Battery Charging (again!) - Am I too full for my alternators?

[Snowgoose35]

Both my alternators (twin engine cat) work properly on a bench test. They are 35a each and connected to the house batteries via a pair of VSRs

I have 1000w of solar and an MPPT charge controller from Morningstar.

At worst the display on the MPPT shows in the morning about 60% state of charge but the inverter reading normally shows around 80% at the same time so neither can be relied upon and indeed Morningstar say its a very rough indication and effected by current drain on the batteries against current charging amps so basically not really to be relied on.

When I motor I notice that we never really get anything to speak of from the alternators - my guess is that we're always too "full" for them to be doing any noticeable amps (especially as the start batteries will be 99% full) - as the alternators do definitely work and all the wiring is new and over sized is this a correct assessment ? If I ever get to having very low batteries (a week of heavy cloud perhaps) should i expect to see 70 amps pumping in through the VSRs?

What I'm trying to decide is if I need to upgrade to expensive DC DC chargers just in case I need an alternative to solar sometimes or will the VSRs do the job of getting the bulk charge in and leave the solar to do the clever absorb and float stuff?

As we live aboard and cruise full time I don't really have any opportunities to test out with heavily discharged batteries to be sure...

[john61ct]

It is easy to get House depleted, just use it without charging a while.

Put some other big loads to work off House as well when you get a chance, heat-producing appliances running off inverters, or a windlass or. . .

Measure amps with a clamp ammeter close to the alt, before things get hot, at decent rpm.

35A might be a short-run peak rating not continuous.

[GordMay]

... When I motor I notice that we never really get anything to speak of from the alternators - my guess is that we're always too "full" for them to be doing drawing any noticeable amps...

Indeed.
Alternators & Chargers don't "push", batteries "suck".
You're charge source will only put out the amperage that the (somewhat depleted) battery demands.

[Sailmonkey]

Are your alternators connected to the start batteries?

Your alternator are stock 35 amp, internally regulated? If yes, even if connected directly to the discharges house bank don’t expect to see much more than 10-15 amps continuously out of either one of them.

[smac999]

Draw a diagram.

Then with engines running. Reved up. With discharged house batteries. Measure voltage of engine and house batteries. With solar off. Night time.

[senormechanico]

Also, measure voltage right at the alternator output and compare that to the voltage right at the battery to which it's connected to see how much voltage drop you have in the associated wiring.

[roland stockham]

It is probable that your alternators are internally regulated as others have said. This means they give an output of 13.4-13.8v. That voltage is set at a point that wil maintain a charged battery and provide DC power to any services in use. it WILL NOT charge a battery. Standard care alternators are not designed as battery chargers. What you need is an external 3 step regulator. This converts your alternator to a battery charger and I would expect to see 40-50a as a reasonable output with the engines on fast idle. Output will depend on bank size. If you divide the bank size in Ahr by 10 that is about what you should see at 50-70%dod. So that is 10a for a 100a/hr bank

[john61ct]

As stated, loads can be increased in order to determine the IRL maximum continuous output current.

[Stu Jackson]

Originally Posted by roland stockham

It is probable that your alternators are internally regulated as others have said. This means they give an output of 13.4-13.8v. >>>>>>>>>>>>>>>

Roland, I used to believe that, but Maine Sail corrected me about a decade ago. His point was that the output voltage of internally regulated alternators could well have "inched up" since I originally thought that.


That said, the rest of your post is absolutely correct.


For the OP, while you're checking under the hood, check the output voltage at cruising rpms.


You may also be interested in these before you go buying an esxternal regulator since you have solar:


Automotive Alternators vs. Deep Cycle Battery banks (by Maine Sail) (NEW March 2016)

http://www.pbase.com/mainecruising/a...ycle_batteries


How Alternators & Regulators Work PLUS External vs. Internal Regulators (by Maine Sail): http://forums.sbo.sailboatowners.com...d.php?t=125392

[john61ct]

Originally Posted by Stu Jackson

Automotive Alternators vs. Deep Cycle Battery banks (by Maine Sail) (NEW March 2016)

http://www.pbase.com/mainecruising/a...ycle_batteries

That link is to his old site, now deprecated.

