IF a relay coil fails it falls to its de-energized state. Meaning the NC will be closed. However, a relay can fail in other ways: The contacts can build up carbon and fail to switch, meaning they switch but do nothing. The other way a relay can fail is the contacts can become welded. They can weld in the NC position OR in the NO position. With welded contacts you lose control of the circuitry. That - at sea - can be a problem.

To a non-nautical guy like me, I can't imagine all the possible dangers. I may even imagine an un-real danger. The point is that nobody here wants you to get hurt. Or for your system to harm others. That's why safety is both the first and last concerns - especially in an area where you can't just pull over and wait for help to arrive.

Over my career I have seen very few relay failures, except in the very cheapest of consumer junk, and cases of extreme mechanical abuse. ( A fork lift ran into a control panel.) When those few failures did occur they were usually caused by abnormal conditions, or gross misapplication of a relay. They do eventually wear out and become unreliable, but that takes many thousands of operations. I have never seen a relay fail because of "carbon build up" on the contacts, although open-frame relays are subject to dust accumulations. Power relays used to switch very low power analog signals will become unreliable, that is a clear case of choosing the wrong kind of relay.
But a correctly sized properly chosen relay for power switching will usually outlast the system that it is installed into.
Finally, consider the RV power converter box that outlasts the travel trailer that it is installed into. So there would be another option.

 

Over my career I have seen very few relay failures, except in the very cheapest of consumer junk, and cases of extreme mechanical abuse. ( A fork lift ran into a control panel.) When those few failures did occur they were usually caused by abnormal conditions, or gross misapplication of a relay. They do eventually wear out and become unreliable, but that takes many thousands of operations. I have never seen a relay fail because of "carbon build up" on the contacts, although open-frame relays are subject to dust accumulations. Power relays used to switch very low power analog signals will become unreliable, that is a clear case of choosing the wrong kind of relay.
But a correctly sized properly chosen relay for power switching will usually outlast the system that it is installed into.
Finally, consider the RV power converter box that outlasts the travel trailer that it is installed into. So there would be another option.

I agree. However, a tug (aircraft), forklift, car or train, failures can be an inconvenience. But a failure in a plane or a boat - or other life / mission critical situation, even the remotest possibility of a failure needs to be considered. I've almost never seen a relay fail, but I have seen them. Mostly it's been welded contacts, either stuck open or stuck closed. But I've seen one coil fail in a car. The sunroof of my wife's old Hyundai failed. It was an open coil on the close circuit. You could open the sunroof but not close it. I actually replaced it with a 24 VDC coil relay and it worked.

I call for safety and request one to consider what happens IF the relay should fail. Does a failure simply mean a long row home? Or does it mean calling the coastguard for a rescue because of a catastrophic fire? If light bulb burns out - it's an inconvenience. But if a wire shorts, that could be a different story.

I worked for MacDonnell Douglas back in the 70's and 80's. We were building a DC9 Series 50 (stubby little jet) for the NAVY. The technician who plugged in the "Thunderstorm Lighting" system, accidentally shorted two pins together in the connector. One pin was 305 volts from a ballast and the other was ground. This rare condition meant there was no circuit breaker to kill the power. The cockpit began to smoke up when I was testing the lighting systems. Unable to immediately determine which circuit was at fault I shut the whole plane down. The hydraulics mechanics got angry because they were depending on the wheel well lighting for light. They threw me off the plane and re-energized the plane, then they went back into the wheel well. Walking down the hallway was a NAVY contingency coming to check on the progress of their plane. They saw the smoke pouring out of the cockpit. Since I was standing there they asked me "Why is our plane burning?" To which I explained the situation and how I got kicked off the plane. The CEO of the company told me to shut down the main generator. I did. The hydraulics supervisor came screaming at me swearing I was about to be walked out by security. He didn't see the NAVY or the CEO. HE was the one who got walked out by security. Took us three months to rewire the cockpit.

