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Thread: 2nd car battery

  1. #11
    Constant Bitrate
    Join Date
    Sep 2009
    if anyone is interested in knowing more about comparing complete dual battery management system, i just did lots of research into this topic:

    and the whole thread where that post is has valuable info:

  2. #12
    Maximum Bitrate bratnetwork's Avatar
    Join Date
    Oct 2007
    Haledon, NJ

  3. #13
    Join Date
    Dec 2008
    that does look awesome, bratnetwork. $100 here-

    and they have a 200a version for $50 more. this looks like a VERY solid way to go.

  4. #14
    Raw Wave
    Join Date
    Nov 2009
    I've just skimmed, but AFAIAConcerned, those "current sharers" etc are a croc of crap and are unecessary.
    I have discussed this ad (my) nauseum on other sites. Nobody has yet explained HOW the systems determine full charge etc. (When somebody does finally answer that, I will ask "why?" and under what circumstances such an approach is useful. LOL!!)

    Your system is best replace by a simple relay which is energised when the system (alternator) is charging.
    This can be done via the charge-lamp circuit from alternators (L or D+ terminal), or an over-voltage sensor where that is not practical (eg, marine stator systems).

    There are then ways that the system can be made to manually disconnect, connect, or "continue" after charging ceases. (Most involve merely diodes an switches or push-buttons added to the relay - very simple stuff. The only complexity is if an undervoltage cutout is desired to preserve the 2nd battery.)

    I might have posted a sample circuit on this site, else maybe

    The basic circuit is the additional relay as per:

    For more info, see - adding a second battery-page 5 which also refers to ( - trailor alarm wiring) Split charge systems etc (~7th post).

    There is also Smart Battery Isolator on this site.

  5. #15
    Join Date
    Feb 2010
    ive tried the relay thing, and it didnt work months ago, unless I did it totally wrong. The blue wire on the psu is not 12v it goes to ground when the carputer powers up. I need the amps to delay

  6. #16
    Raw Wave
    Join Date
    Nov 2009
    Jimmy - can you be more specific...
    Quote Originally Posted by Jimmy1
    ive tried the relay thing, and it didnt work... The blue wire on the psu....
    What relay?
    If it's the relay I talk about, it had nothing to do with any blue PSU wires....

    But after re-reading this thread - man there is a lot of confusion.

    A solenoid is merely a relay. It usually refers to a big relay, but it is also the actuator or coil in any relay. (Then there are contactors....)

    An isolator is something that isolates.
    It may be a relay/solenoid/contactor - they are all effectively the same.
    It may be a solid state circuit that uses FETs or whatever to replace the mechanical contacts in relays.
    It may be a "passive" diode splitter where - to copy from what I wrote at Aux. Battery oops :S (bottom pf page -post #15)...
    ...diode isolators. IMO the worst of the lot - the alternator has to up its output by the diode drop to maintain proper charge voltage - and which battery does it pick for that? Does it overcharge one or undercharge the other....?
    And then the huge heatsinking - the diode (voltage) drop x current - say 0.7V x 10A = 70W.
    And you cannot parallel the batteries....
    Diode isolation is for signaling and small power - not big!
    A solenoid or relay does not (usually) include a diode. Nor does an isolator have to have diodes.

    I have written elsewhere about so-called "current prioritising" systems that charge one battery before the other etc. If nothing else, these systems lengthen the time to charge both/all batteries.

    As to the isolators shown in the links in the prior replies, they are all generally relays (solenoids).
    They merely connect the auxiliary battery(s) as instructed (when signaled etc).

    The "smart isolators" generally include a voltage sensing circuit so that the relay/solenoid is signaled to turn on when the vehicle voltage exceeds some voltage - typically above 13.5V - hence when the system is charging.
    In the diagram above, the "smart sensing" is provided by the alternator regulator's charge lamp circuit.

    Elsewhere I have explained how the charge lamp circuit can be replaced by voltage sensors (such as the MW728 "battery protector) hence being a "smart isolator". These are required for non-charge lamp systems such as marine chargers, some motorbikes & cars, etc.

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