How Long for Alternator to slow charging rate?

[tritonyawl2]

Hi all,

My battery was not charged all winter, but started the engine just fine. I added distilled water to the battery and launched the boat. In last couple weeks engine has run about 3 hours. She is outputting 14.1-4 volts when running. Engine says 15 to 30 amps. But it always shows that amount when running. I would think the charge would be nearly complete by now.

I can take it into the company that rebuilt the old Motorola alternator last year, but that takes at least half a day. They can test it.

I don't want to overcharge the battery (660 cranking amps), but all seems to be running OK.

I wonder if the charging rate will slow soon or am I courting disaster.

Any clues would be helpful.

Thanks!

Hal

[Dave Neptune]

A check or 3

Hal, the voltage seems OK albeit on the high side. After running the amps should be dropping down as the battery gets charged up.

First thing I would do is check the voltages at the battery and compare with the alt output. Also check and inspect the grounds and the all of the positive side connections in the charging system. This includes the exciter from the ignition is so equipped. What I am thinking is the battery is OK but the regulator is not seeing the actual voltage due to a poor connection.

Dave Neptune

[tritonyawl2]

Thanks Dave. I will do those checks tomorrow.

[hanleyclifford]

A battery load tester is a tool that will pay for itself again and again:

[Al Schober]

The voltages seem to be fine, but pull a few battery caps and take a look. If things are merrily bubbling away, you may have a bad meter. Time for a second opinion on the voltage?

[ndutton]

I'd like to see voltage measurements at different points in the system. As Dave mentioned, right at the alternator output post, at the exciter wire (if present), at the battery posts during charging and at the battery posts at rest. If something's wrong, these measurements should uncover it.

Always good to have an accurate meter, or maybe two meters to compare.

[tenders]

What's the battery voltage with the engine off? You could have a failed cell in the battery. You have enough voltage to start the engine but not to taper off the alternator's charging.

[Administrator]

Always good to have an accurate meter, or maybe two meters to compare.

I have what I thought was a very nice Fluke 10 multimeter. Expensive, if not nice, anyway. This spring, I was testing some 12 volt batteries and getting readings of 16 volts+. Wow!

Anyway, I sent it out to be repaired, and now 12 volts is 12 volts again. You would think that an instrument of this alleged quality would read nothing, rather than something grossly erroneous, when things go south.

I guess two meters is not overkill when the numbers really matter.

Bill

[ndutton]

As an electrical professional for over 35 years I've owned a variety of testers: Knopp wiggys, Amprobes, Flukes, one very sensitive Greenlee and a few I cannot remember. My tester of choice these days is the Harbor Freight Tools $20 clamp meter (on sale right now for $13, think I'll get a couple more). I buy a new one at least every year, usually have a couple around.

It does the job just fine and the best thing is when I drop it off a ladder and it's destroyed it's a minor event.

[edwardc]

...It does the job just fine and the best thing is when I drop it off a ladder and it's destroyed it's a minor event.

Wow! What part of your engine compartment do you need a ladder to get to!?

[ndutton]

Wow! What part of your engine compartment do you need a ladder to get to!?

Substitute 'dropping it overboard' in the case of the boat.

[joe_db]

Question: What kind of regulator do you have? If it is the one that comes with the Motorola, it is a simple fixed set-point regulator. It does absolutely nothing to try and decide what voltage is appropriate for conditions. All it ever does is increase the field current whenever the voltage is lower than the set point and decrease it when it is above the set point. The voltage you see at the battery can be lower than the set point if the battery is low and/or the loads are high because the alternator might not be able to put out enough power to reach the set point voltage, but it should never go higher than the set point.
What you might see with a set point of 14.4 volts and big battery that is discharged:
When you start the engine even full blast from the alternator won't reach 14.4 volts. It might be quite a bit lower with a big nearly-dead battery. As the battery charges the voltage will creep up to 14.4 volts and stay there.

Is this what you see?

EDIT - I can take a while! My main battery is 200 AH. If it is 75% dead it needs 150 AH. Running the engine an hour will - at best - maybe add 50 AH to the battery assuming no other loads, so it is still half dead after an hour. If you have the original A4 wiring with the long wire run to an ammeter you can add some more time.

[edwardc]

Substitute 'dropping it overboard' in the case of the boat.

Sorry. I just had this vision of a cavernous engine room. Your boat would have to be dimensionally transcendental (bigger on the inside than it is on the outside).

[JOHN COOKSON]

What's the battery voltage with the engine off? You could have a failed cell in the battery. You have enough voltage to start the engine but not to taper off the alternator's charging.

+1
Been there. Done this.

TRUE GRIT

[ndutton]

Wanted to emphasize the charging rate is not solely determined by the alt/reg. As others have said, the battery condition - its ability to accept and hold a charge - as well as cable terminations are major contributors to charge rate.