DC to AC

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ip485

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I am posting this because I know some of you are brilliantly technical when it comes to this sort of question!

So the question is this;

I am accumulating some of the usual electrical goods on board and originally tried to source items than run on DC so they connect directly to the boats 12V supply. However, it is much more difficult to find consumer items that are designed to run off DC power.

I have an inverter which will comfortably power the lot.

I have therefore decided to run some items off the inverter but interested to know what the typical loss would for example for a TV comparing the amps drawn running it off 240V via the inverter and 12V DC?
 
'fraid its one of those "it depends" areas.
The first place to look is the specification of the inverter, it might have a figure for efficiency.
The other problem is: Is it a Square wave (very unlikely), a modified sine wave (most likely) or a pure sine wave inverter?(again see the inverter specs for this info)
If its modified sine then your TV might need more ommph, it can happen so the TV will be introducing some inefficiency.
If its a pure sine wave inverter, they can be less efficient than a modified sine wave version.

For your own experimentation then you would need to carefully measure the current taken by the TV when connected to household mains (extreme care must be taken as it can be fatal) and then measure the current taken by the inverter when it is powering the TV.
 
ip485 said:
I am posting this because I know some of you are brilliantly technical when it comes to this sort of question!

So the question is this;

I am accumulating some of the usual electrical goods on board and originally tried to source items than run on DC so they connect directly to the boats 12V supply. However, it is much more difficult to find consumer items that are designed to run off DC power.

I have an inverter which will comfortably power the lot.

I have therefore decided to run some items off the inverter but interested to know what the typical loss would for example for a TV comparing the amps drawn running it off 240V via the inverter and 12V DC?
Click to expand...
Do modern TVs run on 240v or are they stepped down like PCs are
 
First, ignoring for the moment losses due to conversion inefficiencies it will take 20 times more amps from the batteries to supply a 240V AC device. So for example you have an appliance that uses 3 amps at 240V it will pull 60 amps at 12V from the batteries. Then as a very rough approximation (it will depend on the quality and efficiency of your inverter but this will be close enough for basic calculations) add another 10% for conversion losses.

Obviously if you plan to run any 240V appliance that draws more than milliamps for more than a minute or two you are going to need some serious battery banks or you will have to be running the engine or other charging source while using the inverter.
 
By the way, about TVs. A lot of the smaller flat panel TVs are powered by a transformer or brick (what do you call them over there) that plugs into the AC socket and converts the power to DC that the TV actually runs on. I have read from a number of boaters that purchased this type of TV that one can cut off the brick and splice the cord into a 12V DC power source on the boat. Just make sure before doing this that the brick is making +/- 12V DC and not 5V or 20V. Should be written right on the brick.
 
skipmac

This is true and quite common. I have often wondered the implications of doing just that. You read horror stories of the results of an unstabilised voltage as the batteries fall below 12.4V. I have tried doing just this with one or two items (for example my on board router) which has continued to work perfectly but the implications with more expensive equipment is of potentially greater concern. Of course with a 24V supply there is an easier solution with a stabilised converter but unfortunately I am not 24V.

Skipmac - thank you, I hadnt appreciated just how much of a penalty there is running off the Inverter. I have a largish Dell PC monitor and a DVD drive at the moment which is all that runs off the 240V system, other than some occasional use of kitchen equipment!. I havent noticed it pulling down the batteries all that quickly (its a 800AH bank) but the bank is a reasonable size.

Thank you for your replies so far.
 
skipmac said:
........are powered by a transformer or brick (what do you call them over there)
Click to expand...

I always refer to them as a 'wall cube' ....(although of course most of them are now nothing like a cube :D). More correctly: 'mains adapter'.

In British English usage in this context a 'brick' tends to refer to a rather large old-fashioned mobile phone.

Wikipedia thinks:
Other common names include plug pack, plug-in adapter, adapter block, domestic mains adapter, line power adapter, wall wart, power brick, and power adapter.

:cool:
 
ip485 said:
Skipmac - thank you, I hadnt appreciated just how much of a penalty there is running off the Inverter. I have a largish Dell PC monitor and a DVD drive at the moment which is all that runs off the 240V system, other than some occasional use of kitchen equipment!. I havent noticed it pulling down the batteries all that quickly (its a 800AH bank) but the bank is a reasonable size.
Click to expand...

If you want to do the calculations for other 240V appliances it's pretty easy. In the simplified form Power is in Watts. Watts = Volts X Amps (ignoring power factors, etc) So a 2400 Watt appliance would use 10 amps at 240 Volts or 200 amps at 12V. Some appliances are rated in amp usage so something that draws 0.5 amp at 240V is 120 Watts or 0.5 amp at 12V is 6 Watts.

The monitor and DVD drive both probably well under 100 Watts so even based on 12V DC that's just a few amps. 800 AH is a good sized battery bank. I've got about 650 and might want to bump that up when I move onboard.
 
The brick referred to is known as a 'Switch mode converter or charger'.

Losses in a 12vdc - 240 vac inverter could be as high as 25%, subject to the load type being applied. Some electronics can't handle square or modified sine wave very well.

AC power is measured in kWH with local draw in watts which = amps x Volts

DC power is measured in Amphours, based upon a typical battery's 20 hour rate, i.e. how many amp hours it will discharge on a load set to keep it going for 20 hours.

