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Thread: Why do car batteries not overcharge??

  1. #1
    Join Date
    Jul 2011
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    Default Why do car batteries not overcharge??

    HI Forum

    We have someone interested in batteries and an inverter for a small boat (the battery is to be for stationary type use, not for starting). The trouble is they want to charge the batteries from the boat alternator.

    I have only worked with solar and wind chargers and am not sure what the requirements would be for an alternator to give a good charge to a solar-type battery of say 200Ah.

    And I was thinking that in a car, the alternator gives around 14V to a 12V battery. This i suppose is equivalent to a bulk charge from a pv system. However, there is no charge controller.... so how do you stop the battery from overcharging once it's reached near full charge?? How do these alternator systems work?

    Thanks
    Larry

  2. #2
    Join Date
    Jun 2008
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    Default Re: Why do car batteries not overcharge??

    Yes, there is a "charge controller" on car alternators.

    In the olden days, it was a separate voltage regulator box, now it's usually built into the alternator. The purpose of the regulator is to increase or decrease the electricity sent to the field windings of the generator, thus increasing or decreasing the power output. It does this by monitoring battery voltage.

    Good little tutorial on alternators: http://www.autoshop101.com/trainmodu...or/alt101.html
    20-175watt Solarworld panels, mounted on two home built single axis powered trackers, 10-245w Solarworld panels, mounted on home built dual axis tracker, 1 MX60, 1 FM60, 1 FM80, 24v bank of Absolyte 2v AGM GNP II cells 1200amp/hr, 2-GTFX2524 2500w Outbacks producing 120/240 AC, Trimetric monitor, Grid tied, produce about 600kw/hrs/mo. 12/2012: Added 10 Solarworld 255w panels and 10 Enphase mirco inverters.

  3. #3
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    Default Re: Why do car batteries not overcharge??

    firstly, there is a regulator on most alternators to prevent overvoltage. many alternators can put out voltages in the area of 100v or so without regulation (don't quote as i am not possitive of that even though it's high).

    as to the car batteries not overcharging i already said it's because it is regulated, but many regulators for car alternators (and i guess boat alternators too) can come up short as some stay in the 14.1v or 14.2v area. most batteries need at least 14.4v or more to get a full charge to them. it will charge up most of the way if low in volts, but being it's consistently undercharging it could set it up for sulfation due to deficit charging. all cars i've had (except my current '96 taurus that i have not checked and is the newest car i've owned) have been about 14.2v or lower. i'm not sure of all makes and models or current millenium makes and models so take this with a grain until you measure the voltage for yourself.

    edit to add---i believe the reason many regulators were put so low was because there's a variety of batteries out there and they are scared to overcharge rather than undercharge them. also note that voltages for a cold battery should be higher yet and is one reason many failures occur in the winter with the battery getting even further undercharged and not just the cold robbing you of capacity.
    NIEL

  4. #4
    Join Date
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    Default Re: Why do car batteries not overcharge??

    i should also add that if you do charge another battery that the batteries should be isolated from one another. it's a case of different batteries with different ages and characteristics don't you know. anyway that is usually accomplished with diodes (battery isolators) and diodes generally put a voltage drop to the batteries and can cause a deficit charge to occur even if the voltage from the alternator was good in the first place.

    another option is the ability to just switch the batteries with a heavy duty switch like this one from blue seas,
    http://www.solar-electric.com/miblseabases.html
    NIEL

  5. #5

    Default Re: Why do car batteries not overcharge??

    I recently bought a cigarette lighter type voltage meter. It has a 3 digit LED display which is easy to see while driving in my Toyota Tundra work truck. I've noticed the votlage getting up to about 14.2 when I first start up and after a half hour or so it settles down to 13.8-13.9 range. If I stop briefly, like at a gas station, it usually starts back up in the 13.8 -13.9 range.

    I think it may be more of a tempurature correction issue than a multi-stage charge deal. I believe the voltage regulator is built into the alternator so it does heat and cool in close conjunction with the engine temp.

  6. #6
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    Default Re: Why do car batteries not overcharge??

    Haven't had reason to check the voltage in the car I have now, but all the cars I've had in the past, regulated at 14.5, perhaps because of the cooler climate here in Canada.
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  7. #7
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    Default Re: Why do car batteries not overcharge??

    Quote Originally Posted by SolaRevolution View Post
    I recently bought a cigarette lighter type voltage meter. It has a 3 digit LED display which is easy to see while driving in my Toyota Tundra work truck. I've noticed the votlage getting up to about 14.2 when I first start up and after a half hour or so it settles down to 13.8-13.9 range. If I stop briefly, like at a gas station, it usually starts back up in the 13.8 -13.9 range.

    I think it may be more of a tempurature correction issue than a multi-stage charge deal. I believe the voltage regulator is built into the alternator so it does heat and cool in close conjunction with the engine temp.
    Older auto alternator regulators only have a single temp compensated voltage usually in 13.8-14.2 volt range. Most newer autos have two stage voltage regulation. The speed at which it switches from 14.2-14.5v to 13.8-13.9 vdc depends on how fast the battery voltage rises after starting car. Having headlights on immediately after starting can make it stay at 14.3 vdc a bit longer then it would normally.

    Alternators are self limiting on current. The field windings approach saturation near the rated current for a given alternator. The very old (pre-mid 60's) brush based D.C. generators have regulators that contain both voltage and current regulation. A common alternator test by techs is to fully ground the alternator control field winding which causes the alternator to produce its full output current capability, pretty much whatever battery voltage is at. This is okay for short period to check alternator. Do this test on an old generator based system and the generator brushes will light up like an arc welder.

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