Charger as Isolation Transformer?

[Jdege]

I'm refitting an older boat that has a minimal electrical system - starter battery, 100Ah 12aV house battery, alternator, and cabin lights.

No charger other than the alternator, no shore power connection, doesn't even have nav lights, though I'll be installing them before I put the masts up for the season.

What I want, eventually, is a fully electric boat - electric propulsion, electric cooking, solar charging, etc. The current propane oven/range is non-functional, and the diesel needs a full rebuild.

I can't, of course, do everything at once, so I've been thinking about what smaller projects might move me towards where I want to be.

I'd been thinking that I might start by installing a shore power connector, feeding an inverter/charger, but something I read in Nigel Calder's "Boatowners Mechanical and Electrical Manual, 4th Ed." rather an interesting possibility.

He suggests that if your batteries and invertors are sufficient to power your AC loads - which I intend mine to be - that connect shore power AC only to battery chargers that are electrically isolated from the boat. That, in essence, you configure your chargers to act as isolation transformers.

From his chapter 2:

Shore-power benefits of an inverter-based boat.
You can wire an inverter-based boat so that its shore-power cord is connected to a large bank of battery chargers and nothing else. The shore-power circuit now terminates at the battery charger(s). The onboard AC circuits create an entirely separate (electrically isolated) AC system that originates at the inverter. When plugged into shore power, the boat’s AC system will run off the inverters, as at sea, but with the battery chargers recharging the batteries.

He discussion of this points out that this violates ABYC standards, but it's within ISO.

Is this commonly done? I'd think that as battery capacities increase and inverters improve, there'd be more choosing this route, particularly with the advent of multi-input chargers that will accept 120V and 240V.

[jt11791]

A lot of people use inverter/chargers that can invert or charge, but can't do both at the same time. You'd need two separate devices for your plan to work. Other than that, it seems a reasonable way to go if your AC needs are not large. Big AC demands (like multiple aircon units) would require really big inverters to run from and big inverters are big in physical size, weight, and idle current. Of course isolation transformers are not small either, so yeah, maybe your plan will become more popular.

[Stu Jackson]

Originally Posted by Jdege

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He suggests that if your batteries and invertors are sufficient to power your AC loads - which I intend mine to be - that connect shore power AC only to battery chargers that are electrically isolated from the boat. That, in essence, you configure your chargers to act as isolation transformers.

...................
Is this commonly done? I'd think that as battery capacities increase and inverters improve, there'd be more choosing this route, particularly with the advent of multi-input chargers that will accept 120V and 240V.

It's unclear to me what you are trying to accomplish here because of the title of your post compared to your discussion.


Sure, people have large battery banks and can choose, as an electrical system management issue, to use their inverters, but they rarely do it at a dock because of the inefficiencies in 12>120>12 conversions and battery voltage sag with larger AC loads (and constant battery use rather than 12V pass through from chargers).


If you are trying to replace an isolation transformer with a battery charger just because the AC shorepower inlet is only connected to a charger(s), it doesn't work that way.

[Jdege]

Originally Posted by jt11791

A lot of people use inverter/chargers that can invert or charge, but can't do both at the same time. You'd need two separate devices for your plan to work. Other than that, it seems a reasonable way to go if your AC needs are not large. Big AC demands (like multiple aircon units) would require really big inverters to run from and big inverters are big in physical size, weight, and idle current. Of course isolation transformers are not small either, so yeah, maybe your plan will become more popular.

The critical question do you have AC devices will you want to run in the slip that you are OK with not being able to run on the hook? If the answer is no, isolated chargers may make sense.

As for what I'm trying to accomplish, I want a live-aboard setup that isn't tied to the dock. But I want a dock-side charging option that doesn't risk galvanic problems.

It's my understanding that galvanic isolators protect from fewer possible issues than isolation transformers, but isolation transformers are bulky, heavy, and expensive, and seemed like overkill when I only want them to run a couple of chargers.

That chargers can be configured to provide isolation is a new idea to me. And I'm wondering how well it works, and what risks it poses.

[Jdege]

Originally Posted by Stu Jackson

It's unclear to me what you are trying to accomplish here because of the title of your post compared to your discussion.

Sure, people have large battery banks and can choose, as an electrical system management issue, to use their inverters, but they rarely do it at a dock because of the inefficiencies in 12>120>12 conversions and battery voltage sag with larger AC loads (and constant battery use rather than 12V pass through from chargers).

If you are trying to replace an isolation transformer with a battery charger just because the AC shorepower inlet is only connected to a charger(s), it doesn't work that way.

If you have modest AC requirements, an inverter capable of providing them, sufficient batteries, and enough generation onboard to keep them charged, you don't need shore power at all, but installing shore power simply as an alternate charging mechanism might make sense.

As for conversion efficiencies, the efficiencies of onboard alternators and generators are worse, so I don't see it as an issue.

[Stu Jackson]

Originally Posted by Jdege

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As for conversion efficiencies, the efficiencies of onboard alternators and generators are worse, so I don't see it as an issue.

Except they ^^^ have nothing to do with his question.

