Batteries slow to charge from alternator - SailNet Community

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Go Back   SailNet Community > On Board > Gear & Maintenance
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  #1  
Old 09-08-2011
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Batteries slow to charge from alternator

Last weekend I went on my first overnight trip. We spent 3 days in Provincetown. I don't have solar panels or a generator so I was using the engine to charge the batteries. It seemed really weak at charging, and barely made any progress in the hour or so I would run it at a time. I was expecting the battery voltage to jump up above 13V when the engine started, but it would merely increase by about 0.1V (as measured by the digital display on my inverter and loosely corroborated by the analog meter on the main panel). Over the hour of running the engine, the voltage would steadily march upwards to perhaps 0.5V more than where it started. The alternator was clearly working, but very poorly. There was no appreciable load on the system except the sporadic cycling of the refrigerator (about 9A). Any idea what might be going on? I've never paid attention to this before so I don't know if it has always been this way. I do know the boat had no functional battery charger when I bought it and the previous owner kept it on a mooring, so he was clearly charging it off the engine.
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Old 09-08-2011
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Originally Posted by rmeador View Post
Last weekend I went on my first overnight trip. We spent 3 days in Provincetown. I don't have solar panels or a generator so I was using the engine to charge the batteries. It seemed really weak at charging, and barely made any progress in the hour or so I would run it at a time. I was expecting the battery voltage to jump up above 13V when the engine started, but it would merely increase by about 0.1V (as measured by the digital display on my inverter and loosely corroborated by the analog meter on the main panel). Over the hour of running the engine, the voltage would steadily march upwards to perhaps 0.5V more than where it started. The alternator was clearly working, but very poorly. There was no appreciable load on the system except the sporadic cycling of the refrigerator (about 9A). Any idea what might be going on? I've never paid attention to this before so I don't know if it has always been this way. I do know the boat had no functional battery charger when I bought it and the previous owner kept it on a mooring, so he was clearly charging it off the engine.
We would need more info to help..

-Size of alternator
-How it is wired
-Size of bank and type
-Depth of discharge when you began charging
-9A is a decent load and one that can help keep the voltage low while in bulk.
Do you have a battery monitor or ammeter?

A deeply discharged deep cycle bank can take hours to reach absorption voltage with a small alternator. Get a cheap DVM from Wal*Mart and check it with that vs. your analog gauge or the one on the inverter.

Check the voltage output at the alternator then check it again across the bank and look for a voltage drop. The voltage at your B+/Alt Output terminal should be within +/- 0.1-0.2V from the battery terminal.
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Old 09-08-2011
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If this is a small boat with an outboard, it might simply have limited charging power.

If this is a larger boat with an inboard, alternator, etc. then I'd suggest the internal regulator in the alternator is shot, pull it and take it to any shop to be tested.

Could be something more complicated or a wiring issue but a bad regulator is probably the most common scenario.
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Old 09-08-2011
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You should see 13+ volts w/ the engine fired up. Do as maine sail says and you will probably find a bad connection somewhere. Don't forget to check grounds.
A battery monitoring system would help you a lot. It will show you how many amp hours you are down and show you the amps your alternator is putting out when you are charging. Makes it pretty simple to monitor your battery banks.
You also didn't state if the alternator is internally or externally regulated.
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My Bristol behaved in the same way when I got it, the cause was a fried alternator. Some places will bench test them for you.

Would be worth eliminating the wiring as the cause before splashing out on a new one.
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Ok, here's some more info as requested:

I do not know the size of the alternator. I don't believe it is very large, and it is certainly not one of the high-output alternators a lot of people have as aftermarket equipment. It dates to at least the last time the engine was rebuilt, if not earlier, which was circa 2003. The engine is a Perkins 4-108.

The alternator is wired to the combined terminal on the battery combiner switch. The wire is probably 0 gauge. The batteries consist of two 110Ah Group 27 wet cell batteries (IIRC). Each battery is wired separately to the battery combiner, but I always run them with the combiner set to "both", so they're treated as one bank.

I do not know the depth of discharge in terms of Ah since I do not have a battery monitor (it's on the list, but I currently need to spend money on more pressing repairs/upgrades). The battery voltage was about 12.4V the first time I began charging, and over the course of the next couple of days, it ended up around 11.5V, despite my efforts at charging. I do have an ammeter and it showed nearly no draw except for when the fridge was cycling, which was 9A for only a short duty cycle (other things on were GPS and wind instruments, together they don't even draw enough to register on the ammeter). As an example, one time the battery voltage was about 12.2V with no load and about 12.1V when the fridge was running, then when I started the engine it went to about 12.3V with no load and 12.1V with the fridge running. Over the next hour or so it climbed up to about 12.5V with no load but would still drop to 12.3V or 12.4V when the fridge kicked on. (the fridge runs frequently with very short on-time. this is not a marine fridge, it's a normal 120V minifridge running on an inverter. yes, I'm measuring it's current draw on the DC side of the inverter. getting a proper marine fridge is one of those higher priority items I need to spend my money on rather than a battery monitor).

When I got home, I plugged in the shore power, and my 40A charger was able to get the voltage from ~11.5V up to ~13V instantly. It stayed in bulk charge mode for 3-4 hours. I don't know how long it was in absorption mode, but it was full by the time I woke up the next morning, so less than 8 hours. I'd expect my alternator to put out at least 40A, so I'd expect it to be able to raise the voltage similar to the shore power charger.

I'll do some more testing of the connections and wiring with a DMM this weekend and I'll report back. From what you guys are saying, I strongly suspect the regulator is shot. The regulator is internal to the alternator, btw. I've never heard of a failure mode where it would output just a little bit of power, I always thought they were all or nothing kinds of devices. Thank you for your help!
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With your batteries full from the shore charger shut the shore charger off and ALL DC loads including the inverter. Then fire up the motor and let it run for about 10-15 minutes. At this point check the alternator output AT the B+/output terminal of the alternator grounding your meter to the engine block, t-stat housing etc.. If it is below 13.6V or so your alternator needs repair or a cleaner case ground connection to the engine block.

Some older alternators are only set to 13.6V to 13.8V absorption voltage. Newer ones are in the 14.2V -14.6V range.

You can check it like this:Measuring Alternator Output Voltage - YouTube
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Check your fan belt tension before doing anything else. If it has been slipping and the belt is polished replace it.

Occams razor.
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