Dual Battery Question
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RWD 19xx-1984 DeVille and Fleetwood,
1985-1996 Fleetwood and Brougham Forum Discussion, Dual Battery Question in Past Cadillac Vehicle Discussion; It's been a while since I've paid much attention to my Fleetwood. The RV project has taken up most of ...
  1. #1
    turbojimmy's Avatar
    turbojimmy is offline Cadillac Owners Connoisseur
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    Dual Battery Question

    It's been a while since I've paid much attention to my Fleetwood. The RV project has taken up most of my time for the past few months. But, the good news is that the RV is ready to roll. New 454 is installed and working great. Just have a few loose ends to tie up before I put it to use.

    Here's some ear candy - 454 through open manifolds. First time firing it up so it's a little choppy:
    https://www.youtube.com/watch?v=zeBq7w3-_GY

    So on the limo - I have 2 batteries. I installed #2 in the trunk. I have an isolator behind the passenger seat that connects and disconnects the front battery and alternator from the rear. There is a 150 amp fuse just off the alternator connected to the cable that feeds the isolator. Back in the trunk, I have a 150 amp fuse right at the battery box. So, the run from the trunk to the alternator is fused at both ends.

    I lost all the power in the rear of the limo, which is fed by the rear battery. I noticed that the ground cable for that rear battery had corroded and disconnected from the frame. I replaced the connector and reinstalled it. Still no power. But, when I switch the isolator to connect the front and rear batteries everything works fine. The isolator connects the alternator and front battery to the rear battery.

    I got looking at it and the 150 amp fuse at the rear battery is blown. It had to be a dead short. Could it be the fact that the rear ground came loose? If there were a short between the front battery and the rear battery the fuse up front would have blown, too.

    I'm hesitant to simply replace the fuse and hook the battery back up. The ground cable is fixed, but I'm not sure that was the problem.

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  3. #2
    The-Dullahan is offline Cadillac Owners Master
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    Well, in theory, yes the loose cable could have caused it. However, it could also have been caused by a short someplace between the rear battery and the isolator. Is there a chance that the terminal on the isolator could have grounded? If not, it could well have been someplace in line between the two.

    An easy way to check this is to disconnect the cable from the rear battery positive, but leave it connected to the isolator and then use a tester to see if it is grounded.

  4. #3
    turbojimmy's Avatar
    turbojimmy is offline Cadillac Owners Connoisseur
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    Re: Dual Battery Question

    Quote Originally Posted by The-Dullahan View Post
    Well, in theory, yes the loose cable could have caused it. However, it could also have been caused by a short someplace between the rear battery and the isolator. Is there a chance that the terminal on the isolator could have grounded? If not, it could well have been someplace in line between the two.

    An easy way to check this is to disconnect the cable from the rear battery positive, but leave it connected to the isolator and then use a tester to see if it is grounded.
    Thanks! I didn't think of disconnecting the positive and checking for ground.

    But here's something more interesting that I forgot to mention. The fuse holder by the rear battery also has a voltmeter. It reads 0 until I throw the switch on the isolator. At that point, it shows the alternator voltage (when the car is running). I assumed, therefore, that the cable from the front battery to rear must be good.

    ----------

    So I checked the positive cable and there was no short to ground. I put a 250 amp fuse in it (all I had laying around) and it's fine.

    I guess the ground cable coming loose baked it? I don't understand the science behind it but I need to make sure that rear battery ground stays properly attached.

    It's nice having the limo back. I dusted it off, washed it and took it to my nephew's first communion. I forget that people don't see all the defects with the car that I see. To people that don't own it, it's an awesome ride. After church there was a party, took 9 kids around the neighborhood so they could stick their heads out the sunroof and yell at people. Dropped them off at a park. Took my daughter to a sleep over party but couldn't leave until everyone got to see inside the limo. Great day remembering why I bought this thing.

  5. #4
    The-Dullahan is offline Cadillac Owners Master
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    Quote Originally Posted by turbojimmy View Post
    So I checked the positive cable and there was no short to ground. I put a 250 amp fuse in it (all I had laying around) and it's fine.
    Seems that way. Definitely plausible.

