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Thread: Cap or not to cap

  1. #41
    Raw Wave
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    Since your 2nd battery is for reserve capacity and not to fight voltage dips, it doesn't matter.
    Just remember that if you want to fit a cap at the amps, a 1.2AH or 2.4AH etc AGM is probably smaller. (In that case, I'd probably mount a temp sensor rather than an isolator.)

    PS - since it's engine bay mounted & for reserve, I suggest a wet cell. It could be the same as your cranker unless you want a deep cycle, or a different size.
    Last edited by OldSpark; 07-22-2013 at 07:10 PM. Reason: PS...

  2. #42
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    Ok - Seriously your have to be one of the most knowledgeable I have talked to on this stuff...I am an advanced-novice like I said. I know enough to build a system but not as much as most. I keep picturing my second battery being the size of a regular battery. That's where I am running into problems. I need to see one of thses batteries your talking about. And I would like to see your setup also.

  3. #43
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    Here is a link http://www.atbatt.com/sealed-lead-ac.../m/12v-2ah.asp to 2.0-2.9AH think of the batteries in a computer UPS. I would try and find a ups that has electronic problems and still has good batteries and use one of those, might be cheaper and save the planet at the same time. SNO

  4. #44
    Maximum Bitrate rray's Avatar
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    Here is a picture of what I did. It's not the prettiest setup, but functional:




    What I have is a Stinger SPV35 35AH sealed battery, and the GM 750CCA vented battery connected through a 200A battery isolator. The stereo, (and carpc when I hook it back up) runs from the Stinger when the car is off, and the battery isolator relay turns on when the engine is running for charging. It all sits in the trunk where a spare tire would normally go.

    The subs fire into the trunk above the battery area:



    And on the other side of the Subs is where all the stereo stuff is mounted:

  5. #45
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    I actually have 3 UPS' at home that last I knew work fine. Will check into that.

  6. #46
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    UPSs often use 12V 15AH or 18AH batteries though smaller UPS may use 12 7AH. (AGM of course.) They probably tend to be "cranking" type rather than deep cycle since they have a high discharge rate - ie, discharged in 10-30 minutes.


    rray - IMO your set up looks good, though that may be with my "functional" bias. I was worried that the yellow SPV35 was a "(semi-) sealed" wet cell, but it is an AGM (dry cell) which can be mounted on any angle (though often not upside down but that is solely due to construction reasons - the plates might fall "down").

    Is your isolator a UIBI - ie, controlled by the charge light? And if so, is the smaller relay a buffer for the 200A relay/solenoid's coil (which probably draws 1-2A)?


    DracoPCGuy - I don't have any set up per se - at least not for audio. I have a mere Alpine HU with its power taken direct from the battery, though with my new Alpine CDE-121 (with USB) I had to relay-switch its IGN +12V (else constant +12V) to prevent the ~120mA drain when both const & IGN +12V were connected to the battery (as per my traditional set up). That relay is triggered by IGN or ACC but can also be switched to batt +12V for any time listening without the IGN key. I also have the relay in an optional latching mode so that once on in IGN/ACC mode, the Alpine stays on even with IGN/ACC off.

    I do have a 2nd battery isolator circuit which was charge-Light controlled though at the moment it's controlled by a tachometric fuel pump relay. It's a typical 555-timer based circuit. It has an initial ~0.8 second turn-on for priming at power up, and then is on only when ignition (else injector) pulses are present. It's probably a 321 906 059C or 3.300.300 or similar.)
    The tachometric aka fuel pump relay is not ideal since it parallels the batteries when cranking (unlike the charge-Light controlled UIBI), but that's not a worry since my interconnect wiring is more than capable and I have 50A self resetting circuit breakers at each end. Not that I am aware of those CBs ever tripping during cranking, but I do have a reduction starter motor that only draws ~140A.
    The fuel pump relay drives the fuel pump and my battery isolator relay which is 60A rated, though sometimes I use a 140A rated relay.

    My 2nd battery (when fitted) has varied from a 40AH or 80AH wet cell, or a 38AH AGM UPS battery.
    My secondary loads have merely been an old 8A cooler and now a newer max 2.5A fridge. (With one exception when I ran an electric winch off the 2nd batt when bogged some years ago.)
    It has also been used to charge whatever 2nd battery has been used in my tent whilst camping to drive lights & fridge (if I haven't set up solar panels etc).
    Since changing from 30A interlink fuses to self-resetting breakers, I have had no problems.