Now lets look at a kettle with a 2kw heater element on the warm up cycle for 5 minutes ...

2kw x 5/60 hours = 0.17 kWH or around 2 pence

To operate same kettle for same period via an inverter ...

2 kW will require ~ 8.3 amps at 240v.
That is 20 x 8.3 or 166 amps at 12 v plus say 10% losses = 184 amps
184 for 5 minutes = 15.3 AH

Then you need to replace the 15.3 AH, and again there will be a charger loss and battery charge efficiency loss. Your 15.3 AH will require around twice the amount to regain the battery charge from the 240v supply to the charger.

Thus your overall efficiency is at best around 40 - 50%.

Bear in mind also that usable AH from your battery is ~ 50% of the rated capacity, so for a typical 100 AH battery one boil of the kettle has just accounted for 1/3 of your usable capacity.
 
Superheat6K explains some of the p[roblems of running 240vAC appliances on the ships 12v supply via an inverter. There are many pitfalls and you must only use AC applainces if you understand the pitfalls.
Almost any appliance involving heatinjg will be a no no. Your computers and Tv may be OK but beware the long operating time will really empty batteries.
Small chargers etc no problem.
As said the inefficencies of the inverter coupled with the huge 12v Dc current with associated wiring losses
can all add up to punishing those batteries.
A friend has been involved in a remote school in Africa. He set up huge solar arrays and inverter systems running on 24v and even 48v batteries. It just doesn't work because he can't get the users to appreciate that the 240vAC from the socket is not an inexhaustable supply. The batteries are dieing prematurely because the supply is not being carefully monitored.
So for the OP every 240v appliance must be carefully considered before being allowed on the boat.
good luck olewill
 
superheat6k said:
............

AC power is measured in kWH with local draw in watts which = amps x Volts

DC power is measured in Amphours, based upon a typical battery's 20 hour rate, i.e. how many amp hours it will discharge on a load set to keep it going for 20 hours.
Click to expand...

You are muddling power with energy units.

Power is a measure of the rate at which energy is used or generated.

The SI unit of Power is the Watt. AC, DC, diesel, petrol or steam, it doesn't matter.

The SI unit of energy is the Joule.

1 Watt = 1 Joule per second.

Electrical energy sold by the utility companies is metered in kWh.
1 kWh = 1000 Wh = 1000 * 60 * 60 Ws = 3,600,000 Joules

In the UK, power for electrical items it is commonly referred to as "wattage". A term I hate but that's my problem.
 
skipmac said:
By the way, about TVs. A lot of the smaller flat panel TVs are powered by a transformer or brick (what do you call them over there) that plugs into the AC socket and converts the power to DC that the TV actually runs on. I have read from a number of boaters that purchased this type of TV that one can cut off the brick and splice the cord into a 12V DC power source on the boat. Just make sure before doing this that the brick is making +/- 12V DC and not 5V or 20V. Should be written right on the brick.
Click to expand...

One of the surest ways to blow your nice expensive TV when you start your engine.
Fine when the voltage is around 12 volts but watch out for the spikes or overvoltage as you will have no protection unless you also add a voltage limiter to prevent this happening!

Too many experts on here with no practical experience!
 
JohnGC said:
In the UK, power for electrical items it is commonly referred to as "wattage". A term I hate but that's my problem.
Click to expand...

It's not quite as horrible as when numpty Americans (including ones giving out advice) write "amperage" when they mean current :)

Pete
 
TSB240 said:
One of the surest ways to blow your nice expensive TV when you start your engine.
Fine when the voltage is around 12 volts but watch out for the spikes or overvoltage as you will have no protection unless you also add a voltage limiter to prevent this happening!
Click to expand...

It's quite common for consumer electronics to use cheap unregulated external power supplies and then have a linear regulator (or a cleverer switch-mode one) inside the case. I believe the maximum permitted input voltage for the classic LM7812 is somewhere north of 30v...

Pete
 
TSB240 said:
One of the surest ways to blow your nice expensive TV when you start your engine.
Fine when the voltage is around 12 volts but watch out for the spikes or overvoltage as you will have no protection unless you also add a voltage limiter to prevent this happening!

Too many experts on here with no practical experience!
Click to expand...

In my case, this expert was reporting merely what he had read a number of others have done as in real world, practical application of the idea, and none have reported blowing up their sets. Perhaps they area all careful to turn off the sets anytime a charging source is connected or disconnected or a large load engaged that can produce a voltage spike. Or perhaps the battery banks act like a large capacitor and smooth out the worst of the spikes. Or perhaps the sensitivity to voltage spikes isn't as big a concern as some think.

By the way, if you think voltage spikes on a boat are a problem have you ever looked at a graph of the voltage spikes found in a typical AC power line to a home?
 
prv said:
All over the Internet...



Errm, yes?

This post is also the only place in this thread (for now) that the word "blancmange" appears.

So what?

Pete
Click to expand...

So what? I just wondered what any of this has to do with the OPs question or this particular discussion. I confess that I am as guilty as any of drifting threads but at least I do try to give it a couple of pages for the original question to be well covered before bringing the entire internet into the mix.

Also not sure why you need to pick on numpties over here in the colonies for poor use of electrical terminology. I've read a number of threads on this forum about boat electrics where the lack of understanding and incorrect use of terminology was no less astounding than I've seen anywhere else on the planet. Lack of knowledge and understanding knows no boundaries.
 
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