Alternator size vs battery bank size

[benzy]

Need some advice on whether I'm going to cook my batteries.

The old engine on our Moody 376 has had it and we are putting in a new Beta 35. We have 300Ah of house batteries and an 80Ah starter (both are wet cell).

Being the clever guy I am , I thought I would get the Beta shipped with the larger 120amp alternator (with poly-v belt), in case we one day decided to increase our house batteries and needed the extra charging capability (standard alternator is 70amps).

Now I know the general rule that charge acceptance rate for wet-cells is approx 25% of capacity. So in our case it would be around 75 amps.

I also know that alternators don't run at 100% efficiency, so the max output is more likely 100-110 amps (not sure on this one?)

Anyway, I figured this combo (120amp alt and 300Ah batts) would be fine, and that the batteries would just "pull" whatever amperage they needed and the alternator would happily deliver with room to spare.

But...

I have since done some more research (including speaking to Beta), and I'm hearing that this might not be a good idea. It seems that the too much amperage might be pushed into the batteries, which could damage them?

So I have a couple of questions:
1. Could the alternator force too much current into the batteries and stuff them up?
2. Will a BMS/charge regulator solve this problem?
3. If a BMS/charge regulator is used, could it stuff the alternator because the alternator "wants" to output more than the BMS will allow?

Ps: Beta say that using a BMS/charge regulator voids the warranty on the alternator (not the rest of the engine) because it can cause the alternator to work too hard for too long, thus burning it out. But I figure since mine is over-sized, this would not be a problem.

Cheers!

[benzy]

Any help on this?

[conachair]

I have the same engine with a 120A alternator.
http://www.iskra-agv.cz/us/pdf/alternators.pdf
AAK model.

I just have 225Ah of trojans but never see more than maybe 40A going in. That's low revs, I haven't looked high revs, the batteries are seldom below 80% state of charge , there and above the amps going in tail of rapidly no matter what size of alternator you have.

Another thing to note, the alternator regulator is set to about 14.6V , if you do go for an external regulator it will need to completely replace the internal one - "piggy backed" on top won't do anything.

Sounds like it might be OK, unless you're regularly running full revs on a heavily discharged bank.

[btrayfors]

The 120A alternator will be fine. The FLA batteries will accept whatever amperage they want, and no more, presuming that the voltage level(s) are OK.

My preference would be for a good external regulator (like the Balmar MC-614) which would provide many options, but as noted you'd have to route around the internal regulator.

Still, I think you'll be fine with the internal regulator. Get a good clamp-on AC/DC ammeter and montitor things closely for awhile.

Bill

[a64pilot]

Even if you only had a motorcycle battery you would not hurt it.
It's too high a voltage that will hurt batteries, current wise they are self limiting.
However, you need a good regulator to make it work well.
You will be fine though it's just not optimum for charging is all.

[benzy]

Thanks for the replies.

So as btrayfors says, assuming voltage levels are ok, then FLA batteries will only take whatever amps they want, and no more.

But just to clarify then, does that mean the alternator only outputs that same amount of amps that the batteries will accept?

As conachair mentioned, if I'm running at full speed on a heavily discharged bank, the internal regulator will read low voltage on the batteries, so will start cranking out the amps. How does the alternator "know" many amps to output? Eg: is there a feedback loop between the batteries and the alternator to manage this? Because otherwise, I'm assuming the alternator will just output it's max rating, which the batteries will not accept, so where does all that current go?

Cheers

[a64pilot]

Alternator only makes what the batteries can accept by maintaining voltage, current output is controlled by voltage. So it's simple if batteries are way down on charge alternator will try to make set voltage, but can't as battery bank can draw more than alternator can make, this is known as bulk charging and if alternator is not thermally protected it can burn up an alternator.
Once voltage set point is reached, its maintained and battery current draw will drop as it becomes charged, eventually at full charge only current being drawn from the alternator is what is being used, well the battery will draw a small amount, but not much at all.
If you have a smart regulator at this point ideally it goes into a lower voltage, this is known as float, not much real charging is going on at float voltages, it's just to maintain charge.

