Interior lights start to dim

Well i have noticed lately that when i am hooked up to shore power and power supplied to 110v and 12v side. that after a while my interior lights start to dim till they will not work anymore ( they dim till no light is visible). This is after a period of a couple hours and they will not work again till morning. I have a dual battery system with an on board charging system. Any suggestions would be helpful.


1986 carver 26 santa cruz 260 merc and alpha 1 outdrive if that helps.

Re: Interior lights start to dim

Have you checked the voltage on the batteries as the lights are being used to make sure that your charger is keeping up to the usage? Maybe there's a bad ground, faulty wiring or a bulb drawing too much current and draining the batteries? What else is being used while the lights are on?

Re: Interior lights start to dim

I have not checked the batteries while the lights are on. I am also running the 12v fridge while i have the lights on too. The fridge seems to work just fine though and keeps things cold. When i check the voltage on the battery gauge both read 14 volts, this is while the charger in on as well. I will check the batteries with a volt meter while under load to check where they are at. I haven't checked the battery grounds because the boat starts strong every time. I think the batteries are old and may need replacing, maybe.

Re: Interior lights start to dim

For the light circuit does it have fuses or breakers? Maybe it has a heat sensitive type breaker for the lights and while they're on, the circuit is getting warm and causing the breaker to slowly open. Then once it's cool it resets itself. I'm just throwing things out there that are easy to check and hoping something makes sense and helps out.

Re: Interior lights start to dim

Sorry, but that's not how breakers or fuses work. They open when to much current is being drawn, that's it.

Sounds like your charger can't keep up with the load. If the batteries are getting old it is probably time to replace them. You may have the voltage there but not the amps.

Before replacing them, make sure that they are topped off with water just in case.

Good luck!

Re: Interior lights start to dim

I'm pretty familiar with the 120 volt AC to 12 volt DC converters from the RV industry. It sounds like the one you have isn't keeping up with the demand. When the batteries are fully charged, most converters are designed to sense that and switch to trickle charge (.5 amp or so). I've seen many instances where the converters don't sense it, never switch out of fast charge mode (around 10 - 20 amp, depending on the model), and end up overcharging and boiling out the batteries. While I've never heard of it, it is possible that the converter is stuck in trickle charge mode. This condition could also be due to your battery condtions/ages. Sometimes, a dead cell in one battery will be a "black hole" for the charge to enter. This black hole will also suck voltage from other batteries in the system. Thus, some charge is going to the black hole, and some to the black hole. The black hole, recharging the good battery, running the lights/other accessories is just too much for any good converter to keep up with.

When the boat is plugged in to shore power, your converter is definitely charging (should be putting out 13.5 - 14.5 volts, depending on the model). This is shown by your meter reading 14 volts. Is that 14 volts while the batteries are fresh, or long after they are dimming down? After dimming down, the converter will still put out 14+ volts, but that voltage will be much lower when the lights/dying batteries demand the bulk of the amperage (thus decreasing the voltage). We'll save the further technical data (voltage/resistance/amperage relationship) for another discussion.

Re: Interior lights start to dim

Blue Crabber said:

Sorry, but that's not how breakers or fuses work.

Click to expand...

Actually that is how fuses/breakers work. Current causes heat which causes a breaker to trip or a fuse to blow. If it's a self re-settable breaker that is old and worn out it can exhibit this kind of behaviour.

Re: Interior lights start to dim

Right, more current draw equals more heat and trips/blows the breakers/fuses. We agree on that.

All that I meant is that they should not slowly open, they are either on or off. (closed or open) But if one is going bad, I suppose it is possible.

Re: Interior lights start to dim

Blue Crabber said:

All that I meant is that they should not slowly open, they are either on or off. (closed or open) But if one is going bad, I suppose it is possible.

Click to expand...

A fuse wouldn't open slowly, and if it did (like on a friend's SAAB), it only happens once. A breaker though, depending on the type just might. I've seen some pretty wonky stuff happen on Harley's because of this which is the only reason I asked. Like I said though, I'm just throwing spaghetti against the wall to see if any of it would stick.

Re: Interior lights start to dim

The gauage reads 14volts all the time when hooked upto shore power and 12 to 13 when boat is running. I am pretty sure i have maintenance free batteries on the boat, i will check when i go down to the harbor this weekend. I did find out that the bulbs in the lights were flasher bulbs with two metal nubs at the base. I am going to replace them and see what happens, but i am pretty sure it has something to do with the charger or the batteries. It is a cylindrical glass fuse type system for the guys who are arguing. Appreciate the feedback!

Re: Interior lights start to dim

Not arguing, just clarifying.

Are you talking barrel style fuses when you say cylindrical? If so, pull each fuse and carefully check the prongs the fuse snaps in to for any kind of corrosion or any kind of lose connections. If the prongs look dull then use a piece of emery paper (basically 800 grit wet/dry sand paper) rolled up in to a tube and clean up the surface. Some dielectric grease on the fuse ends will help prevent them from corroding. Even if that isn't the problem, it should be done anyways to help the system work the way it should. My one motorcycle had problems with the barrel style fuses all the time because the connections sucked. I replaced it with the spade style and haven't had an issue since.

Re: Interior lights start to dim

yes barrel type fuses. I do have to go and remove some light corrosion on the fuses, just not a priority now. I was looking at buying a new charging system, but none of them look like mine. Mine is huge compared to the new ones, what type would be sufficient if i decide to purchase a new charger( at least they look small on the net).

Re: Interior lights start to dim

What is the current output of your present one? From the sound of it, you need a charger that can keep up with your current draw while you are on shore power and using items on the boat correct?

Re: Interior lights start to dim

Well it turns out the previous owner only had the charger hooked up to the cranking battery and not both. So i just connected the other terminal to the other battery and voila. The other battery was completely dead and i guess it would recharge itself just enough to use some lights.

Re: Interior lights start to dim

Nice simple solution... Gotta love that.