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Thread: Dual-battery setup with a smaller battery?

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    Maximum Bitrate kbyrd's Avatar
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    Dual-battery setup with a smaller battery?

    I've done the searching and reading on dual-battery setups, I think I understand the need for an isolator and a few of the various options for setup. Here's something I didn't see.

    I've got a M1-ATX, so I don't have issue surviving crank or any of that. I want a second battery simply to handle the S3 powerstate overnight or for a few days. I don't need to play my PC with the engine off for hours on end, I just want a battery to handle the small drain required of the S3 (suspend to ram) state.

    Does this have to be a huge battery? I'm hoping I can use something smaller (but still sealed as it'll be in the car). Can someone help me with a schematic? I don't really care if my carpc runs off the main battery or secondary battery while the engine is running (or key in accessory) but I'd like it to run off of the secondary when the key is off. I think I this means I'll switch the input of the M1-ATX from the primary electrical system to my small backup battery when I take the key out.

    Has anyone done this? Is this more trouble than it's worth?
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    Once you hook up the carpc to the second battery it will always run off that battery. That's what the isolator does. It isolates the two batteries while still charging them.

  3. #3
    Maximum Bitrate kbyrd's Avatar
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    Quote Originally Posted by undisputed
    Once you hook up the carpc to the second battery it will always run off that battery. That's what the isolator does. It isolates the two batteries while still charging them.
    Ok, so it'll always run off of the second battery. That's fine as long as this smaller battery can handle the load. Is there any issue with having a smaller (considerably less amp hours) than a regular car battery (or a normal deep cycle) in this setup? I've got a new-ish car and a low powered system (90W M1-ATX runs everything), so I'm pretty sure the alternator isn't taxed.

    Can anyone recommend a smaller 12V approriate for this? As I said before, I'll be mounting in the car so it needs to be sealed. By going smaller, I'm hoping to save space and cost.
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    Quote Originally Posted by undisputed
    Once you hook up the carpc to the second battery it will always run off that battery. That's what the isolator does. It isolates the two batteries while still charging them.
    No, this is not true. If the battery is being charged then the charging system will be powering the computer.

    Rob.

  5. #5
    Well, He asked for it. WebDog's Avatar
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    Quote Originally Posted by gommino
    No, this is not true. If the battery is being charged then the charging system will be powering the computer.

    Rob.
    Yes but the computer is always connected to the second battery...

    I would just do a relay that is closed when the IGN in on that way the battery will get a charge but when IGN is off it wont see the other battery, that way you dont need to deal with isolators...
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    Low Bitrate agentloveless's Avatar
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    Plain and simple- The more amp hours you have the longer you can run it that way. The only way you will know what size you need is to come up with your requirement. Example - I wish for the PC to be able to be suspended for one week with out me ever starting the engine. Then what you have to do is measure the draw of your car pc in amps, and then you can decide on the apropriate number of amp hours.

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    Maximum Bitrate kbyrd's Avatar
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    Quote Originally Posted by agentloveless
    Plain and simple- The more amp hours you have the longer you can run it that way.
    That's what I was hoping for. That's great news.
    Well, I was hoping for one more thing....recomendations on "smaller" 12V sealed batteries. I have no idea what brands I'm looking for.
    My worklog.
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  8. #8
    Well, He asked for it. WebDog's Avatar
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    Do you have a batteries plus around you? i would just get any sealed one they have...
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  9. #9
    Maximum Bitrate kbyrd's Avatar
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    Thanks for the Batteries Plus idea, I'll try them. Here's some followup questions:
    I've done a little bit with solar panel charging a battery and running some wireless gear. In that instance we used a big deep cycle gel battery. We also but a charge controller in the middle of the solar panel, the battery, and the load. The charge controller aparently did voltage regulation as well as keep the battery in good condition by pulse charging or some other such thing. It also cut the load if battery voltage dropped to low to prevent a complete discharge.

    Why don't automobile charging systems use charge controllers like this? Would a dual battery setup benifit from one on just the second battery? The charge controller was supposed to manage the battery charging better so you the battery lasted longer and you didn't have to worry so much about how it was discharged/charged.
    My worklog.
    Status: VM GTI sold, got out of the CarPC tinkering hobby, but I still think about getting back in.

  10. #10
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    if you cut the charge from the battery using a relay or aswitch then it would be running from the battery direct

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