|Topic Review (Newest First)|
|09-16-2009 01:26 AM|
Originally Posted by simple72 View Post
You must use the voltmeter function and do a voltage drop test of the circuit. Just do a search on how to do voltage drop testing, youtube has some good videos on that subject.
13.8v is the minimum acceptable voltage for an alternator. Acceptable Alternator voltage output will range from 13.8 to approximately 14.8 volts. While your output is on the lowside it is still in the acceptable range.
|09-15-2009 11:39 PM|
"Volt meters use a small current to check for continuity "
You could confuse some folks that way. Voltage meters don't check for continuity, they check for voltage. Multimeters, when being used as ohmmeters, check to continuity by applying a small current to the leads. And of course these days almost eveyrone uses a digital multimeter, not a dedicated voltmeter. But considering how many sailors will read at face value...be kind to the newbies.
A Zap Stop can be a good thing, it is really a $5 specialty diode if you know where to order one, and with the usual markets and expenses $25 is not unreasonable for one. BUT. Last time I was on a boat with one installed, both leads had broken off the diode because the owner had simply installed it with electrical tape in the middle of the wire, without any strain relief. It MUST be mounted and secured to prevent that failure.
And even better, there are a number of modern alternator designs with protection built into them, which can be disconnected (i.e. switched over) while operating, without taking any damage. You really need to know what your alternator can take, before bothering with something that can add more failure modes to it.
A 1-wire alternator is a kludge job that will never charge batteries properly, since it can only measure alternator output voltage and it never really monitors battery voltage. Good way to charge and cook a cheap battery on a tractor or a cheap boat. Bad way to treat anything that will cost more than $100.
|09-15-2009 09:17 PM|
Volt meters use a small current to check for continuity and should show "0" even for small gage wire. They can barely turn on a red led due to the low voltage and current. A voltage drop should be checked from the engine ground to the ground terminal on the battery when at max charge and the plus alternator output should also be checked for voltage drop to the plus side of the battery.
4 gage is .25 ohms per 1,000 ft so .25 volts for 10ft at 100 amps.
6 gage is .40 ohms per 1,000 ft so .4 volts for 10 ft at 100 amps
8 gage is .64 ohms per 1,000 ft so .64 volts for 10 ft at 100 amps
Excessive voltage drop will prevent 1 wire alternators from achieving full battery charge. Remote sense alternators can compensate for the voltage drop.
I would expect 14.2 to 14.4 volts output at 2,000 rpm and above, if less the alternator should be checked if the belt(s) are in good shape and properly tensioned.
The local Autozone spun mine for free and it was found to be dead. If running at partial power, could be 1 or 2 of the diodes. Install a ZAP stop diode to prevent diode damage.. A small investment/ insurance.
|09-10-2009 04:48 PM|
Just reread your original post. You said that the voltage reads 12.9 volts with the switch on both and 13.2 with the switch on 1 or 2. Where are you measuring this voltage from? The alternator output wire should go to the Guest switch common. From there it would go to whichever battery you have selected (or both). There may be a smaller wire going to the panel if you have a voltmeter. But it should not be the only output wire. I just finished adding a new engine panel and rather than shortening the harness, as I moved the panel 6' closer to the engine, I rewired the engine totally instead. I found that the original wiring (32 years old) was not tinned marine wire like what we use today and there was evidence of corrosion as well as it not being of a large enough size. My guess is that somewhere from the alternator's output to the Guest switch or from the switch to each battery there is more resistance than you want - there should be minimal resistance. This could be either from a bad or loose connection or the wires themselves could be suspect if old. The ground wiring could also be a problem - either the alternator connection, engine ground, or battery posts themselves. If there is more than one issue with older wiring replacing may be a good idea.
