Understanding Boat Electrics And Installing An Inverter

[Ray]

So many questions lol

Having successfully completed a few jobs that I would previously have paid to have done, thanks to advice from forum members, I'm looking into a DIY inverter installation. From the outset I am aware of safety issues and won't do it if strongly advised not to.

Below is an image of the various components of my current system, labelled to the best of my knowledge. In addition there is a galvanic isolater between the shore-power input and the consumer unit.

The questions if any of you have the time and don't mind 🙂

1. Where in this setup should the inverter be placed?

2. Where or how do you safely earth it on a boat.

3. Can the 240v output be directed to the same domestic sockets as the shore-power uses

4. I can't find where the existing solar panels enter the system or know if they even work. I'd upgrade them if I proceed.

5. The most important question...  if undertaken with care after seeking advice is it a job a layman can undertake or is the only sensible course to get it done professionally.

Thanks for any advice 👍

 

 

[grendel]

1. after the battery isolator as close to the battery as possible with suitably large wires (the interconnecting wires between batteries should be the same cross section too)

a 1kw inverter at 12v can draw up to 84A from the battery at full load (12x84=1000)

2. good question, nobody at work has been able to tell me how an inverter is earthed either.

3 it can, but you will need a changeover switch

4 solar panels should be wired to the batteries with a charge controller (MPPT for preference). so panel to charge controller to battery (with maybe an isolating switch)

5 it can be done by a layman, I have wired my shed solar system, there are two heavy duty wires into the inverter from the battery, and then the mains output to the sockets (I understand electrical wiring, but dont hold any qualifications), you may want to get it inspected by a professional if you are unsure, it depends upon how confident you are with your wiring skills, its pretty straightforward, but understandably, it also needs to be correct.

 

[Ray]

Thanks Peter, I can now go back to the boat armed with your answer and get an idea where  and how the installation would go. Then I'll have a clearer idea about whether I'm confident that I can do it properly (or not!)

[Vaughan]

I agree with what Grendel says.

Question 2 : The problem on a Broads boat is that almost all are wired "earth return", just like a car, where the engine is used as the negative return to the batteries, for almost all circuits.  This is why there should be only one main negative connection to the starter batteries.  Both starter and domestic batteries are on a common negative return, even with mixed voltages of 12/24v.

The regulations state that for shore power, all 220v AC circuits must be earthed to the shore.  You are not allowed to earth these circuits to the same return as the DC circuits.  This means you cannot earth to the engine or anything connected to it such as the prop shaft or any raw water or exhaust pipework.  Probably the best place would be the metal skin fitting on the galley sink, but not if this comes out above the waterline.  The rudder post is also a possibility.

All AC circuits in a boat must be installed in a protective gain, or conduit, all fittings must be protected just as they are in a house.  Most boats have a separate circuit and separate plugs for the inverter and the shore power but as Grendel says, a changeover switch can be used.

 I would still like to know more about your boat before I recommend you do this yourself.  For instance, why do you have two sets of domestic batteries?  Are they connected or do they do different jobs?

There's an old saying on boatyards : Mains electricity and water do not mix!

I would still prefer that you had your AC circuits installed by a marine electrician.

[Vaughan]

By the way, am I correct in saying that an 800 watt microwave is actually using about 1200 watts of AC current?

Always read the label on the back of electric appliances.

[ChrisB]

My College lecturer used to say (I was doing Mechanical Engineering) :-

" you can't see it, you can't smell it and when you feel it, It's too bloody late"

With 240ac if you are not 100% sure get a qualified electrician

[Ray]

Thanks Vaughan, the 2 sets of domestic batteries are connected, this setup came with the boat so I can only guess that a previous owner wanted/needed plenty of 12v capacity? 

I'm definitely feeling cautious about the water/electricity mix and have to bear in mind my ability to access everything given my size lol

As for the microwave, I believe you're right. Whether or not I fit it I was planning on a 1.5 or 2 kw inverter.

[ChrisB]

PS. We were all from Machine Tool companies so in those days when we had a machine tool industry we were mostly dealing with 415v 3 phase, which you don't get a second chance with.

[Vaughan]

With a big inverter you are also going to have to think about how you provide the DC power to it and how you re-charge that power.

Most microwave installations have a separate bank of 2 batteries to power the AC circuit alone.  This sometimes means you need a second alternator, to charge them. 

This is also why most boatyards say you should only use the microwave when you are running the engine.  Which presumably means - on a mooring!

