How do you limit amp output from an alternator?

Discussion in 'Off Grid Living' started by Duck, Jun 18, 2014.

  1. Duck

    Duck Monkey

    Like if I wanted to make a 12 volt generator with a 100 amp capability but wanted to be able to turn it down to limit it's output to 30 amps for charging two deep cycle batteries (15 amps each).

    Two deep cycles will draw as much as you give them (theoretically). Ideal rate of charge is typically 10-15% of it's amp hour rating. 100 amp hours, give it 10-15 amps. Two 80 amp hour batteries in a parallel bank, ... give 'em 16-24 amps.

    So if I want to build a generator for charging those batteries, I'll want to be able to reduce the amount of amps they receive so I'm not feeding them 70-100 amps or so.

    How is that done?
  2. melbo

    melbo Hunter Gatherer Administrator Founding Member

    I'm not equipped to answer your question but can tell you that my Outback charge controller will push 80 amps to my (4x 6Volt) battery bank if it's depleted and my panels are in full sun. This is during what they call 'bulk mode' and then it begins to scale back as the voltage rises. I think my charger runs at bulk, absorb and then float but this is all set by the voltages targets from my battery manufactures data sheet.

    I guess I was under the impression that watching voltage was more critical than the amps your pushing in in a battery bank setup. Of corse, for me, this is all automated.

    @BTPost @ghrit
    mechstdr likes this.
  3. ghrit

    ghrit Bad company Administrator Founding Member

    First things first: You need to know the mfrs design charge rates and the voltage wanted to satisfy that. A common voltage regulator (basically a relay with a resistor lashed up to limit the generator output voltage. Every automotive generator or alternator has such a gizmo.) Controlling the voltage differential between the generator voltage and residual battery voltage will control the amperage.

    If you want to do it manually, a variable resistor and an ammeter is all you need. However, you are unlikely to want to sit at the resistor and watch the meter.

    Mo' bettah says me is to get your hands on a charge controller set up to charge the batteries the way the mfrs recommend. There are several charging schemes in common use depending on the type of battery. For examples, consider the common auto battery. High discharge rates, but very intermittent, takes one scheme. Then consider industrial batts that come in several different constructions (and design charging schemes) depending on use. I can't give you much more info without knowing the design parameters of your batteries.

    Bear in mind that the batteries you use have to be balanced, meaning that they are close to the same initial voltage, or one will tend to hog the amps you are sending. In some cases, one will try to charge the other if there is a significant difference, and there will be no control on the voltage difference, hence there could be damaging amps flowing between them.
    mechstdr, Pax Mentis, oldawg and 2 others like this.
  4. BTPost

    BTPost Stumpy Old Fart,Deadman Walking, Snow Monkey Moderator

    In Three State Charging, you set the three different SetPoints by Voltage, and Temp, of the BATTERY...
    in Bulk you pump as many Amps as you can into the Battery, until one of two things happen.
    1. Battery Temp exceeds 70F
    2. Battery Voltage exceeds Battery Type SetPoint.
    Either one sends the Charge Mode to Absorb Mode.
    In Absorb Mode, the Voltage is held constant, and the Amps tapper off, u,ntil all the PBSO4 has been reduced back to PB and H2SO4, and the Specific Gravity of the Electrolyte is that of H2SO4. Then the Charge Mode dumps to Float Mode.
    Float Mode the Voltage is held Constant at the Manufacture's recommended Float Voltage, + whatever
    the Static Battery Load is.

    This is the BEST Charging System ever invented for ReChargeable Batteries.
  5. melbo

    melbo Hunter Gatherer Administrator Founding Member

    Can you tell us more about the generator you're going to build?
  6. BTPost

    BTPost Stumpy Old Fart,Deadman Walking, Snow Monkey Moderator

    Very few Car Alternators will actually Output 100 Amps, for any length of time, and the ones that will, are very expensive. Car Alternators are notoriously inefficient, and subject to Overheating, easily, should you try to get their Rated Output for any Length of time. Marine Alternators are usually better choices. West marine makes some really Good Alternator/Regulator Setups.
    mechstdr and Dunerunner like this.
  7. Tevin

    Tevin Monkey+++

    First off, some clarification: Per Ohm's Law, a battery, or ANY load for that matter, will not "draw as much as you give them". They will draw as much as they need according to their internal resistance and the voltage applied. This is why the current to a battery goes down as the battery charges.

    You can limit how much current is sent to a load, but you cannot "force" it to take more current than R÷E will allow, nor will a load consume more power just because the power source has the extra capacity to supply it. This is a basic rule of physics and there is no wiggle room or exceptions.

    Example: If you plug a A 100 watt lightbulb into a 1000 watt generator, the bulb will consume 100 watts. It will not suddenly draw more watts if you plug it into a 10,000 watt generator.

    Using a variable resistor and an ammeter as previously suggested will not work because resistors drop voltage, not current. Placing the resistor in series with the battery will reduce the voltage to the battery, which is not desirable in a system where the source and the load voltage need to be essentially the same. Placing the resistor in parallel keeps 12 volts on the battery, does not change the current to the battery, but increases the overall current draw of the entire circuit. And either setup produces a lot of heat. You would need a pretty big mofo variable resistor.

    If this were my project, I would connect the alternator directly to the batteries and let Ohm's Law run its course. As long the batteries are not allowed to go completely dead, it will be be ok.