[redneckrob]

Originally Posted by Stu Jackson

It's unclear to me what you are trying to accomplish here because of the title of your post compared to your discussion.


Sure, people have large battery banks and can choose, as an electrical system management issue, to use their inverters, but they rarely do it at a dock because of the inefficiencies in 12>120>12 conversions and battery voltage sag with larger AC loads (and constant battery use rather than 12V pass through from chargers).


If you are trying to replace an isolation transformer with a battery charger just because the AC shorepower inlet is only connected to a charger(s), it doesn't work that way.

He is saying if you have a 3,500 watt inverter/charger acting as a charger hooked up to another 3,500 watt inverter/charger acting as an inverter can you avoid the isolation transformer and the answer is yes, that setup would isolate you from the dock AC after the first inverter. You wouldn't suffer from the sag, but would suffer from the conversion inefficiency. On the plus side you would always effectively have a spare inverter. I will note it's hard to find a pure charger that goes up to beyond 1,000 watts, so you kind of have to go the dual inverter/charger route.

[Jdege]

Originally Posted by redneckrob

He is saying if you have a 3,500 watt inverter/charger acting as a charger hooked up to another 3,500 watt inverter/charger acting as an inverter can you avoid the isolation transformer and the answer is yes, that setup would isolate you from the dock AC after the first inverter.

You don't need a charger that can meet the peak AC demand, you only need to meet the average demand.

[LoudMusic]

I can't imagine it's worth the hassle. Separate inverter and charger is more complex than an inverter/charger and an isolator, surely? Especially considering the capacity options of each design. And if the goal is to be fully electric, you're talking about needing some massive capacity.

I'd also go 48vDC from the beginning.

[PippaB]

We have a world-traveled Euro boat, and this is how we've run for close to a decade. We have a 30A battery charger that has a wide voltage input and doesn't care about frequency. That charges the batteries. Can be plugged into shore power pretty much anywhere in the world just by swapping out the shore end of the power cable.

On the boat end we have an inverter that powers whatever AC appliances we have at 230V/50Hz. Shorepower never meets those circuits. The batteries can deal with whatever current is drawn by the inverter, so not important to us that the charger keep up all the time. The 30A charger can deliver ~10kWh/720Ah per day. We don't use that much, or even close (we do not have air conditioning or electric heat).

We have an RCD/GFCI on the shore power inlet as recommended, and a galvanic isolator on the PE line. Does it meet ABYC? Probably not. Am I (and almost more importantly the surveyor who did the insurance report) happy with it? Yes. I'm comfortable that the system is safe, without having the shore power ground connected to the boat ground/DC negative even though the charger is on the boat and thus we "should" have a connection or an isolation transformer.

[Jdege]

Originally Posted by LoudMusic

And if the goal is to be fully electric, you're talking about needing some massive capacity.

Yes, but enough so that shore power is only an adjunct, an alternate charging mode, not an essential.

It takes a lot of watts to drive a prop, but it's not something you do while connected to shore power. Or run through an inverter, for that matter.

[Jdege]

Originally Posted by PippaB

We have a world-traveled Euro boat, and this is how we've run for close to a decade. We have a 30A battery charger that has a wide voltage input and doesn't care about frequency. That charges the batteries. Can be plugged into shore power pretty much anywhere in the world just by swapping out the shore end of the power cable.

On the boat end we have an inverter that powers whatever AC appliances we have at 230V/50Hz. Shorepower never meets those circuits. The batteries can deal with whatever current is drawn by the inverter, so not important to us that the charger keep up all the time. The 30A charger can deliver ~10kWh/720Ah per day. We don't use that much, or even close (we do not have air conditioning or electric heat).

We have an RCD/GFCI on the shore power inlet as recommended, and a galvanic isolator on the PE line. Does it meet ABYC? Probably not. Am I (and almost more importantly the surveyor who did the insurance report) happy with it? Yes. I'm comfortable that the system is safe, without having the shore power ground connected to the boat ground/DC negative even though the charger is on the boat and thus we "should" have a connection or an isolation transformer.

Can you point me to a description of exactly how this is wired?

[LoudMusic]

Originally Posted by Jdege

Yes, but enough so that shore power is only an adjunct, an alternate charging mode, not an essential.

It takes a lot of watts to drive a prop, but it's not something you do while connected to shore power. Or run through an inverter, for that matter.

That's fair. But it does mean if you arrive to a marina with a low battery and want to fill it you could be there for a long charge time. The cost of a bigger charger could be offset in dock fees rather quickly.

I don't think separate hardware is a bad thing, I'm just not sure it's the best thing

[redneckrob]

Originally Posted by Jdege

You don't need a charger that can meet the peak AC demand, you only need to meet the average demand.

Good point!

[Jdege]

Originally Posted by LoudMusic

That's fair. But it does mean if you arrive to a marina with a low battery and want to fill it you could be there for a long charge time.

If I arrived in a marina and my onboard charging wasn't working I could be there a long time.

Take Uma as an example. They spent years in the Caribbean, living off of solar. It wasn't until they reached northern Europe that they started charging in marinas, and not until they decided to spend a summer in Svalbard that they bought a backup generator.