|09-10-2009 04:29 PM|
I agree the alternator output is low - best to check that out at an alternator shop. But the voltage should not be higher at the alternator than at the engine panel. What gauge is the wire? Most factory wiring on older boats is inadequate in gauge and travels a sometimes convaluted route to get to the batteries. I prefer a straight path as there is less chance of problems. I understand the batteries are set up as bank 1 and bank 2. A single group 27 for house loads while away from the dock is pretty minimal on a 30' sailboat unless you are using kerosene lamps. You would be best to increase the size of the house bank in my opinion. If your batteries are the same age I would suggest paralleling them to create a house bank twice the size and purchasing a start battery for the engine side of the system. The best way to charge batteries is not through the main battery switch. If you were to purchase an Echocharge you could wire the alternator (with a heavier gauge wire) directly to the house bank. The Echocharge is a simple 3 wire installation and is a reliable solid state device (available for $100-$125 US). It automatically takes care of the start battery - when it senses a charge voltage is present it allows up to 15 amps to travel to the start battery. The battery switch would determine which battery is used for which purpose. Leave it on house when on the boat (#1), start (#2) to start the engine and off when leaving the boat. If someone were to switch it off when the engine is running there would be no problem as charging wouldn't be interupted. If you have a shore charger with this scenario its output would also go directly to the house bank. I hope this helps. Please inform us of the results.
|09-10-2009 04:28 PM|
' The alternator output wire joins the harness and travels back to the cockpit engine panel."
That might NOT be the output wire, the output shouldn't be going back to the panel at all, as far as I know. There SHOULD be one or two wires from the alternator going back to the panel, but they would be the excite voltage and key switch that tell the alternator "OK, turn on now".
On many alternators, the output wire is a single heavier wire connected to a stud all by itself, and the other 2-3 wires will all be connected to one wire connector/plug.
Something to doublecheck.
|09-10-2009 04:19 PM|
Originally Posted by captbillc View Post
The part I don't understand is
Is the wire disconnected at both ends when you check for resistance? Are you using the same ground when checking for voltage at the alternator output and at the cockpit engine panel?
|09-10-2009 04:02 PM|
If you read 13.8 volts at the alternator, that's low, but at idle speed it might be normal. Many alternators just don't pout out useful voltage at idle speed. With the engine in neutral, at "crusing rpm" you should read 14.3-14.4 volts at the alternator, and the same at the battery posts.
If you don't read 14.4 at the alternator, it probably has a defective internal regulator or defective diode. Your choice, take it in, replace it, repair it.
the alternator shows 14.4 but you aren't getting that at the batteries, you'll need to trace out the wiring (charge and charge sense leads and ground) to see where the problem is.
In order to do that clearly, it helps to draw out (pencil!) a schematic that shows every "box" and wire along the way, including any a/b switch, any 'zapstop" or other gizmo installed, and so on.
You could just have some bad wiring, but the only way to find out is by first testing the alternator/regulator, and then continuing to veroify every inch of the way back to the batteries.
Of course if you are lucky, it is just an old worn glazed alternator belt (any real shiny spots or piles of black dust?) or the alternator hasn't been tensioned enough, so it is just slipping. First check the belt, and if it is 5 years old replace it regardless. If you can, get a belt tension meter to set the tension. Doing it by feel or by ruler works--but not quite as well, and too little tension lets the alternator slip, while too much can ruin bearings. So the tension guage is a "buy it once, use it for life" investment.
|09-10-2009 03:57 PM|
|captbillc||how long have you been running the engine to charge the batteries? will the voltage rise if you charge one battery for a while? i expect the voltage will not rise for a while because the batteries were so low. are you at a dock where you can use a charger? how did you get the engine started in the first place? if you are at anchor it will take a long time to fully charge the batteries.|
|09-10-2009 03:33 PM|
Not getting enough charge voltage.
I sail a 1984 Tartan 3000 sloop. I'm currently living aboard and fine tuning her for a big cruise.
Here's my issue.
The boat is equipped with a 50amp Motorola alternator and two new group 27 wet batteries.
The batteries are discharged and reading about 12.10 volts with no load.
When I run the engine at about 2000rpms I get about 12.9 volts with the guest battery selector on Both. I get about 13.2 volts with the selector on battery 1 or 2.
According to Don Casey's book this voltage is inadequate for charging.
I'm getting 13.8 volts at the alternator. The alternator output wire joins the harness and travels back to the cockpit engine panel. When I read the voltage there it's 13.2.
I've tested the output wire for resistance and got a zero reading.
Does anyone have any idea why I'm not getting enough voltage? Is my alternator shot??? Should I install an new and larger alt output wire???