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Thread: Secondary Battery Installation

  1. #11
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    Quote Originally Posted by OldSpark View Post
    And with apologies for maintaining this hijack, but a correction....

    canstb - a fully charged 12V lead-acid battery is ~12.7V, not 14.xV. 14V is voltage that the alternator or charger raises the battery terminals to.
    What you say about paralleling of batteries is totally correct. And because both are getting above 13V, it is impossible for one battery to discharge into another (surface charge excluded).
    Thanks for your replay (and support).
    right, I was not clear;
    the voltage level of 14.4V I took as an example in my previous post as "a battery fully charged(14.4V)" is the voltage during charging and charging current almost 0amp. Which btw, 14.4V is actually the highest safe charging voltage for 12V SLA battery. thats the reason I added a note at the end, saying different applications consider different voltage levels as fully charged. (eg. constant voltage charge or constant current charge, float charge or boost charge)
    and yes, after a while the fully charged battery is disconnected from the charger, without any load connected to it, its voltage drops below 14V and after 2-3 hours should settle and keep at 12.7V if the battery is healty and in good condition.

    having done this explanation, I beleive I was again not clear about the parallel connected batteries charged by one alternator;
    let me explain with a scenario; -I will keep the same voltage level as referance.
    if a second, less charged battery is connected parallel with the alternator and the first battery(fully charged, almost 0amp current from alternator to battery and 14.4V), the voltage will drop because less charged battery will act like a load to the charged battery. and alternator will regulate its output up to 14.4V again in seconds maybe shorter time. the charged battery's voltage was 14.4 already, so there will be no/almost 0amp current flow through that battery. but the less charged battery had lower voltage before it was connected, so the alternator will start feeding/charging the less charged battery. so both of the batteries will be fully charged after adequate time.
    If the charging is stopped after both of the parallel connected batteries are fully charged, without any load connected to batteries, both of them should keep their charge level of 12.7 V IF internal resistances of the batteries are same. (same type batteries' internal resistance may vary according to their health and condition)

    I didt mean to support or disagree anyone, I feel quite uncomfortable when forum members start arguing and I seem like to be a side to any of them. I just wanted to share my knowledge and experience, hoping to help finding an answer to the topmost question.

    To keep both of the batteries in the car and safely charge them, I would build a voltage comparator circuit with an op-amp and feed a relay when the primary battery's voltage is above 12.8-13V. 13V is only possible if alternator is charging. and with this relay I would connect both batteries' plus poles to each other (minus poles are already connected to chasis of the car) through a high amp diode so that current only flows to second battery from the alternator, not the other way to the primary battery. this will ensure that the alternator will feed current to secondary battery, but secondary battery will not feed the primary battery in some circumstances. if the alternator is not charging/engine is not running, the voltage will drop below 13V for sure, then this relay would isolate both of the batteries from each other, keeping the main battery charged for the next engine cranking.

  2. #12
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    Alas you support what I have been saying all along...

    Any load change causes a voltage drop or increase that the alternator response to. Modern alternators can respond fast enough to make headlight dimming unnoticeable, but even old alternators usually respond well with one second.

    Yes - 14.4V (2.4V/cell) is the highest charging voltage for lead acid batteries (wet or SLA sealed). Above that, noticeable gassing starts etc. Above 14.4V is used for battery maintenance - eg equalisation & recovery.
    The voltage required to "fully charge a battery" is 13.8V - ie, 2.3V/cell. (But that is insufficient to remove (soft) sulfites.)

    A fully charged battery takes its float current, not 0A (zero Amps). Float currents are typically 100mA - 2A for automotive batteries,.

    Though a (cheap?) battery charger may "stop" charging batteries, an alternator does not.
    Whilst an alternator is operating, it is charging the main battery (etc). Vehicles do not disconnect the alternator from the battery when the battery is full.


    Whether batteries maintain charge or whether one discharges into another parallel battery is independent of their internal resistance. That latter is solely dependent on whether the battery's "internal voltages" match (ie, their OC terminal voltage at any given time).


