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Thread: Dual-battery setup with a smaller battery?

  1. #21
    Maximum Bitrate kbyrd's Avatar
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    Quote Originally Posted by arebelspy
    1) 60AH for VW, that's what I was looking for, thanks. Yeah, if you're leaving it in standby for a long time you'd definitely want a bigger battery. I wonder if there are any power settings to drop a computer from standby to hibernate after X amount of time. There obviously is for laptops, but I wonder about regular desktops. If so you could always use the 12AH battery and then drop it from standyby to hibernate after a day or something.. your boot will then be slower (Still hibernate so not too bad) that one time at the start of the week after not having used the car for the weekend, but when using it al through the week (only 12 hours aftr you last shut it off or whatever) it'll be coming out of standby.
    I thought a lot about how this would be done (hibernate from suspend). It requires support from at least the BIOS. The problem is that in my S3 state everything (including the disk and processor) is powered off. So you'd essentially have to wake from the suspend, run a few instructions on the CPU, hibernate (run a few more instructions on the CPU, page out your RAM ti disk), then turn off. I've heard laptops do this, I've never seen it. I'm not sure it can be done from S3.

    [QUOTE
    Latter numbers: I'm not sure why you think this, it's working perfectly. More reasoning/links to other thoughts or suggestions?
    [/QUOTE]

    So I don't know how the tank circuit is constructed. I'm guessing, that could be the problem. I assume it's a diode and a resistor. Let's label the connections:
    - 1 ,the car side. This input into the tank circuit from the car's electrical system (sometimes car's battery, sometimes car's alternator)
    - 2, the tank battery side.
    - 3, the load. your carPC, inverter, whatever.

    Here's my idea of how the tank circuit works.

    - Car running. The alternator is at 1. It's supply 13-14V. Both 2 and 3 act as loads (2 is being charged by 1, 3 is being powered by 1). The resistor prevents a ton of current from slamming into 2 all at once.

    - Car is starting. Alternator off, big voltage drop on 1 since car is cranking. 2 has higher voltage now and would normally flow into 1 but the diode prevents this, instead 2 powers 3. CarPC runs during crank.

    - Car is off. For me 3 is still a load (because of suspend) Car battery supplies 1, is this voltage higher than 2, then 2 will be charged by 1, and 3 will be powered by 1. 2 will never charge 1, but 3 will either discharge 1 or discharge 2. I think it'll sort of "take turns", The point is with your car off 3 will run off 2 until 2 is drained to a lower voltage than 1. At that point 1 will supply both 2 and 3. This will cycle, draining both 1 and 2. I would still be draining my car's battery.

    As I said, I'm guessing and I'm new to electronics, but this is how I see it.


    azzuro: Be careful with "just a relay". You have to prevent your car's system from charging your 2nd battery two fast. Voltage will try to equalize in the system as fast as it can. So, your car is off. You drain 2nd battery pretty low. You start your car and your charging system is cranking out 14V at whatever amps your alternator can minus what the car needs. Can you recharge that second battery that fast? I've been lead to believe various batteries have current limits for recharging. You have to be careful that they don't overheat. They'll vent gas (inside your car!) when they overheat. sealed lead acid batteries probably aren't completely sealed. They're sealed with a provision for an emergency vent. They vent rather than explode as a last resort.
    My worklog.
    Status: VM GTI sold, got out of the CarPC tinkering hobby, but I still think about getting back in.

  2. #22
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    Quote Originally Posted by kbyrd
    azzuro: Be careful with "just a relay". You have to prevent your car's system from charging your 2nd battery two fast. Voltage will try to equalize in the system as fast as it can. So, your car is off. You drain 2nd battery pretty low. You start your car and your charging system is cranking out 14V at whatever amps your alternator can minus what the car needs. Can you recharge that second battery that fast? I've been lead to believe various batteries have current limits for recharging. You have to be careful that they don't overheat. They'll vent gas (inside your car!) when they overheat. sealed lead acid batteries probably aren't completely sealed. They're sealed with a provision for an emergency vent. They vent rather than explode as a last resort.
    That's what I was worried about. I'm going to do some more research!
    '03 Intensa Blue Pearl Lexus IS300

    Specs: Xenarc 700TS, Opus 150W PSU, CarCPU Case, 2.0 GHz Celeron D, DFI PS-35-BL, SB Audigy2 NX, Centrafuse 1.4

    Progress: Car PC up and running!
    To Do: Navigation, WiFi, AM/FM Radio, DVD

  3. #23
    Maximum Bitrate kbyrd's Avatar
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    Quote Originally Posted by azzurro
    That's what I was worried about. I'm going to do some more research!
    sure power has a writeup on isolators, but they sell them so they might be biased. Google "battery isolator"
    My worklog.
    Status: VM GTI sold, got out of the CarPC tinkering hobby, but I still think about getting back in.

  4. #24
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    Quote Originally Posted by kbyrd
    sure power has a writeup on isolators, but they sell them so they might be biased. Google "battery isolator"
    Doing some reading. Thank you!

    Do you know how much extra strain a second battery will put on the alternator? Is this an issue, even when using an isolator?
    '03 Intensa Blue Pearl Lexus IS300

    Specs: Xenarc 700TS, Opus 150W PSU, CarCPU Case, 2.0 GHz Celeron D, DFI PS-35-BL, SB Audigy2 NX, Centrafuse 1.4

    Progress: Car PC up and running!
    To Do: Navigation, WiFi, AM/FM Radio, DVD

  5. #25
    Maximum Bitrate kbyrd's Avatar
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    Quote Originally Posted by azzurro
    Doing some reading. Thank you!

    Do you know how much extra strain a second battery will put on the alternator? Is this an issue, even when using an isolator?
    To your alternator, it's a load, just like any other. I believe most modern alternators are current limited to some max and drawing that max ammount of current doesn't hurt them it just means there's only 500 amps (or whatever) to go around. I'm not sure about this, someone else should confirm.

    The ammount of current drawn by the 2nd battery probably depends on the difference in voltage between your car's charging system and the 2nd battery. That is, how discharged is your 2nd battery. If it needed more current than the system could provide, I believe it would be a lot like having a too powerful audio system. Other devices in the car would dim, etc.
    My worklog.
    Status: VM GTI sold, got out of the CarPC tinkering hobby, but I still think about getting back in.

  6. #26
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    I havent fully read everything here but make sure that you have some kind of current limiting device on your second battery if it is not a car battery or rated for really high amps. If you run your second battery down to like 10V (that is the cut off of my inverter, I don't know what an Opus cuts off at) and then start your car which essantially connects it to a 12V extremely high current source then the smaller, more dead, battery will try to instantly charge up to 12V resulting in an very high current draw which, if it is higher than the battery's rating, could cause it to overheat and explode. I know some batteries have their charging circuitry built in but just a warning.

    I was also flirting with this idea with a couple small, fully sealed 12V 4Ah batteries I bought from a surplus store (called Princess Auto) but decided the isolator was too much money to be worth it (a diode isolator that is), I'm reconsidering though with the relay isolators (much more simple and cheaper).

    Good luck with the project!!

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