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Old 28-06-2015, 09:37   #1
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Blowing 12 Volt Bulbs

Lately the boat has been burning out many of its 12 Volt Bulbs, the interior cabin lights, anchor light and stern light all blown, replace with new bulbs and blow again. The main ground wire was lose probably due to engine vibration so snugged that all up and haven't had a chance to see if this will stop the issues. Batteries are on a constant charge from a Charles 40 amp charger.

Would appreciate any input please as to a potential source of the issue, all other 12V equipment works fine.
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Old 28-06-2015, 09:58   #2
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Re: Blowing 12 Volt Bulbs

Please use a decent multimeter and measure the DC voltage while the engine(s) are running.

It should be about 12.5 VDC w/ the engines off and the batteries fully charged and 13.8 VDC while the engine(s) are running.

If the voltage is higher, then the power to the bulb goes up by the square of the voltage.
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Old 28-06-2015, 09:59   #3
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Re: Blowing 12 Volt Bulbs

Have you measured the voltage?
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Old 28-06-2015, 10:02   #4
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Re: Blowing 12 Volt Bulbs

Is your charger a good one? You don't want it on all the time if it's not got the proper charging cycles. Yes, check voltage.
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Old 28-06-2015, 17:07   #5
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Re: Blowing 12 Volt Bulbs

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Originally Posted by Cheechako View Post
Is your charger a good one? You don't want it on all the time if it's not got the proper charging cycles. Yes, check voltage.
Its a Charles Charger 40 amp believe a three step unit, as a livaboard boat is constantly running refrigeration and don't want to run batteries down.

I have checked the voltage with the multi meter on the batts many times voltage usually seems to be about 13.6 while the charger has been running, was blowing bulbs over the winter when the motor wasn't run for several months.
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Old 28-06-2015, 20:20   #6
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Re: Blowing 12 Volt Bulbs

Your alternator should put out much more than 13.8. In fact, depending on what batteries you have, it could be adjusted to put out 14.4 to 14.8 volts. If you are not using automotive bulbs then that could be the reason they are burning out on you. Another good reason to replace with LED lamps with built in current regulators.
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Old 28-06-2015, 21:43   #7
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Re: Blowing 12 Volt Bulbs

A poor ground will cause overcharging
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Old 28-06-2015, 23:57   #8
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Re: Blowing 12 Volt Bulbs

You might check out how well your ground wire is actually working also. Measure it's resistance, both, when it's "hot" (charging), & "cold" - where literally, the charger has been switched off for a while.
That, & it wouldn't hurt to check to see if you have any ground leaks while you're at it. Ditto as to measuring the actual voltage at the bulbs, & not just at the charger output or batteries.
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Old 29-06-2015, 10:33   #9
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Re: Blowing 12 Volt Bulbs

You said you tightened the ground. I suggest you remove the ground wire and clean the terminal and anchor point until you see shiny metal. If the ground has been loose for a while there has probably been some arcing going on and buildup has occurred.
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Old 29-06-2015, 11:15   #10
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Re: Blowing 12 Volt Bulbs

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You might check out how well your ground wire is actually working also. Measure it's resistance, both, when it's "hot" (charging), & "cold" - where literally, the charger has been switched off for a while.
That, & it wouldn't hurt to check to see if you have any ground leaks while you're at it. Ditto as to measuring the actual voltage at the bulbs, & not just at the charger output or batteries.
In a DC circuit, ground is usually the cause of most problems. Low voltage needs more amps to accomplish the tasks of given components. Bulbs will run hotter and burnout sooner, motors will do the same. the wires will get hotter too and fires are more likely. check voltage at the fixtures 12.2 is too low and 13v should only happen during bulk charging. Not only can the ground wire be bad the pos wire can be weak too. checking ohms on a wire will vary with length and gauge, but a drop test will tell you all you need. check the voltage at the beginning of the circuit and record the voltage at each connection along its length, on the pos side. Make sure to use a known good ground not the same ground of that circuit. if the voltage drops more that .7 volts in a low amp circuit replace that wire. in a high voltage circuit, such as starter motor, windless or electric winches, the drop should not be more that 1.0 volts. If the pos side is bad change both sides.