You never know when some silly situation can arise and snooker your best of intentions. Safety is job 1 and safety is the LAST thing to be considered. So, does a relay failure mean an inconvenience? Or does it mean catastrophe? Safety. Be safe.

 

The explanation in post #22 is why I suggested that an approach not using a relay is the better choice. I am quite used to being ignored, but seldom a good sport about it. I have been known to say "I told you so" in public, when things go very wrong, after my advice was ignored.
And on a boat, an adequate diode isolation scheme along with proper fuse or circuit breaker protection, will certainly provide the required functionality. There was never the need for a transfer relay.

 

There was never the need for a transfer relay.

I agree. Since the system is powering the lights and other electronics from shore power, there's no need to isolate the battery. As long as shore power does not exceed 14.4 volts. Ideally 13.6 volts from the supply. Let the marine battery take a charge from that source. Of course nothing beats a proper battery charger, but keeping a 13.6 volt present on the battery, assuming the battery is already charged, will maintain a float voltage. The diode would only be needed to prevent current from flowing from the battery back into the power supply.

 

I agree. Since the system is powering the lights and other electronics from shore power, there's no need to isolate the battery. As long as shore power does not exceed 14.4 volts. Ideally 13.6 volts from the supply. Let the marine battery take a charge from that source. Of course nothing beats a proper battery charger, but keeping a 13.6 volt present on the battery, assuming the battery is already charged, will maintain a float voltage. The diode would only be needed to prevent current from flowing from the battery back into the power supply.

Given that DC battery chargers already contain a rectifier, usually a diode or two, to provide the DC, if the charger is properly designed then no additional diode is needed. Using a regulated DC supply with an active over-voltage trip arrangement would be different, but also not in the realm of a normal battery charger.

 

@MisterBill2 I have a 12 volt wall wart, 3 amp, modified for 13.6 volts in the garage. I have an old car battery that is constantly on that charger. The battery is connected to a car radio that I play when I'm out there working on a car or building something in my make-shift wood shop. The battery is constantly connected to 13.6 volts. It's an old battery but it still does well. I got the battery from a friend who burned up his vehicle. He's not good at preventative maintenance. He checked the coolant level in the radiator and topped it off. Then forgot to put the cap back on. Drove it to work and on his way home he burned up the motor. He gave me the battery, a bunch of 12 volt relays and a practically brand new (rebuilt) alternator. That was back in 2011. The battery was already somewhat old by then. Here it is nearly 2020 and the battery still plays the car radio. I have another battery from my wife's Hyundai. Every year I put it on the charger just to top it up so as to prevent sulfating the plates. Every time I work with something in the 12 volt range in the garage I use that battery and it doesn't show any signs of trouble. But then again, I haven't really put it to much of a test. Still, if push came to shove, I believe I could toss it in the truck and it would start it up.

The TS wants to switch the boat electronics from shore power to battery power and back. Using a relay he can achieve that goal quite easily. Though some of us feel it's not necessary. On top of the pre-described arrangement, the TS has a separate battery charger that charges both his marine battery and the starter battery for the gasoline engine (gasoline assumed). Since he has a charger for the batteries, there's no need to have one battery double charged. Hence, the relay setup. My concern with that is that if the boat is out of water and off shore power for a length of time the marine battery could sulfate. Same would be true of the starter battery. I just don't want the TS to get the idea that safety is a cavalier matter. One has to look at every possible way a system could fail and how that failure would affect other things. Being out on the water is no place to have a serious problem. A short, fire, and loss of electrical power could mean the lack of ability to call for help. Hopefully the TS NEVER has a need to call for help. But if that contingency happens, he needs to have a backup plan. Or a failure that can only fail in safe mode.

 

I have an old car battery that is constantly on that charger.

The battery is constantly connected to 13.6 volts.

Hydrogen, an explosive gas, is produced when a lead acid battery is charged. This “gassing” as it’s typically called, can be a safety issue. An air circulation system may be required to remove hydrogen from the battery room.
Your garage could EXPLODE!

