Car battery question.....

Discussion in 'General Electronics Chat' started by Lyonspride, Jan 6, 2014.

  1. Lyonspride

    Thread Starter New Member

    Jan 6, 2014
    23
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    Hello, new to forum :D

    I don't really do small talk so straight down to a question :)

    This will sound crazy, I know I know the answer to this, but car batteries scare the hell out of me ever since one exploded on me 6 years ago (father in law overcharged it).

    I'm putting a new battery into my car, it's 12v 53Ah standard wet cell car battery.

    To prevent me losing my radio (I don't have the security code), I was thinking about connecting a 12v 1.2Ah SLA battery from a little project i've been working on, in parallel with the old car battery before removing it, then connecting the new battery before removing the little SLA.

    The other reason for this is that GM saw fit to have my car flash it's headlights when the battery is reconnected, which at 2x55w causes some serious sparkage whilst trying to twist and tighten the terminal clamps. So i'd be quite happy to trick the car into thinking nothing has happened.

    12.8v car battery and a 12.68v SLA, both fully charged.

    I strongly doubt anything is going to go bang, but i'm also wondering if I should play safe and wire in one of my 2.2ohm 100w resistors just to limit the current to/from the SLA to a maximum of around 5.5A.

    What would you do?
     
  2. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    The parts stores sell an adapter, that plugs into cigarette lighter socket and uses a 9V radio battery. The 9V is enough to keep all of the "memories" in the car alive while changing starting battery.

    PS, this thread will probably get closed soon for TOS, no car stuff. But the adapter is cheap and works.:)
     
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  3. #12

    Expert

    Nov 30, 2010
    16,270
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    I don't think this is a car modification, especially with the cigar lighter memory trick.

    Ps, thanks for the tip. The best way to kill the battery in my car is to get all the radio stations and the clock set just right. :D
     
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  4. JohnInTX

    Moderator

    Jun 26, 2012
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    Pull the bulbs out of the headlights/taillights and use the SLA. You might also check with a dealer to see if the VIN can be crossed with the radio code. The CIG lighter thing will work if its hot all the time. Some cars switch the lighter with the ignition.
     
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  5. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The off load current is minimal for the computer clock and radio, placing an auxiliary battery across the leads before changing the battery should work OK.
    Max.
     
  6. Lyonspride

    Thread Starter New Member

    Jan 6, 2014
    23
    1
    Just as well i'm not talking about a car mod :) More about the potential issues of having a 12.8v 54Ah battery in parallel with a 12.68v 1.2Ah battery (albeit for a very short time). I was concern about what sort of current the larger battery could push down into the smaller one, as I don't have any datasheets for either and I don't know their internal resistances. I'll probably go with the resistor anyway, just to play it safe.
     
  7. PackratKing

    Well-Known Member

    Jul 13, 2008
    850
    215
    I invested in one of those little cig-lighter / 9v gizmos, and it works as well as advertised... A real boon to the shade-tree mechanic !!!
     
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  8. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Not enough for long enough to matter.

    How much current flows through a set of 0 ga jumper cables when you connect them from a running vehicle to one with a battery that is too dead to start it plus crank over the engine on the near dead vehicle?

    That doesn't hurt anything with either battery regardless of eithers size or state of charge. ;)
     
  9. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,782
    941
    If the battery is fully charged and the voltage difference is only .1 volts, the internal resistance of the battery itself will prevent large currents from flowing. the small SLA battery will have an internal resistance of .1 ohm or maybe even higher.

    Current flow over 1 amp would be highly unlikely, and even if the current should double or triple, it would cause no harm if limited to a time scale of less than a minute.
     
  10. NetDoc

    New Member

    Jan 6, 2014
    22
    11
    If you use the cigarette lighter option with the nine volt battery, confirm that the socket is still energized with the key off.
     
  11. Lyonspride

    Thread Starter New Member

    Jan 6, 2014
    23
    1
    Yeah, my cars accessories and lighter sockets are all ignition dependant :(
     
  12. wayneh

    Expert

    Sep 9, 2010
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    I think most are. This is unfortunate when your hood is frozen shut but you need to trickle charge the battery. No can do.

    My van has a second accessory outlet that is not switched - I love it.
     
  13. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    What's so hard about turning ignition to accessory position?:p:):rolleyes:
     
  14. tcmtech

    Well-Known Member

    Nov 4, 2013
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    It might require rational thought and applied common sense. :rolleyes:

    It's similar to why some people don't wear seatbelts. They dont want to get trapped in their vehicle if they get knocked unconscious in an accident. :confused:
     
  15. Lyonspride

    Thread Starter New Member

    Jan 6, 2014
    23
    1
    Because they don't power up until the ignition is on, at which point it's running the fuel pump and other pre-ignition stuffs. But anyhow, I made up a set of leads and used the 1.2Ah SLA + series 2.2ohm resistor. I clamped GND to the engine block and +12 to the main power terminal in the fuse box, it kept everything running nicely while I swapped out the main battery and no scary sparks from the battery terminals :)
     
  16. wayneh

    Expert

    Sep 9, 2010
    12,100
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    Yes, and I get nervous about making and breaking power connections while the car's computer is powered up. On my boat, this will blow the ECU (engine control unit).

    Anyway, well done.
     
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