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Thread: Secondary battery?

  1. #11
    Nic
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    Quote Originally Posted by falconey
    Running this in parallel is the wrong thing to do. Too much strain on the alternator trying to keep both batteries at the same capacity. Over time you will crap your alternator by doing this. Also if one battery is dead, both batteries are dead.
    ^^^ biggest load of crap ever

    like was said before using a relay is a good option, you dont get the voltage drop of a battery isolator.

  2. #12
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    I have done the same thing as Kingmissel and had it set up like that successfully for a very long time. As far as I can see it really is the only way to do it. hooking the batterys in parallel will increase the capacity but they will obviously both go flat at the same time. (mine runs a fridge and an inverter though, not my box)

  3. #13
    Maximum Bitrate falconey's Avatar
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    Quote Originally Posted by Nic
    ^^^ biggest load of crap ever

    like was said before using a relay is a good option, you dont get the voltage drop of a battery isolator.
    Sorry I meant to say running them in parrallel without an isolater. Use a huge relay if you go this route.

  4. #14
    Low Bitrate philstuf's Avatar
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    Quote Originally Posted by falconey
    Running this in parallel is the wrong thing to do. Too much strain on the alternator trying to keep both batteries at the same capacity. Over time you will crap your alternator by doing this. Also if one battery is dead, both batteries are dead.
    Falconey is right about killing the alternator, even if a large relay is used.

    I put in 7 years in a car audio/electronics shop. I cannot count the number of times I have had to replace/upgrade factory alternators simply because the customer's addition of a second battery caused the load on the alternator to increase. Normally, the customer opted for removal of the second battery, or upgrading the alternator to a dual output (or high output w/isolator). In a few cases, they opted to repeat their mistake, and I wound up seeing them again in just a few months.

    A lot of cars with factory high output alternators will not see this trouble for a while. Many cars now have alternators coming with 130+ amp alternators, and they usually take the load in stride. Even mine is 140 amp, and it was built in 1995. Even with a large alternator, they are not up to the task of charging 2 batteries at a time, as well as powering the electrical system of a car.

    Many people will argue that a large, powerful stereo is worse on the alternator. If played at 100% volume 100% of the time with "all bits high" music, yes, it is. But the load of a stereo in almost all circumstances is transient: It is not full tilt all the time. Even a "bass" cd only plays max volume in intervals. Sure, you pull up to the stoplight, blast the guy next to you, then roll on. You just pulled a load for a few seconds or minutes. No big deal. But that second battery is pulling a load the WHOLE TIME the car is running. Not every now or then, not even 50% of the time, but 100% of the time the engine is running.

    The alternator may not die this year, but it will have its life significantly shortened. The bigger ones just take longer to do so.

    Falconey hits the mark on this one.

    -Phil
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  5. #15
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    What will happen to a 100A alternator if you connect like 25 60W lamps (=1500W = 125A@12V) to the car's electrical system (assuming the wiring can handle it)?

    The answer is nothing. When the alternator exceeds it's power supplying capacity, the battery will provide it. What will happen is that the car will eventually stall when the battery runs flat (in that case, 2 hours with a 50Ah batt).

    So, with two batteries, each one will only be charged with half the current (assuming both are the same model and capacity and are on equal condition). No damage to the alternator. It's like replacing, say, a 50Ah battery with a 100Ah one. The storage capacity will increase, not the batteries "charge consumption". I agree that driving the alternator to it's limit all the time might reduce it's life (more heat on the rectifiers and regulators, etc), or better put, not using the alternator to it's full capacity will increase it's life, but doing so (driving it to the limit) should not cause such premature failures.

  6. #16
    Maximum Bitrate falconey's Avatar
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    Quote Originally Posted by crisr
    What will happen to a 100A alternator if you connect like 25 60W lamps (=1500W = 125A@12V) to the car's electrical system (assuming the wiring can handle it)?