 

Hydrogen, an explosive gas, is produced when a lead acid battery is charged. This “gassing” as it’s typically called, can be a safety issue. An air circulation system may be required to remove hydrogen from the battery room.
Your garage could EXPLODE!

Hydrogen is also a much lighter than air gas, and so a battery not enclosed, but adequately vented by sitting in the open, will not experience any hydrogen accumulation outside of the cells. And likewise, the garage. And because the battery is not used for any high current purpose, the high current trigger for an explosion is not present. A far greater explosion danger comes from the gasoline in the tank of the car in the garage.
And given the very low number of garage explosions from cars parked and not being serviced, it seems that the tone of alarm in the post is totally unwarranted.

 

I understand the warning. The battery is on an open shelf and the garage is a 3 car garage. The amount of hydrogen being produced by the - at present - very slight trickle charge is virtually non-existent. And I've messed with HHO before. I've detonated hydrogen in my garage. It's an ear piercing pop in much greater quantities than that produced by a battery charging. OK, yes, batteries DO explode in cars, and more often than not they explode when someone attempts to jump start a car. That's the #1 reason why you're supposed to make the positive connections first, THEN make the negative connections to the engine block or bare metal chassis. Sparks kept away from the battery is the key. Also, when disconnecting a battery from a car you ALWAYS start with the negative lead. IF your wrench strikes ground while touching the negative lead - no problem. But if you're starting with the positive lead first and strike ground you can get a pretty hefty spark. IF hydrogen is present in the battery AND there's enough free hydrogen in the vicinity you COULD ignite the hydrogen in the battery cells. I'm doing none of that.

But being an advocate for safety - I fully understand what it is you're saying. I've even used my HHO system to - well, I probably shouldn't describe it here, would likely be removed from the comment OR the entire comment may be deleted by a moderator.

 

A couple of things spring to mind.

The first relates to warnings of overheating of the relay if it is left 'on' for a long period. When I have needed such functionality, I have used a second set of contacts on the relay to switch in a series resistor to the coil. A 12V relay may well need 10V or more to operate, but will happily hold on 6V, consuming far less power and not getting hot.

The second relates to the idea of just charging the battery and using it simultaneously to power the on-board circuits. Mention has already been made of the 13.6V (roughly) maintenance voltage, but the charging voltage can get to 15V. Will the on-board electrics take this? Even if they do, lamp life could be considerably reduced (though they will be brighter!). Also, I am surprised to hear that all the on-bard systems consume less than 5A. That's only 60W which isn't much for a couple of lights and other ancillary equipment. If the actual drain is more than the charger puts out to charge the battery, then you'll be discharging the battery rather than charging it if you're using a good range of the 12V equipment.

Personally, I like the 12V relay idea. As long as the contact rating is chosen sensibly and there's not some massive inductive load on the system(!) then the likelihood of welded contacts is low, and the failure of the coil is a 'safe' condition. Or use a solid-state relay.

 

A couple of things spring to mind.

The first relates to warnings of overheating of the relay if it is left 'on' for a long period. When I have needed such functionality, I have used a second set of contacts on the relay to switch in a series resistor to the coil. A 12V relay may well need 10V or more to operate, but will happily hold on 6V, consuming far less power and not getting hot.

The second relates to the idea of just charging the battery and using it simultaneously to power the on-board circuits. Mention has already been made of the 13.6V (roughly) maintenance voltage, but the charging voltage can get to 15V. Will the on-board electrics take this? Even if they do, lamp life could be considerably reduced (though they will be brighter!). Also, I am surprised to hear that all the on-bard systems consume less than 5A. That's only 60W which isn't much for a couple of lights and other ancillary equipment. If the actual drain is more than the charger puts out to charge the battery, then you'll be discharging the battery rather than charging it if you're using a good range of the 12V equipment.

