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Thread: Problem with tank battery - any good Plan B?

  1. #1
    Constant Bitrate
    Join Date
    Apr 2003

    Problem with tank battery - any good Plan B?

    I have a Chevy Sub, so I had room for a second battery tray. I put one in, and a battery, and hooked it all up to a battery separator (one of those thingies that looks like a CPU cooler, with one input and two separated outputs). But the extra battery gets drained. I think I know why as well: Right now I have the regular battery hooked directly to the alternator (since it wouldn't charge when it didn't have access to battery power), but the tank battery gets it's charge from that separator. The voltage loss in that separator is around 0.6 V, and that's obviously exactly the loss that will stop it from working. I have an idea, but I wanted to ask if this will work first:

    I can hoop up a thin cable with a relay and a fuse on it from the alternator to the tank battery. The relay will be wired to something (perhaps the driving lights) so it's only open when the keys are in. Then it won't drain the start battery while the keys are out. That will keep the charging voltage up. But will starting the car make that cable burn up or the fuse blow? My main problem and cause to get that extra battery was that every time I started the car the computer had to be shut down (into hibernation). With the extra battery it won't. But that doesen't really help if I can't go into the gas station to pay with the computer running since the tank battery is almost empty. Any help would be appreciated!
    Tor - The MediaSUV

  2. #2
    Variable Bitrate no1knows's Avatar
    Join Date
    Feb 2004
    Guildford, Surrey, UK
    You can setup a relay with yr ACC line which is switched on while the keys are turned, except when the engine is cranking (in most cars).

    So, this will ensure that your second battery will be charged only when your keys are in the ignition but not when cranking which could cause high currents coming from yr tank battery. You will need a resistor inline with this cable to limit the charging current to the backup battery, the size of the resistor depends on the size of yr backup battery. As far as i understand your problem, that will solve everything.

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  3. #3
    Constant Bitrate
    Join Date
    Apr 2003
    Man, you're fast! OK, I understand part of what you're saying, but not all. I will check out that ACC line, and maybe use dual relays since I want the line to be open onlye when the car is running. So if I use one relay on the ACC and one one something like the driving lights (they only have power in on position, not ACC) that may actually work! Thanks! And now that I think about dual relays I have an even better idea: a relay that's hooked up to the starter engine's trigger voltage, so it breaks power when it's cranked! I probably wouldn't have thought about that without your suggestion!

    But why a resistor? The battery is always in use since I have the stereo system and computer hooked up exclusively to that. I don't always use the computer (and my wife seldom uses it), but I always use the stereo. Do I still need a resistor?
    Tor - The MediaSUV

  4. #4
    What can I say? I like serial. Curiosity's Avatar
    Join Date
    Mar 2004
    Florence Yall, BFKY
    Take a look at this and this to get more ideas. I'm working on something right now using both designs if the relays aren't out of stock this time.

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