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Thread: Does anyone know how to create dual battery setup?

  1. #11
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    Thats more like it, but you only need one fuse holder, unless the second one should have been between the second battery and the distribution panel. I would also take the feed from the startermotor terminal.

  2. #12
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    I am going to do the same and bought this thing:
    http://www.kemo-electronic.de/de/module/m102/index.htm

    But now that I am going through all the posts about "tank" I am not so sure. So my question is will it work (the wires on the m102 are a bit thin)? If so this is a very cheap sollution (10$)

  3. #13
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    yes, but you can also use my setup to start your car if the main battery dies. I would hate to see you try to start your car with a 30A relay in line!!! It's great if you leave your interior light, parking lights or something like that on.
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  4. #14
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    So, is that a normally closed relay? Or is the idea that it just charges the battery when you flick a switch. I would think I would want to have a normally closed so that it always keeps the two batterys at the same voltage, but when you turn the key to start it would activate the relay and open the circuit not allowing the starter to pull from the second battery. Perhaps I'm missing something in my logic... if so, please somebody set me straight... Oh, and if a normally closed will work, where could I get one that size... all the stores around here have normally open only.(atleast in that large a amperage)

  5. #15
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    The 150A contactor is normally open. Once the alternator indicate line put out 12V it then closes the 30A relay which then closes the contactor. So once the engine is started and the alternator is charging, the second battery is also charged. You can connect the two batteries at any time by pushing the switch for emergencies. ie flat main battery.
    The micro relay is used so there isn't too much current draw on the indicate line of the alternator and the 30A relay is used to supply the current required to keep the contactor closed.
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  6. #16
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    I don't need to be able to start my car from the battery, I'm looking at using a battery from a ups I have. Its 12 volts and like 7-10 Ah. All I want is to be able to get my inverter through... the inverter isn't running much(just a usb hub), but if the voltage drops it shuts down and when it shuts down I have to reboot anyways cause the computer gets really upset.

    Perhaps I should just wire the small battery to charge only when the key is on. When I turn the key to crank I would have it interrupt the signal and disconnect the relay between the two batterys... anything wrong with this? When it reconnects will it have a really big draw... or like when it reconnects after sitting overnight will it have a huge draw to equalize between the two?

  7. #17
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    That should be fine. The current draw when you connect the smaller battery shouldn't be that great. Similar to when you start you car and the alternator charges the battery. Initially the charge current is high and tapers off. If you are worried about it, get a battery isolater from a 4wd or electronics shop.
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  8. #18
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    Quote Originally Posted by meddler
    try this... Credit for this circuit goes to my mate Darren Pollard. I'll clean is up and repost when I have time. Basically once the alternator puts out a voltage on the indicate line the 12V contactor is closed hence charging the second battery. This contactor can be manually shut by pressing the push button illuminated switch. Disregard prices, they are in AU$
    Nice diagram but it is missing fuses. Someone said that only fuse is needed. That is not correct at all. There needs to be two fuses in this schematic. One next to each battery on the positive line. Also a isolator is used not to prevent fire but to ensure both batteries get a proper charge. An isolator is just a few very large diodes. When one battery runs low on charge the isolator allows current to flow to that battery and charge it. Fuses are used to protect the car from fire. A fuse on the main power does not protect amps or any other electronics. That fuse is to protect the car and it's occupant's from possible fire. I know of shops that have burned cars to the ground because they did not fuse the power wire correctly.

    dmcdlrn....you should check into a car alarm back up battery. DEI makes one that comes with an isolation circuit that is designed for a small battery and the battery.

  9. #19
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    Quote Originally Posted by hortimech
    Meddler's way of charging the second battery is way over the top, you just really need the 30amp relay. If you did connect the second battery with normal starter motor cable, you would not need anything else as this would stand the cranking current.
    It's not over the top. When the engine is running the alternator is powering everything in the car, including everything connected to the second battery. This means that if you have a lot of stuff connected to the secind battery such as high powered amps, they well may draw more than 30amps which would be going through that relay. Therefore the cable connecting the two batteries and the relay must be capabable of passing enough current from the alternator to your equipment, as well as charging the second battery.

    Also, even if you were just using normal starter cable, that would not be enough. You would still need an isolator. The whole point of an isolator is so you don't drain your main battery when you are using your computer/stereo with the engine off, so you can start your car again.

    Rob.

  10. #20
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    Quote Originally Posted by SickVette
    Nice diagram but it is missing fuses. Someone said that only fuse is needed. That is not correct at all. There needs to be two fuses in this schematic. One next to each battery on the positive line. Also a isolator is used not to prevent fire but to ensure both batteries get a proper charge. An isolator is just a few very large diodes. When one battery runs low on charge the isolator allows current to flow to that battery and charge it. Fuses are used to protect the car from fire. A fuse on the main power does not protect amps or any other electronics. That fuse is to protect the car and it's occupant's from possible fire. I know of shops that have burned cars to the ground because they did not fuse the power wire correctly.
    I agree on the fuses. but as for the rest, if it is directed at me...
    Never let the truth get in the way of a good story

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