Personally, I like the 12V relay idea. As long as the contact rating is chosen sensibly and there's not some massive inductive load on the system(!) then the likelihood of welded contacts is low, and the failure of the coil is a 'safe' condition. Or use a solid-state relay.

There are both intermittant duty relays and continuous duty relays. An example in a car is the horn relay and the starter control relay, both intended for very short operation times, and the headlamp relay, intended to be energized for hours at a time. They are not interchangable. That starter relay will overheat and burn out after just a few minutes of being energized. Another constant duty relay handles the high speed running power for the car A/C system.
The inconvenient part is that the constant duty relays are often bigger and usually more expensive.

 

does one still need a bridge rectifier and a capacitor if the transformer is this:
https://www.amazon.ca/ALITOVE-Transformer-Switching-Converter-Security/dp/B07DN8HH8V

* usual draw for lights is currently around 5 amps max . but this is for a new boat so i'll have to test.
a transformer that is twice what i need sounded like a good idea.

What about bilge pump, fresh water pump, and macerator or similar. Does your boat have a refrigerator? Maximum potential load likely exceeds 5A by a fair bit.

Don't you get the feeling you are reinventing the wheel? Boats and RVs the world over run off the charger while connected to shore power. Having the battery in the circuit provides for momentary over-current, and also stabilizes the voltage. I'm not certain the arrangement can be substantially improved upon.

 

I have seen published circuits for battery charging systems that drop the voltage to a trickle-charge value with a light load but switch to a slightly higher current. There are also some diode isolation schemes that will allow the battery to be charged by the higher voltage.
If all of the on-board lighting is LEDs then the 5 amps max is quite believable.

 

Hydrogen, an explosive gas, is produced when a lead acid battery is charged. This “gassing” as it’s typically called, can be a safety issue. An air circulation system may be required to remove hydrogen from the battery room.
Your garage could EXPLODE!

Fortunately, hydrogen rises in air rapidly and can seep through the roof of an unfinished garage easily. Also, it takes greater than a 10% mix of hydrogen and air to burn.

 

I am an electronics engineer and long time boater. While I prefer to work with and use the latest technologies, I feel that some applications are best implemented using old tech - like relays. When you are pitching around in your boat trying to troubleshoot an electrical issue it is way easier to detect and bypass a failed relay rather than a failed rectifier. A relay in a socket is easily replaced or temporarily bypassed by moving some of the wires around with just a screw driver.
Leave the camper/RV isolator-charger gear on land where failure is not requiring a call to the Coast Guard.

 

A simple solar charger controller like this solves all of your problems. It is put between the charger and battery. It stops charging when the battery is full and starts charging when the battery is low. The full and low threshold can be set.

EDIT: I use "charger" for your boat charger and "solar charger" or "solar controller" for the controller in the link down. You already have the boat charger you said, the solar charger is connected to it and controlls everything. You can see in this link how to use it. REMEMBER: the solar controller takes in DC and outputs DC. You can NOT connect it to the mains AC.

Option 1: a good charger will solve all of your problems. Put it before the battery and don't care.
Option 2: put a relay for the charger or for a transformer(properly picked transformer so it does not burn) and don't care.
Option 3_Recommended: use the solar charger I gave you, you can see it on "AliExpress", it will control everything for you. The solar controller has input(this is where you connect your already available or any "DC charger" at about "14V" or more volts. The controller will reduce it for a "12V" or "24V" battery(automatically detected). It has 2 connectors for the battery(the main used battery). It has 2 connectors for the load(your boat). if you connect the solar controller without a battery it will pulse but still supply a current to the load. The battery will filter the current and avoid the flip flops that you mentioned before. EDIT: You must connect the battery first to the middle 2 connectors!

I have this solar charger at home. Unfortunately it measures with +- "0.2V" accuracy, so you need to measure the battery voltage with a multimeter and check should you set the threasholds at "+0.2V" or "-0.2V". Its easy to use and cheap. You have 6 connectors, 2 for the "DC boat charger", 2 for the "battery", 2 for the load(boat).