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Thread: Having two batteries, one solely for start-up, the other deep cycle -- relay switch?

  1. #1
    Constant Bitrate doncarbone's Avatar
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    Having two batteries, one solely for start-up, the other deep cycle -- relay switch?

    Hi, I'm new to this and have virtually no experience working with batteries/ignition, but wat I would like to do is have some sort of a relay that in layman's terms tells battery A (my starter battery) to take a breather once the car is started, then have a deep cycle battery B in the trunk give juice to audio/carputer/lights/in-car electronics

    I am not quite sure if this is the ideal set-up and whether or not my OEM alternator ('03 mitsubushi lancer oz rally) can handle this extra load, but if so, can anyone here recommend a product i can use to facilitate my request?

    Here's my set-up and requirements:

    Focal 165A1 Components - Max power 120W // Nominal 60W
    Focal 165CA1 Coaxials - Max power 120W // Nominal 60W
    Focal 25A1 10" Subwoofer - Max Power 400W // Nominal 200W
    JL Audio HD900/5 900 Watt Amplifier (100 watts RMS x 4 at 4ohms + 500 watts RMS x 1 (at 1.5-4 ohms)

    Then there's a touch screen 10.2", a small HP netbook (powered through a 300W inverter), and just a small cigarette lighter charger for my Blackberry

    Do I even need to consider the parallel battery set-up? Does it look like I need to upgrade my alternator or should I be fine with what I've got (basic run of the mill battery, OEM alternator)?

    Thanks

  2. #2
    Raw Wave
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    No.
    All you do is keep the two batteries separated when the vehicle is not charging.

    When it is charging, connect the second battery.
    I do this with a relay controlled by the alternator's charge light circuit.
    People that don't have charge lights, and other suckers that are stupid enough, buy a "smart battery isolator" which is a voltage controlled relay. (IE - voltage is above 12.5V or 12.8V or 13.5V and the batteries are connected; it disconnects below <whatever> voltage (which is lower than the connect voltage); and uses various time delays.)


    The practice is to keep your main battery always connected. There is no need to disconnect it. (Only the other battery(s) when not being charged.)

    And forget stuff you read about connecting a battery oncethe other is full or nearly charged - that's all colorful bullsh to blow money from suckers.


    Search my posts for more details.
    Don't forget to have protection (circuit breaker or fuse) and EACH END of the battery interconnect.


    PS - you only need an alternator upgrade if it doesn't handle the total load. The extra battery shouldn't change that (each fully charge battery only adds 1-2 Amps - ie, 12-30W).

  3. #3
    Constant Bitrate doncarbone's Avatar
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    I appreciate your time in helping me, sir!

  4. #4
    Raw Wave
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    Don't call me 'sir' - that reminds me of Scatterbrain's 'Don't Call me Dude!'.
    OldFart will do nicely. (wink wink)

    I tried searching for a reasonable post on the issue....
    All I found was How to hook up extra battery? (Reply #8) which has an old diagram and 2 links to elsewhere; also Layout / Power (Reply #7) where I probably try to counter "complicated wiring" issues; and some general background to adding batteries in Battery set up, how do I exactly do it?.

    I thought there were newer posts & diagrams, but maybe they are on other forums....


    But if you have any questions or arguments, just ask etc. (Arguments as to reasons against - whether your own or others....)

  5. #5
    Constant Bitrate doncarbone's Avatar
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    Quote Originally Posted by OldSpark View Post
    Don't call me 'sir' - that reminds me of Scatterbrain's 'Don't Call me Dude!'.
    OldFart will do nicely. (wink wink)

    I tried searching for a reasonable post on the issue....
    All I found was How to hook up extra battery? (Reply #8) which has an old diagram and 2 links to elsewhere; also Layout / Power (Reply #7) where I probably try to counter "complicated wiring" issues; and some general background to adding batteries in Battery set up, how do I exactly do it?.

    I thought there were newer posts & diagrams, but maybe they are on other forums....


    But if you have any questions or arguments, just ask etc. (Arguments as to reasons against - whether your own or others....)
    OldSpark, what is it that you do for a living out of curiosity (or long ago before you retired...heh)

    Can you give me an idea of the equipment, wiring and the amount of $ required to do this?

    How can I locate the alternator's "charge light circuit"?

    Thanks

  6. #6
    FLAC
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    why would only stupid suckers use smart isolators? how is running the trigger off the alternator charge light much different then going with +12.8v and above for a trigger? if the alternator dies, there is no way a battery running an engine will stay above 12.8v for even a minute or so... so its not like there is a risk of shortening your fail-safe drive home. am i missing something?

