2nd car battery- solenoid/relay?

Discussion in 'The Projects Forum' started by LaZyLuke, Sep 4, 2009.

  1. LaZyLuke

    Thread Starter Active Member

    May 10, 2009
    56
    0
    Hi. I wanted to place a second battery in my car due to the audio system. People advised me not to buy a larger capacitor (I already have one farad) but add anothet battery. I have heard that batteries can "drain each-other" when the car is not working, and the safest thing to do is to disconnect them when the car is off. Now I like that idea since I can listen to music when the car is off, and wont have to worry about not being able to start the car when the battery dies. Now I have few questions about this project:

    I have a manual relay but I will prefer something that can be turned on "remotely" with a current - like a starter solenoid. My question is, can I use a 12V starter solenoid for this application? I am not sure if this type of solenoids can be used with current passing through them for longer periods of time.

    The system I have will be using 500Amp max (connected to the secondary battery) so I am not sure how much current will be going through the connection between the front battery and the second battery placed in the trunk. So my next question is - does the solenoid/relay have to be rated for the 500amps or less?

    The cable I have right now that will be connecting two batteries is fused - do I need to keep this fuse? or is it useless after I add another battery?

    Any help will be appreciated, Thank You, Luke.
     
  2. jj_alukkas

    Well-Known Member

    Jan 8, 2009
    751
    5
    Are you sure it is a 500A battery?? A battery of that size would be 5 times a truck battery which is about 100-120Ah... I think you stated the output in watts or is it the other way?

    You can do what you asked by using a relay which is rated MORE than what your battery will draw. If you keep it less, your relay will arc and sometimes will not release after it is denergized. First confirm your battery rating, not the amplifier rating.
    You will need to connect the coils to the car key such that the battery will charge as well as be connected to the car electricals.

    The fuse is better if its not removed as it will save more that the price of a fuse if you accidentally connect things the wrong way.

    A car's starter solenoid is only 90A at 12V and that itself is a heck load of power at 1KW... Confirm loads first.
     
  3. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    I think he's confusing the AH rating of a lead acid battery with Cranking Amps.

    Are you saying that the typical coil current of a starter solenoid is 90 Amps?!?!? :eek: Where did you get that data from?
     
    Last edited: Sep 5, 2009
  4. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    I don't know if auto supply stores carry them but the marine suppliers sell battery isolators. They are heavy duty Diodes mounted in a heat sink.
     
  5. wr8y

    Active Member

    Sep 16, 2008
    232
    1
    You say you want a battery disconnect switch? Well, you COULD go to a supplier of ambulance/emergency vehicle parts and pick one up for around $150.

    OR...........

    Here is a cheap version:
    http://www.jcwhitney.com/COLE-HERSEE-BATTERY-MASTER-CUTOFF-SWITCHES/GP_2006124_N_111+10201+600000926_10101.jcw

    Place it between the two batteries and turn it off when you wish to 'use' the second battery with the engine off!

    Now, if you REALLY want to use a relay, we sell and install them in public safety vehicles (ambulances and police cars). We find that leaving a lot of 'stuff' connected drains batteries. So, we install a "master" relay that is keyed by the ignition switch which controls power to all the accessory equipment in the vehicle. I'd have to check to see the current rating, I can do that on tuesday. These relays are rated to be turned on and left on, most such vehicles run 8 to 12 hour shifts, so you have no need to worry about duty cycle!

    PM me if you are in need of one. I SUSPECT the price will be around $85 assuming UPS charges are $10 or so.

    I have never, in 28 years, heard of putting a fuse in a battery cable! Just what rating of fuse are you going to use for what YOU are doing?

    EDIT: Tell ya what: I'll find the name of the manufacturer of that relay and post it here - you should be able to order it locally.
     
  6. wr8y

    Active Member

    Sep 16, 2008
    232
    1
    I like this idea better!
     
  7. jj_alukkas

    Well-Known Member

    Jan 8, 2009
    751
    5
    He said that he's gonna use the starter solenoid relay. I was just indicating that the relay is only rated for a maximum of 90A. Its not the coil current, just the maximum limit they have put for the relay and the fuse. Source : my car's instruction manual.
     
  8. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    I should have noted that the schematic that I posted neglects to show where the vehicle's voltage regulator (V_Sens) lead is hooked up to. I believe it would be the common anode but these isolators come with complete installation instructions. Inboard engines would use the same charging scheme as an automobile. ;)
     
  9. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    Are you driving a Go-Cart or a motorcycle? Most cars in the US pull a hell of a lot more cranking amps than 90 Amps. Fuses are NEVER found in the cranking (starter motor) circuit. That portion of the vehicle's wiring is always heavy gauge and not fused. Very little voltage drop can be tolerated by the starter motor.
     
  10. jj_alukkas

    Well-Known Member

    Jan 8, 2009
    751
    5
    Ooops, sorry, my alternator is 90A, starter motor is 250+ ... I had a doubt, so checked right now. Starter motor is not stated anywhere in my manual, how much is yours?? You are right, I couldnt find the fuse in the fuse box or engine cabinet, a big mistake. sorry...
     
  11. jj_alukkas

    Well-Known Member

    Jan 8, 2009
    751
    5
    The best part is I have a diesel engine. I think diesel engines require more powerful starters, dont know. It is now a question for me to find the starter rating. Maybe tomorrow.
     
  12. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    The higher the compression the more cranking amps you need, and yes diesels are very high compression.
     
  13. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    I have no idea but it's >>>90A. I drive a Toyota Tundra with a big mill that has never met a gas station it didn't like. Moves like a Bat out of Hell though!!!:D
     
  14. wr8y

    Active Member

    Sep 16, 2008
    232
    1
    YOu can say that again, CD.


    I bought a Chrylser Crossfire two months ago, only a 3.2L engine but they spec a BIG battery. I questioned why on the Crossfire forum - and a guy said, "Same reason as they specify 91 octane gas - you have a 10:1 compression ratio in these engines".
     
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