Alternator size vs battery bank size

[a64pilot]

Depends on use, if your mostly a guy who keeps their boat in a marina and uses the engine primarily to get into an out of the Marina, you have no use for a smart regulator, be a waste of money, but if you depend on your engine to recharge a discharged battery bank, you need a smart regulator.

[conachair]

Originally Posted by a64pilot

Depends on use, if your mostly a guy who keeps their boat in a marina and uses the engine primarily to get into an out of the Marina, you have no use for a smart regulator, be a waste of money, but if you depend on your engine to recharge a discharged battery bank, you need a smart regulator.

The beta engines alternators are regulated at 14.6v anyway so maybe a smart regulator isn't that big a deal for them.

[a64pilot]

Originally Posted by conachair

The beta engines alternators are regulated at 14.6v anyway so maybe a smart regulator isn't that big a deal for them.

I believe on a long motor trip, that may cook my lifeline batteries, so it depends, but probably 90% of boats really don't need one

[conachair]

Originally Posted by a64pilot

I believe on a long motor trip, that may cook my lifeline batteries, so it depends, but probably 90% of boats really don't need one

Indeed very likely not good for agms, but seems most modern alternators are 14.4v or thereabouts without frying millions of car and truck batteries so float can't be that big a deal in the timescales we deal with.

[benzy]

We cruise for 6 months at a time, so will have periods of long motoring and motoring solely for charging purposes. We also have solar which tops the batteries to 100% after the engine has given them a good charge, but can't keep up with daily usage on it's own.

The most I ever saw going in from the old engine was 20 amps, which was a pain when the batteries were really low. From a full charge, we tended to last about 5-6 days of mostly sailing, with a bit of motoring to anchor, etc. But after 5-6 days we would need to motor to charge the batts and it would take many hours (5-6 hours) to get them back to 80%! I'm hoping with the new setup we will need <2hrs to get them from 50% back to 80%, solar will take care of the rest.

conchair I've looked at the link you provided to the Iskra alternators. You said it's the AAK model, but it seems only the AAK Compact has a 120A option. Do you know which it is exactly that comes with the Beta 35?

Cheers

[conachair]

Originally Posted by benzy

conchair I've looked at the link you provided to the Iskra alternators. You said it's the AAK model, but it seems only the AAK Compact has a 120A option. Do you know which it is exactly that comes with the Beta 35?

Cheers

The Datasheet beta sent me was for the AAK model, not the compact. But it had 120a on the Datasheet as well. This was from beta UK.

[Pete7]

Originally Posted by benzy

The most I ever saw going in from the old engine was 20 amps, which was a pain when the batteries were really low. From a full charge, we tended to last about 5-6 days of mostly sailing, with a bit of motoring to anchor, etc. But after 5-6 days we would need to motor to charge the batts and it would take many hours (5-6 hours) to get them back to 80%! I'm hoping with the new setup we will need <2hrs to get them from 50% back to 80%, solar will take care of the rest. Cheers

Benzy, we have been in a similar position, watched the volvo 60a alternator take 10 hours to put the last 30AH into a small batttery bank. We starting buying solar shortly after.

Nice boat btw are you members of the association?

[u4ea32]

You CANNOT overcharge a battery when its less than 80% SoC, and its hard to overcharge a battery before its 95% SoC. Once its over 100%, then its easy to overcharge a battery and damage it.

The battery companies publish recommendations for charging that minimize their warranty returns. Batteries are often damaged when they are overcharged due to cheap battery chargers that charge the battery for a very long time at constant voltage that is above the float voltage.

What is a cheap battery charger? The cheapest is a resistor that limits voltage. If the cheap charger has too little resistance and therefore too much voltage to the battery, then WHEN THE BATTERY IS CHARGED FOR A VERY LONG TIME, the battery can become overcharged.

Cars: start with close to 100% SoC, use an amp-hour to start, then reach 100% SoC again after a few minutes. So the built-in regulators in alternators are designed to barely charge, in order to avoid overcharging.

Sailboats while cruising have a totally different use case from the above, so the battery company recommendations are simply inappropriate.

Of course, the marine industry in general and the cruising sailor in particular are tiny and completely ignored by the big battery companies.

There are two huge consumers of 6v "golf cart" batteries (both flooded and agm): golf carts, and fork lifts. The two groups charge their batteries quite differently: the golf carts get used all day, then charged all night or even all week. The battery company recommendations work for long slow charging, where the risk is overcharging by leaving the charger on all week.

Forklifts, on the other hand, operate much more like cruising sailboats. They operate most of the time between 30% and 80% SoC, and only occasionally (every weekend in the case of industrial forklifts), the batteries get topped up to 100% SoC. During the week, they charge at any opportunity: change of shift, breaks, lunch. The forklifts automatically shut down if the SoC drops to 20%.

For the opportunistic charging during the week, the forklift industry uses very high current to charge, using voltages far above "battery bank voltage" to enable that current flow. They never exceed 80% SoC during these opportunistic charging periods, so THERE IS ZERO RISK OF OVERCHARGE.