In short, having a higher capacity alternator should mean that on average it will run cooler, charge your battery bank faster and last longer, however it will draw more power from the motor of course when it's making high power

Just for comparisons sake after a few min. My 120 amp alternator can only make about 85 or so as it's thermally protected and the regulator cuts it back to prevent overheating.

[smattie]

Something easy to remember, voltage is "pushed". Whatever voltage is pushed is what the battery gets. Amps are "drawn". Above post is 100% correct. You can connect a 12V 240A alternator to a lawnmower battery without hurting it. The voltage must be matched, don't worry about the amperage.

[Suijin]

Originally Posted by benzy

But just to clarify then, does that mean the alternator only outputs that same amount of amps that the batteries will accept?

Consider this. You're motoring long distance on a fully charged bank (size irrelevant), which happens in a lot of circumstances. This creates the scenario that you're concerned about, no matter the alternator size or bank capacity. The batteries regulate what they can accept and the regulator will protect the alternator.

[GordMay]

Greetings and welcome aboard the CF, smattie.
Well said.

[smattie]

Originally Posted by Suijin

Consider this. You're motoring long distance on a fully charged bank (size irrelevant), which happens in a lot of circumstances. This creates the scenario that you're concerned about, no matter the alternator size or bank capacity. The batteries regulate what they can accept and the regulator will protect the alternator.

Actually, the voltage regulator regulates the voltage and has nothing to do with the amperage

[btrayfors]

Consider this:

1. You have a 15A 120VAC circuit at home, governed by a 15A breaker or fuse.

2. That means that it is possible to draw up to 15A from the receptacle on this circuit.

3. You plug in a 5-watt nite light. The total draw on this 15A circuit is:

5 watts / 120 volts = .042 amps

So the total draw on this circuit is .042 amps, even though it is capable of delivering up to 15 amps.

So, how does it know how much to deliver?

Simple: it's called Ohm's Law. In any circuit, E=IR or volts=amps times resistance. And, watts = volts times amps.

A battery represents a variable load, depending on its state of charge, condition, temperature, etc.

When you connect a battery to an alternator with regulated voltage control, e.g., 14.4VDC, how much current will flow depends on the internal resistance of the battery (the load). Resistance is low when the battery's SOC is low, and as it charges up its resistance increases, thereby dialing back the amperage flow.

There's no question of "extra amperage" going somewhere....it's simply not generated.

Bottom line: you can't have an alternator or battery charger or solar panels, or wind generator which is "too large" for your battery bank, PROVIDED that the output of these devices is properly voltage regulated.

Bill

[Scott Berg]

What Bill said; and make sure that you install temp probes on both the alternator cage (to keep it from overheating) and on the battery posts (to allow for temp dependent charging. And yes, the Balmar MC 614 is the gold standard here... Good Luck.

[benzy]

Okay think I get it now. So...

Voltage regulator maintains voltage at a set level - it does this by changing the magnitude of current produced (V=IR, thanks btrayfors).

The danger for the batteries is related to the voltage regulator. If this is set too high or craps out all together, then the batteries are in danger of having too much voltage "pushed" into them, leading to over-charging and gassing, even going into thermal runaway.

The danger to the alternator is that the "draw" from the batteries is more than the alternator can produce, so it over heats and burns out (not a problem in my scenario as alternator is more than big enough).

Smart regulator is recommended as it does a better job (in theory) of regulating voltage to maximise the amount of charge into the battery without damaging it, thereby reducing engine running time. Also has the benefit of temperature monitoring of both batteries and alternator, offering additional protection.

Thanks for all the help.

[donradcliffe]

You have pretty much got it now. In addition, with a smart regulator you can reduce the alternator output if you want, so it does not overload a small engine, etc.

Beta sold me a 'brush block' for my Iskra alternator which replaced the internal regulator and let me hook up an external smart regulator. The brush block was easy to install.