  6. #5
    limo1 is offline Cadillac Owners Member
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    Re: Dual Battery Question

    In order to properly connect two 12V batteries together off one alternator you need to use a dual output diode isolator rated above the alternator output current. center post on diode isolator is fed by the alternator output. each output of the diode isolator goes to respective positive battery post. The rear battery can have a resettable breaker used instead of fuse like a wiper motor breaker. Do not allow the battery positives to connect to each other directly or you can have very large currents going between batteries. that is why you blew the fuse.
    Problem with diode isolator is they have a voltage drop so batteries do not get the full alternator output voltage at the battery terminal. Alternator would need remote sense lead connected to battery to overcome this.
    Here is a link to a 3 phase rectifier type:http://store.mwands.com/pma-parts-co...rectifier-kit/


    My limo I overcome this problem by opening up alternator bring out 3 wires off the stator connectors inside alternator bring the 3 wires out to a remoter 3 phase bridge rectifier and this is the source for the rear battery.


    the internal regulator will still sense the normal car battery voltage and maintain proper voltage 14.1V.
    be sure to use a large wire going back to the rear battery #6 or #8 wire use a resettable breaker http://www.littelfuse.com/products/f.../813050bp.aspx just off the remote rectifier say 50 amps and you will have a dual battery system with totally isolated charging system as well as maintain correct battery voltages.

  7. #6
    csbuckn is offline Cadillac Owners Master
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    Interesting. I'm not getting full voltage to my batteries with the isolator. Might have to look into the rectifier thing. You have pics or a diagram of your rectifier/alternator? Or pics of what you did to get full 14v to the batteries? My batteries are getting about 13 and 13.3v. Seems to work just fine but would love to get above 14v.

  8. #7
    turbojimmy's Avatar
    turbojimmy is offline Cadillac Owners Connoisseur
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    Re: Dual Battery Question

    Thanks for the explanation! I'm using a simple Painless isolator rated at 250 amps. I have 0-gauge wire from both batteries to the isolator, and have both batteries grounded with 0-gauge cable. The front battery is grounded to the frame and the engine; the rear battery is grounded at the frame in the back (which is where it came loose). With this setup I get very close to alternator voltage at the back battery (14.3-14.4 volts).

    Interestingly, and admittedly unrelated, I just noticed yesterday that the isolator in my RV was a different style. It has cooling fins and more connections than the painless solenoid-type does. The RV has a regular automotive chassis battery and 2 deep cycle coach batteries. I haven't looked into it, but I'm hoping that the isolator somehow isolates the 120V converter that charges the coach batteries from the alternator. I know when I start the engine that the alternator charges the coach batteries - I can see the lights get brighter. I'm concerned, though, about idling the engine while plugged into shore power. The coach batteries would be theoretically charging from 2 sources. I'm thinking it has to isolate the converter somehow. It's a "vintage" (1984) rig so I'm not sure how sophisticated it is. I replaced the converter with a modern one earlier this year, though.

  9. #8
    the APEMAN is offline Cadillac Owners Fanatic
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    Re: Dual Battery Question

    Sounds like your fuse blew when starting the engine.

    Most people refer to a diode device as an isolator and a switch or solenoid as a cut-off switch.

    Diode isolators need heat sinks (fins) to dissipate the heat energy associated with voltage drop across the diodes.

  10. #9
    turbojimmy's Avatar
    turbojimmy is offline Cadillac Owners Connoisseur
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    Re: Dual Battery Question

    Quote Originally Posted by the APEMAN View Post
    Sounds like your fuse blew when starting the engine.

    Most people refer to a diode device as an isolator and a switch or solenoid as a cut-off switch.

    Diode isolators need heat sinks (fins) to dissipate the heat energy associated with voltage drop across the diodes.
    That sounds like what's in my RV.

    Not sure when the limo fuse blew. I noticed that I had no juice from the rear battery at all. Then I noticed the ground cable hanging down and figured that was the problem. Upon fixing the cable, I still had no juice so I checked the fuse and it was cooked. I have the solenoid set up to connect the 2 batteries when I turn the key on - it probably cooked the first time I turned the key without the ground cable hooked up.

  11. #10
    the APEMAN is offline Cadillac Owners Fanatic
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    Re: Dual Battery Question

    Having a poor ground connection will not make anything in the system draw enough extra current so it should not pop the fuse.

    Car forensics.......

  12. #11
    turbojimmy's Avatar
    turbojimmy is offline Cadillac Owners Connoisseur
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    Re: Dual Battery Question

    Quote Originally Posted by the APEMAN View Post
    Having a poor ground connection will not make anything in the system draw enough extra current so it should not pop the fuse.

