is most often overlooked. If the battery is in a temperature-controlled environment and its charge/discharge rates are conservative, temperature compensation isn't necessary. If the battery's environment is not controlled and you have significant temperature variations during the year, compensation becomes a concern.

    Batteries represent a large thermal mass, and can take several days to adjust to the ambient temp.

    I posted a spreadsheet in the "Tips and Tricks" thread (reply #36) in the General Discussions forum. You should have a look at it.
     
  13. wayneh

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    Sep 9, 2010
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    I downloaded Sarge's spreadsheet a while back but finally looked at it in detail just now.

    If I read it correctly, when my garage approaches freezing, I should be taking my float voltage (under LM317 control) up a few tenths to ~13.1v instead of the 12.8v given me by the manufacturer. The manufacturer didn't mention temperature, so probably gave me "cool room temp." advice; which matches the table value of 12.79v for 100% charge at 20°C. Adjusting that advice for freezing temps adds ~0.3v.

    Sarge, I don't exactly understand the ~0.7v difference between the recommended float voltage and the 100%-charge voltage. If my charger is set to 12.8v, the current eventually drops to <10mA. Charger disconnected or not, the battery then gives 12.8v. If I boosted the setting up by 0.7v, I would have max current (<1A) for a few hours and then it would drop again. But wouldn't that be overcharged? Or is it simply an acceptable degree of overcharge?
     
  14. SgtWookie

    Expert

    Jul 17, 2007
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    It's typical to maintain lead-acid batteries at a "float" level above the battery's rated voltage. Referencing the battery datasheet should give you that information. You need to enter that data into the spreadsheet, in the yellow cells.

    You should be looking at the "float" voltage instead of the 100% charged voltage.

    The float charge is a "surface charge"; once placed under load, the batterys' surface charge quickly drains off to the 100% charge voltage.

    Lead-acid batteries have a negative temperature coefficient of ~-3mV per degree Centigrade per cell, from 25°C; so a 12v 6-cell battery has a temperature coefficient of (25°C-BatteryTempC)*6*3mV, or TempCoeff = (25°C-BatteryTempC)*18mV.
     
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