  7. #7
    Constant Bitrate doncarbone's Avatar
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    Quote Originally Posted by trader007 View Post
    why would only stupid suckers use smart isolators? how is running the trigger off the alternator charge light much different then going with +12.8v and above for a trigger? if the alternator dies, there is no way a battery running an engine will stay above 12.8v for even a minute or so... so its not like there is a risk of shortening your fail-safe drive home. am i missing something?
    i think its the fact that smart isolators are very expensive as opposed to OldSpark's solution which is much cheaper.

    im not entirely sure though, waiting on the old guy to give me some info on required materials/pricing.

    an additional question (related) - what battery do you recommend for the secondary battery? a deep cycle optima, or a stinger...maybe a kinetik?? is deep cycle the way to go?

  8. #8
    FLAC freeflashstuff's Avatar
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    The charging light in most vehicles comes on at a certain voltage in the 12VDC range, IIRC. So it pretty much does the "smart" isolator's job when you tie into it.


    Sent from my iPhone using Tapatalk
    I have bad luck with vehicles...

  9. #9
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    Charge lights are rarely voltage sensitive, they are usually controlled by by the alternator charging whether its output is 1V or 15V.


    The problem with voltage sensing isolators is the complexity in avoiding complex situations.

    Generally speaking....

    They all require minimum on times, minimum off times, and delays before switching on or off (if not included in the on & off time).

    Then there is the voltage threshold issue...
    Do you want it on at 12.5V or 13.5V?
    Should it switch off at 13V or 12.5V?
    (Hence why all should have adjustable thresholds - preferably for both else a fixed hysteresis.)

    You may not see the need for the above. I do.
    But the charge light overcomes all that. It doesn't care about surface charge, extended sags etc - if it is charging, the batteries will be interconnected.

    And having analysed problem & failure modes, I too concluded the charge-lamp solution to have far fewer risks & problems.
    (EG - one I have at the moment is a charge light that extinguishes as soon as the engine spins, hence joining batteries during cranking. Although that is actually a benefit, it is not the usual practice for smart isolators. But it is easily defeated.)

    doncarbone - the charge light circuit is easiest found on the alternator. Unless it is a newer DP type alternator (that interacts with the vehicles EMS), it should have at least one wire - namely the D+ to the charge lamp. (That's in addition to the heavy+12V "B" or "B+" power to the battery and its chassis ground.)
    If it is a multi-wire connection, then it's usually labelled "L".
    The charge lamp (circuit) often controls electric fuel pumps or fuel cut-off valves in carbied vehicles.
    And of course the charge lamp (circuit) goes to a lamp in the dash.


    Apart from the batteries and the cable and the 2 breakers/fuses, the cost is that of the relay. IE - $5 for 30A, $15 for 60A-120A; $25 for 200A-400A - noting that a small relay ($5 15A-30A) may be required to drive a BIG relay - the alternator's (regulator's) charge Lamp circuit may not be able to supply enough current for a BIG relay solenoid.

    Keep in mind that a "smart isolator" can have the same type of relay, but it adds voltage sensing circuitry (with hysteresis and time delays) instead of using the alternator's signal.
    (Isn't it strange - people going to all that trouble to try to determine if the system is charging? It's almost as if they are trying to make money...?)


    As to battery types, that depends in part on the system (eg, the main battery is a cranker for cranking; the 2nd battery would be a cranker for audio surges else deep cycle if deep flattening is required); whether external or internal/vented may determine wet/flooded versus sealed AGM/VRLA.
    Despite my love for AGMs, I have decided to use two identical deep-cycle wet cells in my 4WD. They are 110AH & ~780CCA, but the 2nd will be mounted under the body (ie, externally). My desire is a solar when long-term stationary; otherwise redundancy, refrigeration, and winching as required. Their $200 was far cheaper than AGM equivalents.


    As too my living... LOL - I just applied for the dole (welfare) after several years not working. My last employment was in IT, but I have a long history with vehicles and electrics/electronics. And a longer one for quashing falsities.

  10. #10
    Variable Bitrate
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    I don't think I've owned a vehicle WITH a charge light in quite some time. They all have 'Not Charging' lights. Aside from that, the cost of the automatic charging relays is not THAT expensive - I think the Blue Sea one runs about $65, is rated for 120Amps, and is very ruggedly built.
    2000 Ford Expedition Eddie Bauer - Bi-Fuel Gasoline/CNG
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