[Ray]

I wonder if splitting off the 2 port side domestic batteries as a discrete system just for the inverter would work. A decent pair of solar panels may be enough to keep them topped up? A second alternator is an option I hadn't thought of.

It is beginning to sound like a job that's a bit too complex for my level of experience!

[Vaughan]

Using 2 of your existing batteries is a good possibility but you may then need a third battery on the "domestic" bank. Don't forget that one electric fridge will need the capacity of one battery all to itself, over 24 hours.  If you also have electric toilets, then maybe this was why the previous owner installed plenty of batteries!

I would fit the solar panels to the domestic batteries.  This gives them a top up, but also means they are on a trickle charge all the time in daylight, even if you are not using the boat.  So long as batteries are "on charge", they will not suffer plate sulfation and will last a lot longer.

Although your inverter circuit will use a lot of amps, it will only be for short periods, so it should be re-charged all right during a day's cruising.

[grendel]

28 minutes ago, Ray said:

I was planning on a 1.5 or 2 kw inverter.

in this case you will need cables rated for at least 170Amps (probably 200Amps to be on the safe side)

thats going to be a big cable, as I know a 35mm2 cross section is only good to about 150A and volt drop will come into your calculations depending upon your cable lengths. 

https://www.12voltplanet.co.uk/cable-sizing-selection.html

on my 1.5m inverter/ battery connection I used some 16mm2 jump leads as the connector cables for the 1 kw inverter, and they are probably on the small side (I need to visit my friends in the stores at work and get some thicker cables)

the calculator on the link shows that on volt drop, the smallest you could get away with is 25mm2 (for a 1.5m length of cable)

 

[NeilB]

I noticed you had AGM batteries, most alternators require an additional controller when charging anything different to traditional lead acid batteries.  They will still charge but could be overcharged which will shorten the lifespan.

My 240V installation is work in progress due to the previous owners bodges, mostly the use of household wiring on some sockets.  Rest of it was boatyard installed with 1500 kW inverter / 50 ah charger with auto changeover from shore power / batteries.

i’v used the microwave for a couple of 5 min bursts without running the engine as and had no issues.

[grendel]

This is the main panel of my solar install, the batteries are directly below this, and the solar input ( on 6mm2 cable) comes in the window on the far right. I chose to output just to that  switched socket bar, so wiring was fairly simple.

[Ray]

That's a nice looking job 👍

[Vaughan]

Just remembered - ventilation!

Things like inverters and battery chargers give off heat, and they must have good ventilation.  If you stick them in a cupboard, or up under the helm seat, they will get too hot and fail.  This is the most common cause of problems with battery chargers.

Same applies of course, to fridges.

[Vaughan]

2 hours ago, grendel said:

Going off topic for just a moment, I have been thinking of installing a generator, in case of power cuts in these days of uncertain electric supply.

But with a set-up like that, you could run a fridge freezer and the electrics for the central heating, whether gas or oil fired. So long as there is still a fresh water supply, you are independent!

[Meantime]

2 hours ago, Ray said:

I wonder if splitting off the 2 port side domestic batteries as a discrete system just for the inverter would work. A decent pair of solar panels may be enough to keep them topped up? A second alternator is an option I hadn't thought of.

It is beginning to sound like a job that's a bit too complex for my level of experience!

I would caution against splitting off two of the batteries for sole use of the invertor. If you are planning on a 2kw invertor as Grendel said you are looking at around 170 amp draw, or 85 amp per battery. Leisure batteries are not really designed for such high current drain. Your 130ah batteries should never be drained below 50% which means in reality you have a usable capacity of 65ah which means that at full load on your invertor the batteries would last less than an hour.

Leisure batteries are normally specified at 20ah and 5ah rates. This is the amount of capacity that can be drawn over 5 hours and the amount of capacity that can be drained over 20 hours. The 20 hour rate gives you more overall capacity because your drain rate is much lower and kinder to the battery. So your 130ah battery will provide close to its rated capacity over 20 hours, but much less if you try to draw it all in 5 hours. It will be considerably less if you try to drain it all in less than an hour. Bear in mind you have sealed batteries you are more likely to boil the acid of and kill your batteries in a very short period of time.

Keeping all four batteries connected halves the current drain per battery to 42.5 amps per battery, which is still not a kind discharge rate for leisure batteries. I would reconsider if you really need an invertor that large? If you do, then I would add additional batteries just for the invertor installation and consider using cheaper FLA Flooded Lead Acid batteries because you have the chance to monitor and top up the water. Discharging batteries at that kind of rate will use water. Maintenance free batteries really mean, you cannot top up the water and therefore need to be kinder to them.

