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Thread: How, where to add a second battery to a car?

  1. #1
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    Default How, where to add a second battery to a car?

    Man, the web is such a vast useless pit of info at times. I really want to add a second battery to my ford e250 van and for the most part, the few kits Ive found online are just plastic or fiberglass boxes you screw to the floor or just sit on the floor and use a thin fabric strap with a plastic clip.

    I really want somewhere to secure the battery as Ive knowm batteries in wheelchairs to destroy the chair and cause a lot of damage in the cab of a van. And for a 50 pound battery, I would want it secured to the floor or under the hood much like the stock battery.

    Now where I work, they use the plastic box and just screw them to the bed of the truck near behind the cab. If its a flatbed body, right behind the cab, if its a full tool body its either in the bed area behind the cab or in a compartment box with the inverter.

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    Default Re: How, where to add a second battery to a car?

    Depending what year E250, some of them are available with dual batteries from the factory. If that's the case, under the hood in a factory bracket is the safest. Also remember if you don't replace both batteries with the same size/rating you should use an isolater.

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    Flashaholic* snakebite's Avatar
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    Default Re: How, where to add a second battery to a car?

    since the diesel version uses 2 batteries look at the service manual for it.
    my olds wagon has a diesel option and the empty battery tray was already there after relocating a few things.
    i got 2 new group 27 agm mk wheelchair batts and dropped them in.
    tied them in paralel with 2 ga cable run across the core support.
    on a -18 f morning mine was the only car that started around here.
    even brand new ones failed.
    i run a bunch of ham gear so i needed the extra.changed from a 10si to a cs144 alt too.

  4. #4

    Default Re: How, where to add a second battery to a car?

    Do you have room under the hood anywhere near the existing battery? All you need is to make a good sturdy shelf with a lip around the edge, and a pair of solid hold downs. Are you able to make the new wiring harness?

    Do you want both batteries always on at the same time? If you have two batteries in parallel, and one is weak and the other is strong, the strong one will sit there when the van is off and try to charge the weak one. It will keep trying until they are both equally weak. You used to be able to buy a kit for around $50 US that would keep them from doing this. It used diodes(?) so that they would power the car but not each other.

    If space is tight, or you want to keep things simple, you could just measure the existing battery spot and go out and buy the biggest 12 volt battery that will physically fit in that spot, maybe the battery from a medium truck or something. This will give you more amp hours for the least total fuss.

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    Flashaholic* BIGIRON's Avatar
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    Default Re: How, where to add a second battery to a car?

    You should have a second battery deck from the factory. It probably won't have any fittings or hardware, but that shouldn't be much of a problem.

    You can, if you wish, buy a factory type dual battery cable for a diesel truck which should fit in just fine (OEM is expensive - aftermarket probably $70-80) I replaced mine last year and had it made, along with the negative cables at a local starter/battery rebuilder. $100 for all.

    Advice about differing strength batteries is correct. I keep matching batteries along with a solar float charger on mine (this is my f250 camper truck). If you have to use differing batteries, you might pick up a cheap disconnect switch (either the knife switch or the "green knob" switch) and break the circuit when the vehicle will be idle for a while. Any good auto parts store should have them. Harbor Freight has both.

    If you do go to the additional batt, you might also consider upgrading the voltage regulator. If both batteries are fairly deeply discharged, the initial charging will probably fry the small regulator and the fusible link. I bought several $12.99 regulators and links before I learned this. Not one since.

    One thing you didn't take into consideration when you were debating whether to buy this truck -- you're in for a heckuva learning experience.

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    *Flashaholic* carrot's Avatar
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    Default Re: How, where to add a second battery to a car?

    Quote Originally Posted by cobb
    Man, the web is such a vast useless pit of info at times.
    You do know CPF is a part of the web... right?
    [gearcarrot.com] Collector and distributor of (mis)information.
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    Default Re: How, where to add a second battery to a car?

    Once you tackle the physical mounting of the battery, you will need to electrically isolate it from the existing battery... you cannot simply wire them in parallel.

    The device you need is (oddly enough) called a battery isolator, and you can find them at nearly any garden-variety auto parts store. I wouldn't go with the $35 versions though, since if it fails then you have NO electrical power. But easy to work around; simply reconnect one battery as usual.

    If you don't mind parting with a little extra cash, get a marine battery isolator. They're WAY better, not only in terms of reliability, but also in terms of efficiency. A battery isolator uses huge diodes, and as with any diode, it presents some resistance when forward-biased. Better isolators will use better internals which will minimize this loss. A lot of the marine isolators will come in sizes to allow use of two or more batteries.

    As far as the physical mounting of the battery, summitracing.com has a real good selection of battery boxes. I have one of their aluminum boxes in my track-day car and it's very heavy-duty. (My track-day car is a hatchback with the battery mounted inside the passenger area, so the metal box was required to pass tech inspection... they don't allow plastic.)

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    Default Re: How, where to add a second battery to a car?

    Quote Originally Posted by scott.cr
    Once you tackle the physical mounting of the battery, you will need to electrically isolate it from the existing battery... you cannot simply wire them in parallel.

