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Thread: Cap or not to cap

  1. #31
    Raw Wave
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    On the basis that you are not overflowing with money, do NOT get a cap. As most knowledgeable will agree, a cap is useless. And if it does do any good, then it is the wrong solution. And as I say, why get a cap when a 12V 0.7AH or 1.4AH battery ($10-$15?) will do much better?
    And since you intend to implement a dual battery setup, why spend money on an "interim" cap?

    The above is merely the "logical" elimination of the cap without the need for detailed knowledge. (Such knowledge being yet more cap-elimination fodder.)
    But for now, you do not have a 3kW system, nor a nearby AGM battery, and do not have any voltage dip issues, so there is no need for a cap.


    SMPS (Switched Mode Power Supply) is the heart of almost all modern PSUs (Power Supply Units) whether AC-DC, DC-DC or DC-AC - eg, AC & DC supplied shavers, screen, phones; dc-dc converters, and dc-ac inverters. Even AC-AC transformers are now often replaced by SMPS.
    A basic SMPS circuit uses high frequency switching of or thru an inductor to transform voltages. It's essentially a dc-dc circuit. AC sources (if applicable) are first rectified from AC to DC. AC outputs are usually obtained from the SMPS DC output (eg - stepped-wave or pseudo-sine inverters).


    For big amps, high power requires more than 12V to drive the speakers. A common "rail" voltage is +/-35V (ie +35V, -35V, and 0V aka GND) - ie, speakers are driven from a 35V or 70V supply, hence up to nearly 5kW into a 1 Ohm load. (A vehicle's 12V voltage is limited to about 200W into 1R.)

    Anyhow, one of the beauties of SMPS is that it can provide a constant voltage output irrespective of voltage input. Even if a car's voltage varies from 8V to 16V, it is easy for an SMPS to hold a constant 3V or 35V 450V output (reaction times are typically well under 50uSecs).
    So if an SMPS's input voltage dips, no problems - it merely increases its duty cycle to draw more current to keep a constant voltage (or power) output.
    So, why does an amp's SMPS not do the same?


    As to caps, IMO if going to that trouble, then definitely SMPS output caps - unless it is known that it is NOT the amp SMPS's inductor or input that is adding to the dip. And since amps apparently do suffer "sags", it is obvious that something is not behaving as it should. Maybe severe input power choking (no pun intended - ie, and inductor is aka a choke or coil etc), or maybe insufficient internal copper on the high voltage side.
    The point is, if dips are occurring thru the amp's PSU itself, then caps are better placed on the output rails. Plus of course they can be a smaller capacitance (because of lower current at 70V etc than at 12V) at the expense of voltage rating (eg, 100V caps), but then lower copper & ESR losses etc.


    If you want to lead the next revolution, split your amp's PSU from the amp. Mount the PSU up front (near the battery and alternator) and run the HV (+/-35V etc) to the amp nearer the speakers. That means at least a 6-fold decrease in current from the front to the back. (EG - from over ~120A down to 20A for 1.4kW output system.)


    PS - headlight dipping should be a legacy of older tungsten & halogen lamps. HIDs and LEDs should not suffer such dipping (they use SMPS! {Else current limiting for LEDs}).
    Last edited by OldSpark; 07-18-2013 at 06:44 AM. Reason: PS...

  2. #32
    FLAC PhilG's Avatar
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    Quote Originally Posted by RAWPWR View Post
    Oldspark,.....I guess they didn't get your memo. lol

    Well, crap, they already drank 3 of them......
    My 2007 Ford F350 Work Log located HERE

  3. #33
    Constant Bitrate
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    OK I have to say ALOT of good info here. Old Spark...can you recommend a good battery to look at?

    Second question...I was just looing at my fuses...I have a 100...150....and a 250...so looks like I will have to grab myself a 200...

  4. #34
    Raw Wave
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    My usual recommendation is a wet cell cranking battery for the main/cranking battery.
    The 2nd battery usually being housed internally - ie, cabin or trunk/boot (if NOT the engine bay) - therefore must be an AGM (sealed) unless it's a wet cell in a sealed vented enclosure.

    The 2nd battery should be another cranker if peak loads are the issue, else a deep cycle if long reserve time without the engine (charging) is required. (Crankers should not be discharged more than 20% & deep cycles not more than 50%.)
    Again, wet cells are more robust but AGMs should be fine for your (relatively) low power audio.

