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Thread: 2nd Battery not preforming like I expected.

  1. #1
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    2nd Battery not preforming like I expected.

    Alright guys here's the deal:

    I take my car to the local drive in regularly, and was using my carputer for the radio, but my problem is after about 1.5 movies the battery is dead. Im running an i5 with a 40w kenwood amp and hd radio. So my thoughts were simple as this. Wire in a 2nd battery, extend computer life to last 2-2.5 movies. Well, I wired it in, and the computer still cuts off about 2 hours into the night.


    My batteries are wired directly to each other. + to + / - to - with fuses in the appropriate places. No isolator or switcher, because Im using the same sized battery, with power being pulled off the + terminal of the 2nd battery.


    Am I just stupid for thinking it would work this way? Or.... is something wrong?

  2. #2
    FLAC SNOtwistR's Avatar
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    You are making both batteries half dead as when one goes down it draws current from the other battery til they are both below starting voltage you really need to isolate them and also work on your amperage draw from amp, hd radio, computer p/s, hubs ect. SNO

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    Quote Originally Posted by SNOtwistR View Post
    You are making both batteries half dead as when one goes down it draws current from the other battery til they are both below starting voltage you really need to isolate them and also work on your amperage draw from amp, hd radio, computer p/s, hubs ect. SNO
    Trick is the car starts fine. When I say dead I mean to the point where the computer cuts off, I can turn the car and it starts without a hickup. So Im guessing the m4 is reading below 12v and shutting it off? I dont think there is much I can do to reduce the draw from my current setup. I was hoping as 1 got lower, the other would re-charge it. But that apparently not the way I thought.

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    Correct me if I am wrong:

    But wouldn't installing a isolator cut off the batteries from each other, giving me the 1.5 hour problem I was having originally?

  5. #5
    Maximum Bitrate Mickz's Avatar
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    Iím not going to comment on the batteries as that subject has been done to death on here.

    The M4 has adjustable low voltage cut-off. Almost everything is adjustable with the M4 app and USB connection.

    BTW There are two M4 apps, one is more powerful (dangerous) than that other.
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    Quote Originally Posted by raggedrebel92 View Post
    No isolator or switcher, because Im using the same sized battery...?
    The size of the battery has little to do with using an isolator.

    I'd suggest an isolator (search mp3car for "UIBI" if you have a charge light) and a bigger 2nd battery. At least then you will still be able to crank.


    Be aware that as a general rule to meet warrantied life, cranking batteries should not be discharged more than 20% (~0.2V) and deep cycles 50% (~0.5V).

    Another PSU might help noting that some operate down to 6V which is way lower than you would want.

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    Crap. I just realized, I've been saying m4. Thats from my original setup. I have the m3 now. Sorry guys!



    Oldspark, Just the guy I was looking to hear from. Both my batteries are 65 Class, so I don't want to go much bigger as I had a hard enough time finding a 65 class battery box. (and I'm not really sure what is available in bigger sizes).

    I can crank and start my car fine. The car starts just as if it has a full battery, its just my pc cuts off and will not restart unless the car is started.

    Im kinda thinking like what was said above that I may just need to reprogram my m3.


    By the way, nice to see someone else up this early!

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    Thanks for the compliment, though Zulu time is ~9:20pm. My last reply was 7:30pm (Monday 10Sep2012).

    And I missed the m4 altogether! My bad. In that case, yeah - reprogram its dropout voltage.


    FYI...
    Group 65 - ie, ~70-75AH. By my reckoning, that suggests a 15A load for 1.5hrs to reach an open-circuit voltage of 12.0V (~50-60% discharged). But the actual voltage will be lower due to battery internal resistance so I expect your load is much less... maybe 5A - 10A?

    To use the ROT that each 0.1V drop is ~10% discharge (ie, assumes flat is ~11.6V & full is ~12.6-12.7V), I measure the full open-circuit voltage (after resting - ie, no surface charge) to confirm ~12.7V (12.67V in theory). I then add the load and remeasure to get the "loaded voltage reading" with the fully charged battery. EG - if full OC = 12.6V which drops to 12.3 with the load, I know that the loaded 12.3V corresponds to 100% fully charged.
    If I want to limit it to 30% discharge, then I'll set the cut-out voltage for 12.0V (ie, 12.3V full LESS (3 x 0.1V = ) 0.3V = 12.0V. (And I might go to 30% for a cranker knowing that it will soon be recharged by the alternator, else the battery won't live as long as intended.

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    Things to realize when using two batteries.. They MUST be the same brand and model and hopefully same age/condition or you will have one being preferred over the other and your system will only be marginally better. This bias also happens with charging as well. When running two batteries you also double the internal resistance. If you separate the batteries and use a constant rated solenoid to charge them you may find it actually lasts longer due to the less internal resistance. You can also use ultra high quality batteries as well. Some of these are about 50% the size of a standard battery of the same rating and some can be used like a deep cycle battery as well. Lead Acid batteries are a fickle thing and unless you are familiar with how they are setup they will surprise you in how they interact. If you use a Passive battery isolator they generally use large Diodes to separate batteries and the problem with diodes is that they generally drop .7 volts out of the circuit per Diode. Most alternators that are setup to charge 2 batteries have the diodes internally and are designed to charge at the proper voltage. Do not get an Optima Battery, these are NOT designed for automotive use although you will find many people using them.

    Ultra high quality batteries can generally be purchased in the neighborhood of $200 but they are much more durable than a standard car battery and you can do things with them you can't do with a standard car battery. You do NOT want to use a Ultra high quality battery with a standard car battery.

  10. #10
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    redheadrod, surely you jest...

    Although battery differences are why you do not keep them paralleled (nor mix in series), those difference mean little during charging. And when being loaded, paralleling will increase capacity unless one is faulty or at a different capacity (percentage-wise, not AH-wise).


    Two batteries in parallel halves the internal resistance. Or rather, the internal resistance is the same but are combined the same as 2 parallel resistances (which is always LESS than either of the individual resistances).


    I am unaware of any alternator that uses diodes to charge 2 batteries. That requires 2 rotors and stators - ie, 2 alternators.
    Alternators output up to 14.4V (steady-state) for 12V lead acids. [ 14.2V is a common set point. The older 13.8V spec has long been discarded. ] Alternators don't care what loads they power - it could be several paralleled 12V batteries - provided the alternator is loaded within its specs.

    Who the heck uses diode isolators? Even in the old days, clever people used relays for battery isolation. (Which battery's voltage should the alternator sense?)
    These days MOSFETs might be used instead of diodes, though I would still ask why (when a relay can be used...)?


    Your other points seem subjective else without any definition (eg "quality", or what outlasts a car battery in a car {and why would it be preferred}?).

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