    Car forensics.......
    Now I'm concerned....something baked a 150 amp fuse. Had to be a dead short. I don't have anything that draws that much.

  13. #12
    limo1 is offline Cadillac Owners Member
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    Re: Dual Battery Question

    Quote Originally Posted by turbojimmy View Post
    That sounds like what's in my RV.

    Not sure when the limo fuse blew. I noticed that I had no juice from the rear battery at all. Then I noticed the ground cable hanging down and figured that was the problem. Upon fixing the cable, I still had no juice so I checked the fuse and it was cooked. I have the solenoid set up to connect the 2 batteries when I turn the key on - it probably cooked the first time I turned the key without the ground cable hooked up.

    When the solenoid closed the two batteries were directly connected together if one battery is not at same charge level or voltage as second battery due to week cell you can have hundreds of amps going between the two batteries.
    The internal resistance in the batteries is the only limiting factor also your solenoid will take a beating probably arc closed. Batteries should normally be charged at 10% of the rated capacity of battery or the plates will start to flake off and eventually cook the battery. Most Auto batteries are about 50-70 amp-hours. Deep cycle just have thicker plates but less plates. Using a solenoid to connect batteries is a bad idea. Use the diode isolator to connect batteries to the alternator. You only want batteries to be charged from alternator not from the other battery.
    If you have an alternator with the remote sense pin this can be connected to the fuse bank and will compensate for the series diode isolator and increase alternator output voltage.
    attached is a diagram of dual rectifier alternator setup.

    I can not seem to find how to paste in a picture? I guess forum rules
    the APEMAN likes this.

  14. #13
    turbojimmy's Avatar
    turbojimmy is offline Cadillac Owners Connoisseur
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    Re: Dual Battery Question

    Quote Originally Posted by limo1 View Post
    When the solenoid closed the two batteries were directly connected together if one battery is not at same charge level or voltage as second battery due to week cell you can have hundreds of amps going between the two batteries.
    That makes sense. The reason I installed the second battery in the limo was because the sound system was killing the front battery. Neither are deep cycle, but I routinely kill the back battery and charge it back up through the isolator (and I guess using current from the front battery).

    I never had any trouble with this arrangement until the ground cable for the rear battery corroded off of the frame rail.

  15. #14
    limo1 is offline Cadillac Owners Member
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    Re: Dual Battery Question

    I would use a deep cycle for the back battery as they have thicker plates and can take the continual charge discharge cycles. I have a TV and other items so car sits also you need the front battery in good charge to start the car.
    do not leave batteries in a discharged state too long, as the sulfur is absorbed by plates during discharge and forms sulfate crystals and this eventually causes the batteries to become useless. always keep batteries fully charged even a trickle charger is a good idea. fully charged batteries last for years.

  16. #15
    The-Dullahan is offline Cadillac Owners Master
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    Re: Dual Battery Question

    I prefer to use AGM batteries for the rear/auxillary. They can function as deep cycle, can be positioned however you like and meet my power needs (allowing for several hundred amp hours, as I use much larger batteries)

    As for a rectifier/diode, yes, you cannot have both batteries directly feeding one another. This is bad for both of them.

    There are however a number of ways that you can solve this. The easiest method is to utilize an isolator designed for your application. Many companies make very nice isolators that can be custom programmed to almost any scenario or voltage (though ours tend to be, not all vehicles are 12v). Personally, I use a RedArc isolator, set to only connect the batteries when the front battery (which actually usually only operates my starter) is at 12.6 Volts or higher. At 12.5 Volts, it disconnects the batteries. At each end, connected to the front battery and to the rear/aux batteries, I have a 300 AMP circuit breaker, allowing me to disconnect the circuit altogether, if I ever felt the desire, as well as providing the safety needed in the event of something. Naturally, you would use much lower amperages, depending on alternator output.

    This system has functioned flawlessly for the last five years, with all of my batteries remaining optimally charged, even when I am returning to the state after a long time away (AGM deep cycle batteries do not have as much of a habit to discharge on their own as a standard car battery, not that I have had a problem with either type)

    As Limo1 pointed out, a trickle charger is greatly beneficial if you are not using the vehicle daily, as it allows the batteries salvation from discharge and the eventual dysfunction that it causes.

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