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Car wiring project - backup battery

Discussion in 'Power Electronics' started by van0014, Mar 15, 2012.

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  1. van0014


    Mar 15, 2012
    So ive got a problem with my car battery going flat every now and then. Pretty standard, but it annoys me when i have to jumpstart my car. So my idea is that next time my car goes flat and wont crank over, i can flick a switch from the comfort of my seat then it will start. Effectively, flicking the switch gives me a jumpstart.

    So to do this, i took apart a 17ah 20hr jumpstarter. With the case removed, the battery was a lot more compact. Then i wired it in parallel to my car battery. I took a shortcut or two, though, and wired the jumpstarter to the same wire as my 600w amp. Its earthed to one of the bolts that holds the drivers seat down. I left the switch that it came with in tact, since its designed to handle jumpstarter amperage.

    It may be a problem, though, that im using the amp wire for both the amp and backup battery charging. The second battery will only charge when the switch is on. Now i must mention that im bypassing the fuse from the battery to the amp and jumpstarter. Im using copper wire in duct tape. I know, i should know better than that, but i have no spare fuse and wont get one soon.

    after all the wiring, i did some voltage tests to check if it was all running sweet. Came up with these results and just wanted to know if from these results it seems like my setup works.

    I took all readings from the main battery terminals. B1=car battery and B2=car battery and jumpstarter

    first test
    B1 = 12.64v-12.65v
    flicking the switch
    B2 = 12.65v to 12.66v
    i gained .01v. Seems fine so far.

    test while idling
    B1 = 14.08v-14.09v
    B2 = 14.07v-14.08v.
    i lost .01v, probably because the backup battery was charging very slowly.

    after a 1 minute charge.
    B1 = 12.84v
    B2 = 12.83v

    Before charging i gained ~.02v after flicking the switch. After a short charge i lost ~.02v
    It seems strange that the backup battery makes the voltage rise, but after a small charge it makes the voltage drop. I think i should test it again just to make sure but this could be a problem.

    Im thinking that maybe the small voltage drop is because the main battery could be the jumpstarter when its connected. I wanted to confirm i havn't missed anything, and that these readings seem reasonable.

    Attached Files:

    Last edited: Mar 15, 2012
  2. duke37


    Jan 9, 2011
    Your description is long and detailed so I will not deal with it all.

    1. A battery needs to be charged more than it is discharged. The voltage of 14V or so you measure is about right but these measurements must be made at the battery posts.

    2. The currents used in a car are huge so any little resistance will give a large voltage drop. Make sure that all earths are good.

    3. I would keep all large currents well away from an audio amplifier.

    4. Fuses do not fail just because they are grumpy. Make sure you find the fault before replacing the fuse.

    5. The voltages are low so a short fuse is adequate, winding a long coil of wire as a fuse is just making a furnace, with the risk of fire.

    6. A fuse should be rated properly, bits of foil or random dog ends of wire will lead to disaster.
  3. GreenGiant


    Feb 9, 2012
    I feel as though you are spending way too much time and effort to fix this...

    Just get a new battery/alternator, and that should fix the issue

    you may just need a bigger alternator to compensate for the subs

    you should probably replace the wire for the subs, that looks like 8 or 6awg cable and you would want 4 ideally, also the fuse for a 600W amp should be rated at 50Amps, not just copper wire stuffed in the connectors.

    In my experience with car audio and cars in general you should run separate wires for the charger, and the subs, and THICK wires for each (like 6 or 4 awg)

    Duke: The audio amp will connect directly to the battery and it will have a current limiting resistance to only allow for a certain wattage (600 as stated) at 12 volts therefor it wont hurt it to run a lot of current through those lines, distance doesnt matter, you run it to the battery and its running to the amp.
  4. van0014


    Mar 15, 2012
    im so glad you took the time to read my extensive post. Its long because at the time i had a lot of caffeine and tend to be extremely excessively descriptive due to slight personality issues.

    Aside from that, i never knew small resistances, such as with bad fuses, can cause large voltage drops. My sub seemed to perform over expectations even with the bad fuse, but i was dissapointed when i ran a larger amp with 2 kicker comps, and a 500w pioneer with the bad fuse. It wasnt as loud as i wanted. It also was distorted.

    So now i know the bad fuse was probably why the 3 sub setup didn't run well. That or the stock '93 mitsubishi magna alternator cant handle the huge power draw.
  5. GreenGiant


    Feb 9, 2012
    You have to remember that when it comes to resistance the smaller it is the more current it draws (I=V/R) so ideally you either want no resistance or a very large one
    The more current draw on a battery the faster the voltage drops
  6. van0014


    Mar 15, 2012
    So just to clarify, i should either use a fuse, or not ideally, just use a straight wire from the battery to the other battery and amp. I know, its not recommended and ill only regret it in the future but i like things to work straight away and dont think long term.

    Also it would pay to use 4 gauge or less? but going less may just be overkill. Hoping to get more out of my cheap 500w sub. It goes loud but my mate has the same sub with a different amp and it goes harder.
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