Hope this helps
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Old 29-06-2015, 11:19   #11
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Re: Blowing 12 Volt Bulbs

Oh yah, if you find a voltage drop at a certain connection clean and reattach it and check it again.
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Old 29-06-2015, 12:49   #12
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Re: Blowing 12 Volt Bulbs

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Please use a decent multimeter and measure the DC voltage while the engine(s) are running.

It should be about 12.5 VDC w/ the engines off and the batteries fully charged and 13.8 VDC while the engine(s) are running.

If the voltage is higher, then the power to the bulb goes up by the square of the voltage.
It may be higher than this. A charging voltage can be anywhere from 14.4 up as high as 15V depending on the battery type, charging system and whether they are temperature-compensated. Likewise, 12.5V is not a full battery. Most manufacturers give a number of 12.8V or greater for a full battery.

The likely cause of your bulbs blowing is low voltage, not high voltage, probably due to a bad connection somewhere or just old wiring or wires that are too small, both of which increase resistance. The power to the bulbs will always be the same. A 20W bulb will always draw 20W. Wattage is the product of voltage and current. So, at 12.8V, a 20W blub will draw a current of 20/12.8 = 1.56 amps. If there is a lot of resistance in the wiring however, the voltage will drop. Let's say the voltage at the bulbs is only 10V. Now your current is 20/10 = 2A. It is that high current that will cause the bulbs to blow prematurely.

Since the problem is not isolated to one set of bulbs, but rather a wide range, the problem is likely early on in the distribution system. Check the connections (both positive and negative) feeding the panel and tighten them up and clean them.

You may be getting a proper voltage at the batteries but the voltage at the bulbs is lower. So, check the voltage that is actually making it to the bulbs. Just take a bulb out, switch on the socket and touch the multimeter pins to the contacts in the socket. If it's low, work your way back towards the battery until you have identified the problem area.

You may have already fixed it with that negative connection. Good luck!
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Old 29-06-2015, 12:53   #13
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Re: Blowing 12 Volt Bulbs

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in a high voltage circuit, such as starter motor, windless or electric winches, the drop should not be more that 1.0 volts.
Unless you have some kind of step-up DC transformer for some reason, there are no high-voltage circuits. The circuits for the starter and windlass are high-current circuits, but the voltage is exactly the same as everything else on the boat; 12.8V(ish) when the battery is fully charged, less voltage drop for those particular circuits.
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Old 29-06-2015, 20:16   #14
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Re: Blowing 12 Volt Bulbs

Quote:
Originally Posted by DeepFrz View Post
Your alternator should put out much more than 13.8. In fact, depending on what batteries you have, it could be adjusted to put out 14.4 to 14.8 volts. If you are not using automotive bulbs then that could be the reason they are burning out on you. Another good reason to replace with LED lamps with built in current regulators.
LED current regulation - that is PWM for the uninitiated. . Read up on the technical bulletin at Marine Beam's site. Good ones will often list input power of 12 & 24. Frequently, they will operate from 9 to 32 VDC with no change in output. Imtra is another source of quality bulbs. Not looking for a fight but Super Bright LEDs has had problems
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Old 29-06-2015, 20:24   #15
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Re: Blowing 12 Volt Bulbs

Quote:
The power to the bulbs will always be the same. A 20W bulb will always draw 20W. Wattage is the product of voltage and current. So, at 12.8V, a 20W blub will draw a current of 20/12.8 = 1.56 amps. If there is a lot of resistance in the wiring however, the voltage will drop. Let's say the voltage at the bulbs is only 10V. Now your current is 20/10 = 2A. It is that high current that will cause the bulbs to blow prematurely.
I don't know where you learned your electrical theory but I think you need to go back and reread your books. A lamp, which is a resistance, will not draw more current if there is a high resistance in the wiring. A high resistance in the wiring may cause a fire but it will not be the cause of the bulbs burning out. A lamp does not "draw" power. The current running through the lamp is a product of voltage and resistance. Note that the resistance is of the whole circuit not the lamp alone.
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