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Using a car to recharge a low battery

Discussion in 'Off Grid Living' started by oil pan 4, Aug 10, 2016.

  1. oil pan 4

    oil pan 4 Monkey+

    Well you can pretty much forget using a car as a way to recharge batteries. I always figured it would be less than ideal.
    This week I was away from home and had the alternator quit on my car.
    While I was changing the low battery with my wife's car while I worked on the alternator change. The healthy sized group 48 size battery, roughly around 60ah was at 11.8v, it bulk charged for about 1.5 hours and absorb charged for at least a half hour and still wasn't all the way fully charged. The other car was only able to produce 25 amps with everything off at idle. Roughly 2 hours of charging used most of a gallon of gas and only produced around a half kwh.
    So relying on a car for battery charging looks like at best that its a bad plan.
    Oltymer and Salted Weapon like this.
  2. Bandit99

    Bandit99 Monkey++

    Are you saying that the car alternator (the good one) that you were using could not produce enough amps to charge its battery plus the one you were trying to charge?
    Salted Weapon likes this.
  3. Oltymer

    Oltymer Monkey+

    Didn't realize that, always assumed it worked, but assuming is something that can get a person in trouble. Thanks for the information.
  4. oil pan 4

    oil pan 4 Monkey+

    The alternator in my car was not doing anything. I think the voltage regular failed. I took the battery out of my car and set it on the ground in front of her car and charged it with via jumper cables while I was replacing the alternator in my car.

    It does work but just barely. The amount of gas you burn and potentially pushing your car's alternator the failure is not worth it.
  5. john316

    john316 Monkey+

    if the battery is old,it may be worn out and will not a charge

    if the battery is new,it may be defected and will not take a charge
    my truck will charge an external battery

    be sure to use large cables..........#1 or #2 copper is best, #4 copper is the smallest you should use
    #6,8, and 10 guage are a joke.............just because something is made and sold does not mean it is any good
    Last edited: Aug 10, 2016
    Homer Simpson and Tully Mars like this.
  6. AD1

    AD1 Monkey+++

    I could be wrong but Usually you need the RPMs to be at crushing level 3000-ish RPMs to provide the volts/amps necessary.
    Homer Simpson and john316 like this.
  7. Airtime

    Airtime Monkey+++ Site Supporter

    That is not a surprise. Take a look at the output curve for a couple Delco alternators:


    Note that the pulley on the alternator is generally quite a bit smaller than the pulley on the harmonic balancer or crankshaft that drives the alternator. So the alternator will generally be turning 2-3 times the engine speed. A common idle speed is 550-600 rpms so your wife's alternator was maybe turning only 1100-1500 rpms. As you can see from the performance curve, the alternator needs to turn about double that to triple or quadruple the power output. Idle the engine a good bit higher and your results will significantly improve.

    Will the engine burn more fuel? Yeah. Interestingly many cars these days also have rev limiters built into the engine controls. While some in the past used to cut ignition to limit speed, most these days simply cut current to the fuel injector preventing injection and a combustion event. Once speed it back down below the control point it will resume injection. If the car is in neutral, I have seen these limiters kick in at just a few thousand RPMs. So, if the wife's car has a rev limiter like that, just block the accelerator pedal down and let it charge away. It will put you in a good power range on the alternator, will automatically not over rev the engine and it will cut back on fuel consumption as the battery charges and alternator load drops off. YMMV.

    Last edited: Aug 10, 2016
    Yard Dart, 3cyl, Tully Mars and 6 others like this.
  8. Altoidfishfins

    Altoidfishfins Monkey++

    As I understand it, automobile alternators aren't overly efficient at low speeds. This is why they make such lousy wind generators.
  9. Tempstar

    Tempstar Losing Patience Site Supporter+

    Alternators generally are pulley driven to reach full charge RPM at 2200 engine RPM. As Airtime posted, 4500 rpm on the alternator is the sweet spot. Probably could have ran the engine at that speed for 15 minutes and had the battery mostly charged on less fuel.
    Homer Simpson likes this.
  10. Tully Mars