    BTW - 1.2AH seem more common for 6V though I have specs for Yuasa 12V 0.8AH, 1.2AH, 2AH etc.
    They are (LxWxH mm) 96 x 25 x 62; 97 x 48 x 55; & 150 x 20 x 89 respectively.
    That compares to the better "bang for buck$" 7AH & 12 or 17AH of 151 x 65 x 93; 151 x 98; x 93 & 181 x 76 x 167 respectively.
    That's in case you want to compare cap or available volumes etc.

  7. #47
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    oldSpark. the staff that PWROQR (or whatever:P ) sed in his first post,are all completly right, and cover most of the things needed to know.

    OF course there is a change in ressistance. it is called Impendence, and is to AC what Resistance is to DC. only diference is, it is dependend on the frequency of the Current. So when the bass drops, the impendence goes up, and so on.. and by god i dont think that a car battery could do the job of a capacitor. at best it would explode. Capacitors can discharge ALL their current at once. thats what they are for. Batteries are nowehre near there.

  8. #48
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    Ok, I'll rephrase what I said. The impedance does NOT change with varying (voltage &) current. Just like a speaker impedance doe not change with volume (current).
    { I used to use the term "impedance" for speakers and rippled DC loads but people complained, so I changed to their "resistance" terminology. I just can't win can I? }

    Batteries are no different to capacitors in terms of discharge - that is a function of the voltage drop & their path resistance. EG, if the cap or batt both have an ESR of 5mR and are at 14.0V and the amp "drops" to 13.9V, then an "extra" current of I = V/R = 0.1/.005 = 20A flows. For a 1V drop it'd be 200A.
    The reason they say caps discharge ALL their current at once is because they have so little current to discharge (compared to a battery). It's like saying that when cranking an engine, 0.8AH battery discharges all its current at once compared to a 40AH or 80AH battery.
    And that's the point - a cap does not have the (extra) reserve that a battery has. A typical automotive battery is the equivalent of hundreds of thousands of Farads.

    And a battery won't explode. Do you know of any that have exploded when supplying cranking or winching & amp currents of hundreds of Amps? No even a 0.8AH battery will explode in that situation. However large caps are liable to explode when initially charged (eg, from 0V) if they are not current limited.


    There is more to it than the simple example I have given, but since people aren't arguing at that level, I'll omit such discussion. But if they do, I'll tackle them at that level.
    But if people are arguing that things "discharge slower" because of their ESR (ie, that electricity travels slower through a circuit resistance), then they are way below even a basic level of understanding of electricity. (I mention that because that's what many arguments seem to infer.)

  9. #49
    Maximum Bitrate rray's Avatar
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    I just used a 30A relay to turn on the 200A relay when the ignition is on for battery isolation.

    Sent from my SCH-I535 using Tapatalk 2

  10. #50
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    Quote Originally Posted by OldSpark View Post
    Ok, I'll rephrase what I said. The impedance does NOT change with varying (voltage &) current. Just like a speaker impedance doe not change with volume (current). {)
    WHAT? that is the whole meaning of impendence. http://en.wikipedia.org/wiki/Electrical_impedance
    ZL is the impendence for inductors and ω stands for 2πF it DOES change with varaying current, and there is absolutly the slightest conversation that can be done against it.

    Quote Originally Posted by OldSpark View Post
    Batteries are no different to capacitors in terms of discharge . {)
    Of course they are. Voltage across battery poles, depends on its charge. the less charge left, the less volt, hence, slower discharge. AND YES, i have seen batteries explode, because as anything else that conducts electricity, they heat up. if you try to draw LOTS of amperes, then the chemicals inside act as a cable and BOOM. (if you dont bealive me, then just sort circuit a battary). Capacitors on the other hand drop voltage only after almost all their curent is drawn. that is becouse they are not designed for the pathetic 12v, but 200 or more. And they are small, becouse that is their job. they dont need to have more capacity. Its enough for each seperate bass drop, and they need to be easily rechargable, so they can supply the next bass. a battary would propably just die.

    Not to mantion that a capacitor will clean the system from any noise that the altarnator may produce, (that is the reason most altarnators arledy do have a capacitor on them), where a battery will most probably die.

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