    Who says you can't wire them in parallel? If they batteries are the same model and purchased at the same time it's perfectly fine to run them parallel. This is what all the Diesel light duty trucks do from the factory. If you go with a different size 2nd battery then you will need the isolater I mentioned in my first reply.

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    Default Re: How, where to add a second battery to a car?

    then you will need the isolater I mentioned in my first reply.
    Sorry VWTim, I didn't notice that. I read all the replies but missed that mention.

    Anyway, the factory trucks with dual parallel batteries have a ~140 amp alternator. If Cobb's van has a standard 70-80 amp alternator, charging two parallel batteries full time will be very hard on the brushes and regulator. It "could" be done, but unless the rest of the charging system is upgraded for constant dual battery duty, the isolator is the way to go.

  10. #10
    Flashaholic* chevrofreak's Avatar
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    Default Re: How, where to add a second battery to a car?

    The larger John Deere tractors very effectively use two masive 6v batteries connected in series to supply 12v and massive amounts of current and capacity. Maybe you could rig something like that up with a cross cable between two 6V batteries.

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    Default Re: How, where to add a second battery to a car?

    Quote Originally Posted by scott.cr
    Sorry VWTim, I didn't notice that. I read all the replies but missed that mention.

    Anyway, the factory trucks with dual parallel batteries have a ~140 amp alternator. If Cobb's van has a standard 70-80 amp alternator, charging two parallel batteries full time will be very hard on the brushes and regulator. It "could" be done, but unless the rest of the charging system is upgraded for constant dual battery duty, the isolator is the way to go.
    Well, unless the batteries are regularly ran down then there's nothing to worry about with smaller alternators. Starting the car will drain a battery X amount, starting a car with 2 parallel batteries will drain each battery X/2. So the alternator is simply charging the same amount. Lots of car audio people do this in instances where larger alternators just aren't feasible. And for that matter a large alternator for the van would be a bolt in away.

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    Default Re: How, where to add a second battery to a car?

    Thanks, will check out the racing website. They may have something beefy enough. Other idea was to mount the battery on the floor behind my seat. I just used google and did come across the mechanical and electric isolators, diodes vs solonoids.

    I think I did recall seeing some holes on the other side behind the other headlight where a battery could be mounted. I think it just had the two holes. I would need the tray, rods and cross over bar to hold it down.

    I am mixed on setup. Having them in parallel sounds nice to have more power avaliable for everything, then again, having one battery protected to be good if the other fails sounds even better. I could leave loads on it til it died and not worry about not being able to start. Also if the stock battery dies, I would use a pair of jumper cables and jump start the first battery.

    Sounds like the isolator is a must. Now, if I use it, couldnt I use a deep cycle as the second battery and size wouldnt matter? Wouldnt a deep cycle be great for loads while the starter one does the starter?

  13. #13

    Default Re: How, where to add a second battery to a car?

    Quote Originally Posted by cobb
    Sounds like the isolator is a must. Now, if I use it, couldnt I use a deep cycle as the second battery and size wouldnt matter? Wouldnt a deep cycle be great for loads while the starter one does the starter?
    Yup, a deep cycle would be perfect.
    Got Biodiesel?

  14. #14
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    Default Re: How, where to add a second battery to a car?

    Note, if you use a battery until it dies, then it will hardly last you any time at all (a matter of weeks or months).

    If you are planning on running loads a lot (like camping/alternate living conditions) then a deep cycle battery, that is never discharged much more than 50% is a very good idea. Automotive batteries are designed for starting (heavy, short loads), not for lots of deep discharge/charge cycles.

    You may wish to research the charging voltages/profiles for deep cycle vs the typical automotive battery. It may be OK, but I am not sure if it is a good idea (for long battery life) to parallel charge the two different types of batteries.

    How are you going to recharge the second battery? Are you going to drive enough or have AC power available to recharge it when needed? Or are you going to have to run the van to recharge the battery (such as camping for days/weeks in one spot)?

    A gas generator may be more efficient for those types of charging conditions (Honda's eu1000i and eu2000i are pretty small, quiet, and fuel efficient) plus you can use them for larger loads (like cooking/watching TV/using computer in the evenings). The 2kW unit may even power a small AC unit. The 2kW (peak) generator uses about 1.1 gallons for four hours of at 100% (1.6kW) or 15 hours at 50% (800 watts).

    -Bill

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    Default Re: How, where to add a second battery to a car?

    Ive assumed the stock alternator would charge both batteries through the isolator and at its 70-80 amp capacity, that shouldnt take long.

    I do not plan on camping, but to run accessories like a thermal electric cooler, sirius radio with fm transmitter to hear it at work and home from the van on an fm radio and maybe an internal white led light.

    I think the radio takes half an amp and the cooler 3 amps. It needs to run a few hours to cool down before it work well, you just cant plug it in and get things instantly cool. At room temp with 9 room temp cans of cola it can take 6 hours to cool them.

    So, for the most part the radio would be on 24/7, the cooler maybe the same. Wonder if they make a timer for electrical stuff in your car? Could I turn the cooler off when I arrive home and have it turn on a few hours before i leave?