    Tully Mars Metal weldin' monkey Site Supporter+

    Others have already mentioned the idle speed, but charging the battery from jumper cables is another weak link. if completely dead, charge for a bit with the jumpers, them swap the battery with the one in the car to charge. Hooked up to the car's cables it will take the charge better. Having a diesel, I've taken out one of the two batteries and put the discharged one in it's place. Charges the dead one in very short order.
    Homer Simpson likes this.
  11. If you use a batery isolater it will work fine just drive around for a few min people use them for rv and camp trailers
  12. Bandit99

    Bandit99 Monkey++

    @oil pan 4 good post! I learned something because I didn't know a car alternator went as high as 100-140 amps. Always learning something here swinging through the trees. :)
  13. oil pan 4

    oil pan 4 Monkey+

    This specifically applies to car alternators large case heavy duty truck alternators are different. I believe the CS144 series is more of a "heavy duty" pickup and SUV alternator. I know the CS130 is smaller and was put in a lot of GM cars.
    I am sure the alternator in my wifes car is rated for at least 80 amps at full load. But if you charge a lead acid battery too fast when low it will over heat and damage it, running a light duty car alternator at full output in engine bay heat will likely damage the alternator. Trying to charge it faster seemed like a no win situation and I wasn't in a hurry.
    I would have preferred to charge it from my 120v powered charger at 10 amps or less and equalized it.
    The jumper cables were likely the weak link, they were only light duty 6 gauge car jumper cables. I measured almost a 1 volt drop between her vehicle and my battery during peak bulk charging. So larger cables would have charged faster, but then it would have charged the battery faster, causing it to over heat and I would have had to disconnect and gave it a rest.

    The battery in my car seems fine. Likely because I only ran it down to 11.8 volts. Which is actually pretty low for a flooded starting battery.
    I got the new alternator in, $180 later. Went with the optional 120 amp unit. The stock VW unit should have been a 90 amp unit.
    Normally I use AD244/DR44 and 12SI series in my suburban, Camaro and engine driven toys.
  14. chimo

    chimo the few, the proud, the jarhead monkey crowd Site Supporter+

    Motor vehicle alternators are sized to power the vehicle and supply a modest charge to the battery...there isn't enough spare capacity to do more than barely trickle charge a second battery.
  15. Asia-Off-Grid

    Asia-Off-Grid God Bless those who have served. Site Supporter

    I used to own a 1980 Ford Truck. I pulled a 31' travel trailer with it. The trailer had dual deep cycle batteries to run all necessary appliances. I have always believed in "overkill", especially when it comes to auto-electric needs. A company in Savannah, Georgia, Lair's Auto Electric (I can only assume they are still in business, as this was 20+ years ago) used to sort out all of my auto-electric modifications. They could perform miracles when it came to auto-electric, guys and gals, I have to tell ya. Second to none.

    Anyway, on that particular truck, I had an adjustable voltage regulator. When pulling the trailer, I could increase the output. When not, I could turn it back a bit.

    Similarly, I later owned a 1990 Jeep YJ (rectangular headlamps) that I had added other electrical loads to - front and rear winches, decent CD player and amp, ham and cb radio equipment, lighting that would make the sun blush, and tons of meters, gauges and switches to monitor / control it all.

    I removed a single stock battery and added two Optima Yellow Tops, a 120 ampere alternator with large external diodes, and an adjustable regulator. I personally rewired the entire Jeep with marine grade wire and fittings. I also had the batteries on a dual battery switch, so I could isolate or combine them, as necessary, from the driver's seat. (Made great for those dusk days when I would forget to turn off the headlamps. Later return to the Jeep, realize the battery was dead, switch to the secondary battery to start, switch back to charge the dead battery, and off I went.)

    Now, these were not like new-fangled vehicles nowadays. These were easily worked on by any shade tree mechanic. But, what I am getting at here is, I never had any trouble keeping two or three deep cycle batteries charged at any given time, on either of those vehicles. However, it definitely took "after-market" products, rather than anything I would have had stock in either vehicle, to do so.

    Aside: I recall being at Lair's once, when a guy was having a second alternator installed on his truck. He had a switch inside that he used to turn it on and off, as necessary. A nice little touch, so it isn't working the engine any harder than necessary, during times it isn't needed.
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