    The answer is nothing. When the alternator exceeds it's power supplying capacity, the battery will provide it. What will happen is that the car will eventually stall when the battery runs flat (in that case, 2 hours with a 50Ah batt).

    So, with two batteries, each one will only be charged with half the current (assuming both are the same model and capacity and are on equal condition). No damage to the alternator. It's like replacing, say, a 50Ah battery with a 100Ah one. The storage capacity will increase, not the batteries "charge consumption". I agree that driving the alternator to it's limit all the time might reduce it's life (more heat on the rectifiers and regulators, etc), or better put, not using the alternator to it's full capacity will increase it's life, but doing so (driving it to the limit) should not cause such premature failures.
    I agree with you to a point. Here's where I disagree: A second battery is another load on the alternator. If his second battery is something small just to survive cranks that's one thing, but adding a second battery like a yellow or blue top should be a last resort. There are people using way more power intensive sound systems than just a mere carputer can produce, that don't have to resort to a second battery. One thing about audio systems though is the current draw isn't constant, it's in and out with volume(high to low) and bass hits. A second battery on the other hand is a constant draw. You have to factor in that the life of your alternator would be reduced. That said as long as you're using an isolater, everything is set up properly and the alternator is putting out 14.4V at 2500 rpms then you shouldn't have too many issues. I would say, though if you check the readings on the alternator before the second battery and you aren't getting at least 14-14.4V at 2500 rpms you better worry about the charging system before adding a battery. Basically in short I'd just buy a yellow top or red top as a starter battery and either of those two should be able to handle whatever you throw at them. Although not cheap or easy dual alternators is always better than dual batteries, because again a second battery is just another load while a second alty is another generator.

  7. #17
    Jesus Freak antimatter's Avatar
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    Damn.... Good discussion. I am still trying to sort out my power problems. One day i will have a computer that will survive crank.
    -Jesus- King of Kings Lord of Lords

  8. #18
    Maximum Bitrate eugenen's Avatar
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    The only problem I see falconey is the second battery won't always be pulling a heavy load. If/when its discharged then it will put quite a load on the alternator, when it gets charged up the load will decrease. I added an ammeter on my old truck and could watch the current draw start out at 20-30A after I started it and gradually die down to just a couple A after a few minutes. If I were to hook up another dead battery (say I jump started someone) I could see a 30A draw and it would gradually go down as the second battery charged. Basically the same thing happens with dual batteries and an isolator. Might be interesting to setup a relay controlled by a time delay or even a current sensor to the second battery would only be connected after the load on the alternator has went down. A current limiter or trickle charging circuit would even work better, real expensive systems use a fancy charge controller to do this.

  9. #19
    Maximum Bitrate falconey's Avatar
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    Quote Originally Posted by eugenen
    The only problem I see falconey is the second battery won't always be pulling a heavy load. If/when its discharged then it will put quite a load on the alternator, when it gets charged up the load will decrease. I added an ammeter on my old truck and could watch the current draw start out at 20-30A after I started it and gradually die down to just a couple A after a few minutes. If I were to hook up another dead battery (say I jump started someone) I could see a 30A draw and it would gradually go down as the second battery charged. Basically the same thing happens with dual batteries and an isolator. Might be interesting to setup a relay controlled by a time delay or even a current sensor to the second battery would only be connected after the load on the alternator has went down. A current limiter or trickle charging circuit would even work better, real expensive systems use a fancy charge controller to do this.
    Great line from 'Catch me if you can'..."I concur"... Seriously I agree, but he's talking about using the extra battery so he can have the carputer on for periods of time when the car is off. There in lies the issue. It will be a discharged battery. My opinion is if you want the car off and carputer on, just get you a yellow top(very slow discharge). No need for a second battery.

  10. #20
    Maximum Bitrate eugenen's Avatar
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    Ok, just wanted to make sure. I'm planning to use mine for my carputer, but its very low curent (I've ran the thing off a little 12v wall wart in the house with no problem)

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