As Vaughan said, you also need to look at how you are going to recharge those batteries as well. A second alternator is a very good option. Barnes solution was to fit a second 24V alternator and fit batteries in series to provide 24V. Doubling the voltage halves the current needed and therefore makes things like cables a lot more manageable as they don't need to be quite so thick.

To answer your Q3, not at the same time. You can fit a changeover switch either manual or automatic, but you need to ensure that it is not possible for you to have the sockets powered by the invertor if you are likely to plug into shore power. The safest way is dedicated sockets for the invertor, or a changeover switch. A manual one is simple, you just rotate a switch and move from shore power to invertor power.  More expensive, but more elegant is an automatic switch. These normally work on a priority of inputs. So you wire up your shore power and then the invertor to a lower priority input. The switch will always try and use the highest priority input first, i.e. the shore power and will then fall back to the next input, the invertor.

One other observation I would make from your picture is that your split diode and fuse holder have exposed terminals. You can get Red protective boots that would fit over those connections. I would strongly suggest fitting some. See picture below.

[Vaughan]

3 minutes ago, Meantime said:

Barnes solution was to fit a second 24V alternator and fit batteries in series to provide 24V. Doubling the voltage halves the current needed and therefore makes things like cables a lot more manageable as they don't need to be quite so thick.

This is absolutely true. I think we are talking of only a 1KW microwave here, but all the same . . .

Trucks have 24v circuits as they need the CCA (cold cranking amps) to turn over a big 6 cylinder diesel.

There is nothing wrong with having both 12 and 24 volt circuits in a boat, but just remember that for battery "capacity" on a slow discharge, if you connect two batteries in parallel, you double the amp/hours.  If you connect them in series, you only double the voltage.

[Meantime]

One more thing I've just spotted Ray, in your picture the one top right it looks like you have some very thin wires connected directly to your battery. I'm assuming this is for a bilge pump and is meant to bypass the main battery isolator for when you leave the boat unattended? There should most definitely be a fuse close to the battery to protect the rest of the cable run which with a potential 520ah in reserve would glow very hot very quickly in the event of a short somewhere. Also I personally wouldn't be happy with those crimp connectors. Crimps can work loose and then start to arc. I wouldn't want any source of a spark anywhere near batteries that will gas when being charged.

[Ray]

I need to check more carefully whether I need such a high capacity inverter. It is going to be for the sole use of the galley and I want to be able to use a toaster, a kettle and a microwave... specifically one at a time. Perhaps a 1kw inverter would be adequate, it certainly seems to be a less complicated installation.

[Meantime]

3 minutes ago, Ray said:

I need to check more carefully whether I need such a high capacity inverter. It is going to be for the sole use of the galley and I want to be able to use a toaster, a kettle and a microwave... specifically one at a time. Perhaps a 1kw inverter would be adequate, it certainly seems to be a less complicated installation.

I'd start by looking at the power consumption of those appliances and looking at alternatives. A domestic kettle is normally 3kw and therefore boiling a kettle on gas is by far the best alternative, but you can also get 12V camping kettles and even 12V coffee makers. I have a 24v coffee maker on board which makes very good filtered coffee.

A microwave would be up to 1kw, but you can get smaller 600w microwaves.

The toaster, again I would seriously consider the gas grill, or even one of those stove top toasters you put over a gas ring.

https://www.amazon.co.uk/litre-Plug-Portable-Electric-Kettle/dp/B07D9N6DTR/ref=sr_1_4?crid=T53K9KI8C1HQ&keywords=12v%2Bkettle&qid=1654332934&sprefix=12v%2Bkettle%2Caps%2C67&sr=8-4&th=1

 

[Meantime]

I use the 24V version of this onboard for making coffee, and that still gives the batteries a bit of a whack, but is kinder than using an invertor to provide 240V to use a normal coffee maker.

[Ray]

Having just got my gas grill working perfectly by changing the thermocouple (thanks to advice from forum members) Perhaps my thinking should come full circle and wonder if I really need an inverter at all 🙂

[Vaughan]

2 minutes ago, Ray said:

Perhaps my thinking should come full circle and wonder if I really need an inverter at all 🙂

I quite agree!  Or perhaps have one for shore power only.