    As to AGMs, from my stateside reading & feedback I'd recommend Kinetik or Deka. Deka has also been recommended here downunder.
    I do not like Optima, nor does my battery supplier who also reckons Odyssey aren't that good - he ditched both brands years ago after high failure rates.

    As usual, the bigger the battery the better (longer life; less strain; etc), but that depends on budget and space. (And as I have often said, a 1.2AH (AGM) battery will crap on a cap, though 7AH was the best bang for bucks (~$25) though the formerly relatively expensive 15AH may now have that honor.)


    And as usual, for battery isolator I recommend the UIBI (the Ultimate Intelligence Battery Isolator - a direct comment/attack on so-called "intelligent" voltage sensing isolators) which is merely a relay controlled by the alternator's charge light terminal/circuit. The UIBI has none of the problems or shortfalls of voltage sensing isolators, and it is (obviously?) cheap being either the same rated relay that voltage sensing isolators use, else whatever relay rating you need. (A smaller relay may be required to buffer limited current D+/L outputs, else a buffer circuit (transistor or MOSFET) which can also be used for newer DP etc & ECU controlled alternators.)
    Of course if you have no D+ or L "charge light" circuit, then voltage sensing is the only commercially available option (ie, for boats, some motorcycles & RVs etc).
    Last edited by OldSpark; 07-20-2013 at 02:51 AM.

  5. #35
    Constant Bitrate
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    Due to space I would like to try and fit it in the engine bay. I guess it wouldn't have to match the car's battery exactly right?

  6. #36
    Constant Bitrate
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    I want to do the dual battery. going to find a way to fit under the hood. I may hook up a cap for a temp. fix since I can get one probably for free...then in the next couple months hook up the second battery...then a swap out of the alternator.

  7. #37
    Raw Wave
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    Why do you want the 2nd battery?
    If it's to run the amps without the engine, fine, but then a cap is no use. And I'd suggest a wet cell since it's under the hood.
    If it's for sags etc, then be aware that it is probably useless so far from the amp. And forget the cap since the main battery is so close by.


    You seem to be worrying about problems you are not experiencing.
    Install the amps and wiring. Adding batts & isolator if needed can be done later - just allow for the amp's +12V run.
    IMO it's pointless discussing caps & (AGM) batts if they can't be mounted near the amps. The reserve or emergency cranking aspect can be done later.


    And no, for auto-isolated systems like this, battery matching makes no difference. Hence the ability to mix a wet cell cranker for best main battery performance & life, and deep or cranking AGM(s) for appropriate audio winch, PC, etc usage. It's only when batteries are connected in series, or when connected in parallel long-term when NOT charging that battery matching is an issue.



    PS - also bear in mind that each 10C rise in temp HALVES battery life, hence the undesirablility of under hood mounting - not that trunks are necessarily better in sunny climates. (I thinks it's 10C for batts; otherwise it's every 15C.)
    Last edited by OldSpark; 07-21-2013 at 06:47 PM. Reason: PS...

  8. #38
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    Well tahts kinda what I am saying...I am waiting for another wire kit to get here...Need 1 more fuse holder and some more length of 4 gauge...so I just ordered a kit. going to install all that see how the 2 amps run...IF I end up having an issue I have a guy I can get a 2 farad amp off for free..put that in until I can get the finding organized for a second battery and all the goodies to go with it. I am not a rich man and when it comes to stuff for my car. I make things happen money wise so it doesn't impact my family. Just my way of having fun but not impacting money I need to take care of my kids and wife...just my thing. I do sid jobs and such. Nothing for my "improvments" comes from out paychecks. So if the cap will hold me over and eliminate the problems for the temp. I will get the second battery. See I have more planned for later...Air horn...the carputer...possible air bag system....I would like to upgrade to LED headlights...I also am a person that over kills everything

  9. #39
    Raw Wave
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    So you want the 2nd battery for cranking battery independent reserve time.
    In that case, though the cap is free, I wouldn't bother - unless it's mounted with the amp (at no extra cost).

  10. #40
    Constant Bitrate
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    I just see no way to put the bettery with the amps Car batteries are just to darn big and I am trying to keep as much of the storage space I can

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