By high rate of charge, this varies from a low of 1C (if you have a 400 amp hour battery bank, they charge at 400 amps) to a high of 9C (same 400 amp bank charged at 3600 amps!!). Many, many forklift and electric vehicle industry studies over the past 20 years using these charging rates have shown that battery life is IMPROVED by these high rate charging regimens below 80% SoC.

As mentioned earlier, the amount of current a battery can accept is a function of the voltage and the battery resistance. In order to get these high currents, the voltage is allowed TO INCREASE WITHOUT LIMIT. This is called CONSTANT CURRENT CHARGING or CI (I being the symbol for current).

So the right voltage regulator for the one hour in the morning charging that is common on voyaging sailboats? Simply tie the field to the battery voltage -- uncontrolled voltage! Then the alternator provides all the current the alternator windings can generate.

Now, this is potentially DANGEROUS unless the SoC and temperature of the battery is monitored. The battery should not increase in temperature above about 20 degrees F (10 degrees C), and charging should be stopped if the battery is above 55 degrees C. And again, this high rate (constant current) charging must stop at about 80% SoC.

This 80% is not a magic number. The thing that MUST be avoided is thermal runaway caused by the exothermic chemical reactions that start to occur at about 95% SoC, when gassing begins.

The expensive forklift opportunistic chargers are smart enough to monitor these things. They have to, because the charging always happens during a break, which means nobody is supervising the charging process: connect and forget.

On a sailboat, when you run the engine for an hour in the morning, you WILL be there, you MUST supervise the charging, and you MUST stop this constant current (limit of alternator current) charging when SoC reaches 80%.

Then, let your solar panels slowly fill up the batteries the rest of the way, the rest of the day.

But even then, there is a difference between sailboat battery use and forklifts and golf carts: sailboat 12v systems continue to be active at all times, even during charging.

The published limits of 14.4 or 14.6 (again, depending on specific battery) apply ONLY WHEN THERE ARE NO ACTIVE LOADS. And that is NOT the situation we have on boats. There ARE active loads during charging!

Therefore, its not just the battery that is consuming amps, its also the refrigeration, autopilot, instruments, pumps, etc., that continue to consume amps. Therefore, more than 14.6 volts may well be appropriate even during solar charging.

And the final situation: You must occasionally, like once a week, achieve 100% SoC. When you do so after many days of only achieving 80 or 90 or 95% SoC, you MUST equalize the batteries. This generally requires 15.5 volts AFTER achieving 100% SoC, for one or two hours.

Unfortunately, the Balmar regulators do none of these things. Therefore, calling the MC-614 a "gold standard" is probably inappropriate. Its much, much better than the built-in regulators that are suitable for cars, that is for sure. But its still a constant voltage, rather than constant current, regulator, and therefore VERY FAR from optimal for sailboats. Good for powerboats, very suboptimal for sailboats.

What we sailors really need is a constant current charger through 80% (easy, a switch that ties field to battery positive), then a solar charger that detects full charge properly using zero voltage change detection (again, Balmar does not do this), and then follow the 100% SoC once a week (or so) with equalization.

Zero voltage change detection requires a pretty smart charger: the charger can tell that 100% SoC is approaching, and then periodically stops charging for a minute or so to let the battery voltage settle, measure and record that voltage, and re-enable charging (this is low rate, voltage regulated charging, probably 14.4 to 14.6v). When the recorded voltages stop changing for several (say, a half dozen or a dozen) rest periods, then the battery can be considered 100% SoC.

As batteries age, the voltage achieved will slowly decrease. When the voltage drops too low, then replace the batteries. The replacement voltage depends on the particular type of battery.

[btrayfors]

u4ea32 has provided an excellent summary of battery type and charging characteristics in different usage situations.

I agree with MOST of what h/she said.

However, I certainly disagree most strongly with the implied recommendation for cruising sailboats to ignore voltage limits when recharging for the first hour or so, allowing voltage to rise well above the recommended 14.8VDC maximum for flooded LA batteries. Why?

Quite apart from the hands-on requirement to measure and monitor voltages closely, and the not-mentioned ease of achieving these voltages for a typical sailor on a cruising sailboat, there is the problem of damaging equipment which may well be connected and in use....sensitive electronics, lighting, etc.

Further, the fact that loads may be connected while charging does not mean that charging voltage must be increased. Rather, it should be maintained -- at 14.8VDC or whatever -- as measured at the battery bank.

For me, the MC-612 & MC-614 regulators do about as good a job as can be expected on a cruising sailboat, in the face of all the concerns mentioned in u4ea32's post.

Bill

[u4ea32]

I agree with Bill: if the constant current approach is followed, the DC loads must be connected via a DC-DC converter to ensure the DC loads never see voltages above about 15 volts (14.6 is a good limit).

Thanks for pointing that out, Bill! Its a VERY important point I forgot to mention.