How do you limit amp output from an alternator?

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?

 

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.

 

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.

 

E=IxR

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.

 

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.

 

Guilty as charged (pun).

 

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.

 

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.

 

Haha, I read it as 70C (160F) thinking you were on the conservative side temp wise.
Cheers,
a.