  16. #16
    Enlightened
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    Default Re: How, where to add a second battery to a car?

    but if I remember correctly from doing my own homebrew car stereo projects and researching said projects, that even if the alternator is rated at 70-80 amps, that is only at a much higher RPM than idle or even driving. Case in point, my car has a factory rated 160amp alternator (Chrysler T&C minivan with factory upgraded alternator) and 2 450 watt amps in the back. Both amps are fused at 70 amps on the *amp* (god that sounds redundant ) and there is a 100 amp fuse under the hood near the battery. I have never ever blown the 100 amp fuse in the 2 years of having the stereo (I mostly listen to classic rock and the like its more of an SQ thing the subs are for if any of you follow car stereo), so I can be *mostly* assured that I have not drawn more than 100 amps to the subs in the back. Theoretically if the alternator was able to put out 160 amps and I have never come close to blowing the fuse, then the alternator *should* be able to power the draw from the rest of the car and the stereo at the same time without the battery sagging to much. I have also done what is known as the "Big 3" in car stereo, replace the alternator to battery power cable, the battery negative to chassis ground and the chassis to engine ground cable with much heaver gauge than the pitiful factory wire and yet I can still dimm the headlights and draw down the battery voltage with out *too* much volume, listening to bass heavy music (and it gets way worse at idle). Basically what im trying to ramble on about is that the stock alternator, no matter its rating will put out less amps than it is rated at, especially at idle. What is needed to be remembnered is that highway speeds with a properly geared overdrive transmission will only run the engine at some where around 2000-2700 rpm's, still under the rpm's that the alternator is rated at. If you are going to be charging 2 batteries, i would heartilly recommend doing the "Big 3" to your car and upgrading the alternator, as the stock one may not have the "oomph" for both batteries and the draw from the rest of the car electrical system. The "Big 3" is also good as it reduces the resistance in your charging system, making the alternator seem "bigger" than it is. Sorry for the long post, if anyone stuck around till the end........ Good for You

  17. #17
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    Default Re: How, where to add a second battery to a car?

    Pretty straight forward to calculate the amount of discharge the second battery can support...

    Look at the 20 hour rate (or, if you have the specs., the rate closest to your load). If the 20 hour rate is 5 amps... That would be a 100 amp*hour storage capacity. (This should be the approximate capacity of an average "car sized" storage battery--IIRC).

    1/2 of that, would be 50 amp hours. Call your loads 5 amps, then you can run your loads for roughly 10 hours to 50%--much more, and your battery life will suffer.

    Your alternator would need to operate at roughly 50 amps for one hour to bring your battery back up to, approximately, full charge...

    In real life, long term charging maintenance of a storage battery would probably like to see a slower charge rate (~0.1C or 10 amps for 5 hours).

    Can you plug in an AC cord from your house to your van? That would let you put the warm drinks in the night before, run the cooler and top off the battery with a lower charge rate... If you choose to plug the van in at night, use a standard AC plug, not a "twist lock" style--You would prefer that the plug disconnects safely if you forget when you back out in the morning.

    To run the loads 24 hours, you should probably use two to three batteries (or a single larger battery) and you would probably have to drive 2-3 hours per day to keep the charge up with just your van's engine... (Assuming a 5 amp continuous load at 12 volts).

    By the way, some of your car accessories may include a low-voltage cut-off to help protect your battery... Something that you may wish to look for if you spend a lot of time and money on this.

    The typical cooler(s) are going to be a big electricity hog... I used to just freeze some of the drinks or put ice in a good igloo type cooler. Works well and probably would cost you much less in the long run (smaller battery, better gas mileage, less weight). Using your home fridge is probably going to be more energy efficient.

    Good Luck,
    -Bill
    Last edited by BB; 04-29-2006 at 08:54 PM.

  18. #18
    *Flashaholic* PlayboyJoeShmoe's Avatar
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    Default Re: How, where to add a second battery to a car?

    What they said about Diesel "Light" trucks using "identical" twin batteries.

    I used to plug my block-heater in on my 1990 F350 and it was a no brainer to unplug it in the morning. I even had it on a timer to come on about 4 hours before I left each day.

    Something along those lines would work for a battery charger for an isolated deep cycle battery....
    http://img32.imageshack.us/img32/2640/usflaghalfmastmdwht.gif

    PBJS



  19. #19
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    Default Re: How, where to add a second battery to a car?

    Man, you cant win can you? I know from personal experience a car/truck uses about 37 amps at idle to run the stuff like lights, flashers, defrosters, etc that you may run at night and with everything on. I assumed I would draw that or less and have a good 37-40 amps to go above idle. Assuming I have the 70 amp stock non ac alternator. A new alternator is out of the question right now, too much money, maybe a junkyard one.

    Yeah, I currently freeze a bottle of water to keep the cool drinks cool already and thought I could quit doing this. I use to have a cooler on my wheelchair that ran all day and it didnt effect my battery power that much. THe electric 300 watt heater would draw a bit of power, but I never used it that much as the cooler that runs all day.

    Thanks, maybe I should put this on hold?

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