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Thread: Battery woes and fix ideas

  1. #1
    Variable Bitrate
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    Jul 2010
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    Battery woes and fix ideas

    Well, I guess it was bound to happen at some point... drained battery .

    This got me thinking, researching, planning, etc. Browsing these forums, there are a couple threads I read through, but nothing too definitive to completely make up my mind.
    Basically, I'm wanting to add a second battery for the electronics (laptop/amp/sub/etc.). I read about wiring up through an isolator or a relay, and I guess isolator is the way to go for electronical components.

    2nd battery will run all the electronics, so main battery will act normal, as if no extra electronical components were added (this should prevent main battery from draining again). Since all the power is hooked straight to the battery and not the IGN or ACC lines, I control the power on/off via dash switch. I should've known better, but the girlfriend forgot to flip the switches to power off the electronics and drained the battery overnight .

    Now, since adding the 2nd battery I want to get wiring setup correct before I try anything. If the 2nd battery is wired through an isolator, I am led to believe the batteries won't see each other which is good, but will the 2nd battery charge when the car is on? Will the alternator charge the 2nd battery as well as the main battery? If the 2nd battery discharges/drains and the main does charge the 2nd battery, is there any manual operation I would need to 'start' the 2nd battery again or would it be 'act' just like normal?

  2. #2
    Maximum Bitrate jmullan99's Avatar
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    Quote Originally Posted by detlion1643 View Post
    If the 2nd battery is wired through an isolator, I am led to believe the batteries won't see each other which is good, but will the 2nd battery charge when the car is on? Will the alternator charge the 2nd battery as well as the main battery?
    Yes.
    Quote Originally Posted by detlion1643 View Post
    If the 2nd battery discharges/drains and the main does charge the 2nd battery, is there any manual operation I would need to 'start' the 2nd battery again or would it be 'act' just like normal?
    With an isolator, the MAIN battery will not charge the 2ND. They are seperate. An isolator is a pair of high-current diodes. The current from alternator feeds in the "middle", one diode each feeds their respective battery. This prevents each battery from "seeing" each other and yet the alternator sees both. Keep in mind the capabilities of your alternator. Sometimes two batteries is too much, especially if both are fairly low in charge.
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  3. #3
    Raw Wave
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    Nov 2009
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    Ho hum.

    If you have a charge lamp on the dash (on when not charging), all you need is a relay.
    That's what is now being called "The Ultimate Intelligence Battery Isolator" in response to "Smart Isolators".

    It is fully automatic, simple & cheap.
    The only catch is sometimes if the relay's coil requires too much current, but that is rare in older alternators without EMS interaction. Even then, a MOSFET can be used in place of the relay, else to drive the relay (or one relay per extra battery).

    The relay should be sized for your load - ie ~$3 for 30A or ~$25 for 200A-400A. (And of course (self-resetting) circuit breakers or fuses at each battery end!)


    The only other recommendation is a low voltage cutout (LVCO) to protect the 2nd battery.
    I often recommend the MW728 (~$20) with its 10A capacity & 11.2V shutdown.

    In other posts I explain both above and the various tricks (voltage increase; latching output; manual on & off; complete LVCO isolation after dropping out; higher LVCO currents).


    PS - I recommend against diode isolators. IMO they are the worst choice except where small currents are involved etc.

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