The current one, which does get additions and updates: https://marinehowto.com/automotive-a...cle-batteries/

[CarlF]

Since you've got a lot of solar (1000 watts is a lot - sure it's not 100 watts?). Check it first without adding in the alternator issues. I'm assuming the boat is not plugged into the dock. Ignore the % charge on the Morningstar. What absorption voltage are the solar controllers set to? 14.4v? On a sunny morning before you start the engine, see if the voltage slowly rises to 14.4v (or your setting) as the solar panels charge the batteries from the night's use. If it does, your batteries are at least 80%-90% charged. After several hours at the absorption level, the solar controller will go to float and you'll see the battery bank voltage drop to the float setting of around 13.5V. When this happens, your batteries are fully charged. If this is all set, turn to the alternators.

To test the alternator, disconnect the solar panels and run the batteries down so the battery voltage with the engines off is about 12.5v. This is still plenty of voltage to start the engine. Start the engines (still no solar) and run at 2000 RPM. You should see at least 25 amps from each alternator (they never give the rated amperage). As the batteries charge and the alternator heats up, this amperage will go down. That's normal.

If you still don't get much amperage from the alternator, you have to determine if the problem is with the alternator, the regulator, or the wiring. That's a tougher diagnosis and probably requires a mechanic or marine electrician to come take a look.

[Stu Jackson]

Originally Posted by john61ct

That link is to his old site, now deprecated.

The current one, which does get additions and updates: https://marinehowto.com/automotive-a...cle-batteries/

Thank you, john. I think. Over the years I have gathered tons of links in my 101 Topics on our C34 website forum. I KNOW Maine Sail has begun his own new website, I do not intend to go back and change all those links. Perhaps you could convince him to keep the old site up, like, forever. Would save me a TON of work!


And quite frankly there's not much to add or change to what he wrote. Is there?

[TrentePieds]

GordMay's post seems to be an implicit confirmation that your suspicion that your batteries are "too full" to accept addition charging is right. One way of determining the "fullness" of your batteries is to test them with a battery hygrometer that tells you the specific gravity of the electrolyte.

But that, obviously, only works with "flooded" batteries. Once you get into hy-tech stuff, you just make your life difficult. Because I hate technology, generally, I am sticking with batteries that will talk to me via the hygrometer. I certainly have no difficulty, using these antediluvian devices, installing enough capacity to meet my modest energy budget. They are cheap to buy and to maintain, so who cares if you have to replace them every five or six years? In the long run they6 are cheaper than hy-tech stuff, and less worrisome.

There are nifty little ammeters, "Wemyss" is one brand, that have a sensor that splices into the positive lead between the battery and the load on it. They tell you how many amps are going into the battery or out of it. The sensor is made so that the installation will tolerate the high ampage starting current flowing to it, while only a tiny ampage current drive the actual read-out whether it be digital of analogue. The read-out is, of course, installed remotely from the batt.s, i.e. in your instrument panel or wherever is convenient. I find it soothing to see a "discharge" reading change to a 10 Amp "charge" reading, or thereabouts, for a few minutes after starting while putting back what was taken out of the batteries, and then settle down to what is basically a "zero" reading because the alternator is now feeding my electrical devices. After engine shutdown the "discharge" reading shows me via quick mental math how much time I have to play with before I must do whatever it takes to make the reading go to "charge". That "whatever" can be engine alternator, solar panels or a ProNautic charger (taking power from the shore) that I think is just the cat's pajamas.

TrentePieds

[john61ct]

Originally Posted by Stu Jackson

And quite frankly there's not much to add or change to what he wrote. Is there?

He has years ago asked that people don't link to the old one anymore.

And as I said, updates / additions are only being made to the new location.

He should shut it down ASAP, it hurts his SEO having both.

But likely he doesn't have the admin access to auto-redirect (code 301) from the old URL to new.

If you just replace them one at a time pretty soon they're all done.

Just launch the old, plug a bit of text (up to 32 significant words) from there into google and you should see the direct marinehowto.com link among the top results.

[senormechanico]

HyGrometer is for checking relative humidity in the atmosphere.


HyDrometer is for checking the density of a liquid, in this case, in a lead acid battery.