    Be aware that the above merely adds to your info - it does/should not contradict it.
    I knew what you meant by "fully charged" (whether 14.4V or 12.7V) but some would read your first reply as saying that a fully charged 12V lead-acid battery is 14.4V. (Whereas it should be 12.67V at STP - ie, 25C.)
    AFAIAConcerned, in no way do you disagree what I have been saying for years.



    As to your last paragraph, that's what the UIBI/charge-light controlled and voltage controlled isolator do - they only interconnect batteries when charging - hence no "high discharge currents" from battery(s) to other battery(s).
    However diode isolators are a different and independent issue. But see Simbalage22's problem and other Simbalage discussions for ramblings about that.


    But I see no need to continue this side debate unless there is a supporter for Red.
    Should Red or anyone else wish to continue this paralleling discussion, then start a NEW thread specifically for that purpose.
    This constant hijacking of threads is pathetic.

  3. #13
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    Today I was able to correctly setup my remote line. The second the relay connected the two batteries, they share the same voltage (well at least as fast as I can read both volt meters). I couldnt be happier with the setup. I feel like i have a bit of insurance for the next time my car battery dies. Thanks for everyone's input. Now its time to fix the headlight problem...
    Car PC parts Purchased [90%]
    Car PC software Setup [80%]
    Car Audio setup [60%]

  4. #14
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    Great to hear! Like I said, the batteries will have the same voltage (as the alternator - which will be charging when the batteries are interconnected assuming a UIBI (charge-light controlled) or "smart" (LOL; voltage sensing) isolator).
    And yes, I like the "redundancy" or back-up offered by a secondary battery.

    Let us know how you go with the headlight fix. It's your thread so you could continue any issue here, or start a new thread.

  5. #15
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    The headlight issue has been taken care of as well! I took your advice OldSpark and removed/reconnected while cleaning all of the connections. Well I say it has been taken care of, since it seemed to be random before I can not confirm this for sure. But I have left my headlights in the "on" position everyday for over a month and haven't seen them left on (I now consciously check it before I walk away from my car). Thanks again for everyone's input!
    Car PC parts Purchased [90%]
    Car PC software Setup [80%]
    Car Audio setup [60%]

  6. #16
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    That's great news!!

    And I'm glad again that my advice or opinion solved a problem and saved money. I have often stated that IMO 90-95% of vehicle electrical problems are poor contacts etc. Hence my 2 simple rules:
    - rotate all fuses (of the same size, else remove & refit);
    - disconnect & reconnect all involved connections.

    The above are almost 100% successful for intermittents.
    The break & make usually re-establishes the electrical connection. In some cases extra contact cleaning may be required (or replacement of heated/burnt contacts that have lost their spring & integrity).
    The fuse rotation does the same, but also moves the rare occurrence of an intermittent fuse to another circuit

    Of course that advice makes me incredibly unpopular with auto electricians etc - imaging losing over 90% of that "non-fault" business and being able to add in a replacement part or two! But I'm used to battery suppliers already hating me so wtf. (Not to mention Australian sellers of high-current dc-dc converters, "priority charging" systems, smart isolators... )


    I wish systems like your headlights had a better "fail safe" system (like timers on "frozen" wiper systems) to avoid flatteries etc.

    Whilst IMO a dual battery setup is overkill in case of a once in x-year battery failure (1 < x <10 years), it gives a bluddy nice feeling of security eh? In hazardous environments (remote, arctic, alpine, desert, or wonderland) it can be a relatively cheap lifesaver.
    And it's also protection against most alternator failures (tho that's where IMO a dash voltmeter else voltage alarm is essential!). When you figure out your alternator was dead after stalling with a flattery, you swap over to the spare to limp home etc.


    Anyhow, congrats again.
    I look forward to hearing of future developments.
    Last edited by OldSpark; 08-20-2013 at 07:32 PM.

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