Charging Trouble, alternator or wiring?

[Tkenopic]

Hi everyone,
Currently out cruising in the North Channel of Georgian Bay and had starting trouble yesterday. Batteries were both low. Finally started, voltages at batteries were increasing very slowly. Ran the engine until voltage crept up to 13

Checked the voltage at the alternator (an old Motorola with a new regulator last year), measuring from positive terminal to ground and its reading 13.6.

Ammeter in cockpit is reading low, maybe 5 amps.

Wondering how to diagnose and fix?
Thanks in advance....
Trevor

[edwardc]

13.6 is more like a float level than a charge level. My best guess (and it is a guess, without further measurements) is that the batteries are toast. This kind of behavior (low battery reaches voltage without absorbing much charge) is typical of batteries that are shot. The only way to know for sure is to do a capacity test. (This is NOT the quick "starter test" that most auto shops perform).


WARNING: Math Content

A true capacity test is tricky to do properly, but a crude one can be performed with an inverter and a light bulb. Select an incandescent bulb of the proper wattage (more on that later), plug it into the inverter, and hook it up to the batteries. Make sure it's the only load. The test is over when either of two conditions occur:

1. 20 hours has elapsed. or
2. The voltage falls to 10.5 V

If the batteries make it to 20 hours, they are at 100% capacity.
More likely, the test will end before that. The ratio of the time elapsed to 20 hours, times 100, gives you the battery capacity in percent.

For example, if the test only ran for 18 hours, 18/20 * 100 = 90%

Once the batteries are down to 80%, they are generally considered shot.

Now for selecting the wattage of the light bulb. Batteries have an Amp-Hour rating, but the amount of power you can get out of them varies depending on how hard you discharge them. The harder you pull power, the fewer Amp-Hours you get out total.

The published Amp-Hour spec is for a standard 20-hour rate. So, in order to figure out what current that corresponds to, you take the battery's Amp-Hour rating and divide it by 20. So, for example, if you have a 100 Amp-Hour battery bank, 100 Amp-Hours / 20 Hours = 5 Amps.

But light bulbs are labeled in Wattage. Watts = Volts x Amps. So, on the battery side, 12 Volts x 5 Amps = 60 Watts. Now, its actually a little more complicated than this, as we're starting with 13.6 V and ending up with 10.5 Volts, but I did say in the beginning that this was a crude test. So, in this example, a 60 Watt bulb would be the answer.

EXTRA CREDIT

The above ignores the losses in the inverter, so you'll actually be pulling more than 5 Amps. To select a better light bulb, measure the battery current actually being consumed by the inverter (a clamp-on ammeter is good for this), and reduce the light bulb wattage by trial & error to get the battery current down to 5 Amps. This may require using multiple small bulbs to get to odd, non-standard Wattages. For example, a 40 Watt bulb plus a 7 Watt night-light bulb.

As a final note, any time you discharge lead-acid batteries below 50% of their capacity, you impact the overall lifetime of the batteries. Because of this, a capacity test should not be performed very often.

[Tkenopic]

Thanks Ed, I should have added that info. Battery 2 is brand new and battery 1 is 90%. Just had a load test at a specialty battery shop before the trip...

[Al Schober]

I'd check the output wire from the regulator to see what voltage is being sent to the field. Should be close to 12V to get max alternator output. You can file a needle point on your voltmeter probe and stick it through the insulation or just use a sewing needle.

[Tkenopic]

I managed to get a hold of the mechanic that worked on my alternator last year and he walked me through a few tests to determine that the alternator is working fine. I used the wiring schematic from MM to trace the orange wire to the back of the ammeter. Tested the voltage on both sides so it doesn't look like it's the problem. After that the factory wiring is a spaghetti mess so it's hard to figure out where things go. I've also checked for heat at various connections but found none.

MM is closed this week for vacation so wasnt able to get a hold of anyone there. If anyone has any other ideas of how to find the voltage drop, let me know...

On a positive note the weather and sailing have been great up here this year!

Cheers,
Trevor

[Dave Neptune]

Did you check the "field" voltage as per Al's suggestion? This is very important because it controls the alternators output. If the alt is OK it has to be a bad connection in the "feed" side, the alt or the "absorb" side the connections and battery condition.
Another check would be the "batteries" connections to the block (the grounds) and starter (positive connection)?

Dave Neptune

[joe_db]

I get results similar to what you are seeing when some of the diodes go bad.
Here is a test.
1. Remove the regulator.
2. You will notice the field wire going into the back of the regulator.
3. Put a test lead on it but don't connect it to anything.
4. Make SURE there are no fumes, there is now no ignition protection.
5. Run the engine but make sure sensitive loads like electronics are off.
6. Touch the test lead to +12 volts.
The voltage should immediately jump up and you should hear the engine load up. On full batteries you can easily get 15 volts.
If this does not happen, you have dead diodes or some other internal issue.

[sastanley]

13.8 is about what those regulators are set at, unless you have a newer one that could be as high as 15v. Do you have any more details on that regulator? With an 8 gauge wire going from the alt, up to the ammeter, and back to probably the 'common' post on the battery switch, I'd expect about 13.6v charge voltage. They are a little harder to find, but there is an adjustable version of the 105-197 (or M5-197A) regulator. I have mine set to provide 14.1-14.2v at the batteries. I also removed my cockpit ammeter to reduce the run, and beefed up my output wire at the alternator post to 4 gauge in an attempt to reduce voltage loss. (I added a shunt style ammeter from Blue Seas to the house battery so I can watch in & out current flow.)