    If you are not comfortable with any of this, there are commercially made charge controllers that will limit the current for you.
    Last edited: Jun 19, 2014
    mechstdr, Tempstar and kellory like this.
  8. tacmotusn

    tacmotusn RIP 1/13/21

    When in Florida, except in the wee hours of the morning, the temperature of a car battery outside less than 70 degrees F? Almost never! So, do we have specially designed for high temps batteries, or what. What is the reference on the 70 degrees F factor? Confused, in the hot zone. PS; we do not put our auto batteries in the frig to charge them and at the same time keep them cooler than 70 degrees F.
  9. BTPost

    BTPost Stumpy Old Fart,Deadman Walking, Snow Monkey Moderator

    I lost the "1' on that Temp... My Bad.... it is fixed now.... 70F should read 170F
  10. ghrit

    ghrit Bad company Administrator Founding Member

    I daresay it will work, simply because adding resistance (assuming constant supply voltage, a reasonable assumption in a battery charging situation) over and above the internal resistance of the battery surely reduces current as Ohm's Law says. Granted, the resistor may have to be able to withstand considerable heat (especially) early in a charging cycle, and must be selected accordingly
    mechstdr likes this.
  11. HK_User

    HK_User A Productive Monkey is a Happy Monkey


    You have to overcome the internal resistance of the battery. This takes voltage above the present state of charge. The higher the voltage the higher the charge rate aka current flow. Other concerns, as mentioned, is heat within the battery and the production of explosive gas.

    High dollar batteries have listed rates at which to best maintain a long life for the battery.

    Purchase a quality charger to fit your battery bank, you'll save money and batteries in the long run.

    Along with that are finishing rates and equilizer charges after a test discharge.
    Last edited: Jun 19, 2014
    mechstdr likes this.
  12. Tevin

    Tevin Monkey+++

    I had a feeling someone would bring this up, but I didn't "go there" because I felt it was too off-topic and detailed. But ok, here we are.

    It will not work and here is why: Adding a resistor in series will reduce current to the battery, but it will also reduce the voltage. As they taught us in Electronics 101 and fully vetted by Ohm's Law:

    1. In a series circuit, the sum of the voltage drops across each resistance will always equal the voltage of the source.
    2. In a series circuit, the current will always be the same at all points in the circuit.

    For example: If your resistor drops 3 volts, that means the battery gets only nine. Yes, the current to the battery is lowered but only because you gave up three volts to get there.

    Think of voltage in a series circuit like a pie. You can cut the pie up into as many slices as you want, and the slices do not all have to be the same size, but the pie as a whole (source voltage) is a finite size that cannot be changed. The more guests (resistances) you want to serve, the smaller each slice will be.

    This explanation applies only to series circuits; if anyone whats to know why a parallel configuration will not work either, just say so and I'll "go there" too.

    Last edited: Jun 19, 2014
    mechstdr likes this.
  13. ghrit

    ghrit Bad company Administrator Founding Member

    You need not quote Kirchoff's laws here. I'm done.
    mechstdr likes this.
  14. ghrit

    ghrit Bad company Administrator Founding Member

    Spoken like a sub sailor.
  15. HK_User

    HK_User A Productive Monkey is a Happy Monkey

    Guilty as charged (pun).
    mechstdr and Coyote like this.
  16. ghrit

    ghrit Bad company Administrator Founding Member

    The equalizers were a significant PIA, even if in port. The EMs (nucs, fwd aux and ICs) were nearly intolerably short tempered during those exercises, not to mention the acid burned dungs. (I think it had more to do with them being deprived of their fave pastimes of acey-deucy and dominoes than with the work involved.)
    mechstdr likes this.
  17. HK_User

    HK_User A Productive Monkey is a Happy Monkey

    Nucs were a delight compared to a Diesel with 504 cells. I'd rather be beat with a fathom of rope with Monkey Fist tails than do another charge, well unless I could pick the Home Port. Yup I was a Charging Electrician on a Fast Attack Diesel and a Battery Well diver on the nucs.
    mechstdr likes this.
  18. ghrit

    ghrit Bad company Administrator Founding Member

    Heh. Yep, you wuz der. My time was all nuc, (plank owner on 615 hull, after a stint on staff as ELT at S1C.) Only one battery compartment with just enough capacity for a cold startup. We did one just for s&g alongside the pier during shakedown, it worked. Some limited cell imbalance when we got steam in the ER, but enough to warrant an equalizer (that was coming due anyway.) Back then, the charge was manually regulated (by voltage, IIRC) according to procedures in place. As a nuc MM, I didn't have the pleasure of twisting the knobs in switchboard alley after a very brief qualification.

    S1C and 615 are both long gone --

    With all the advances in battery technology, I've often wondered if they still use lead-acid cans on the boats.

    Pick your own port, aye. Had my dream sheet filled out before leaving S1C, Charleston, Pearl, or Seattle. Got NLon. Lovely. Too close to MIL, only an hour.
    mechstdr likes this.
  19. HK_User

    HK_User A Productive Monkey is a Happy Monkey

    Did my stint a bit A$$ Bacwards. Planker on 625 and a short stay on 644 in NNSBDDCo. until a guy non voled on the 641. The 641 being on Trials needed an experienced hand so I got moved and became a two planker. Did trips on the 641 and then raised my hand for more excitment. Being a non Nuc IC type gave me a lot of choices. Ended up in Pearl and then a bit farther west and sometimes north on the 563.

    As to "still use lead-acid cans on the boats." I'll ask my kids, they both served on Nucs.
    mechstdr likes this.
  20. Coyote

    Coyote Monkey

    Haha, I read it as 70C (160F) thinking you were on the conservative side temp wise.
    mechstdr and Dunerunner like this.
survivalmonkey SSL seal warrant canary