Electricity for an idiot

[Ricco1]

I don't always describe myself as an idiot but when it comes to electricity, I am. I've spent hours reading through posts on here but still struggle to grasp the basics.

 

What do I know: that boat batteries are 12 volt and are charged either through an alternator or via a generator and charger. That an invertor can convert 12 volt to 230 volt. Boats often have 12 volt and 230 volt circuits.

 

What don't I know: Just about everything else. I hear of amps and amp hours, watts. The more I read the more confused I get.

 

Could anyone take the trouble to explain the basics to me, or perhaps point me in the direction of some reading that will clear this in my mind?

 

Thanks.

[Trento]

Beg, borrow or steal a book called: Boatowner's Mechanical and Electrical Manual. By Nigel Calder. ISBN. 978-0-7136-7226-8. Everything you need to know is there and in simple language and sketches..

Enjoy

[DeanS]

Ricco, it is possibly wise to not try and understand "everything", but to ask "per appliance". For example, if you ask...."how is boat lighting wired up", the answer would be something like this.

 

1. A red wire from the batteries is wired to a 12V isolator switch, which is then wired to a bank of fuses. From one of these fuses, a red wire is connected to one side of some light bulbs. The other side of those light bulbs would have a black wire, which is then fed back to the battery neg terminal.

 

For each appliance, a similar explanation, might help you grasp a fuller understanding of "the whole boat's wiring".

 

 

Basically all 12V appliances such as fridges, pumps, lights, horns, would be connected to one of the fuses in the fusebank (a positive supply from the batteries) and also to the negative of the batteries.

 

Shorepower or the invertor output (220V) is normally fed to a switchbox which is then wired to plugboxes, into which various appliances could be plugged.

 

The 2 systems are never joined in any way. Some appliances (such as a fridges, TVs, Laptops, lights) could be bought to connect to one or the other system.

 

 

Hope that helps a little.

 

 

ETA.

 

A bank of batteries has a capacity of supplying a certain amount of current. This is called AmpHours. The more 12V appliances you have, the faster your batteries will lose their capacity and will need recharging.

 

Watts, is an indicatoion of how much demand an appliance will place on the batteries. A laptop is about 75Watts. An LED light bulb is possibly 2Watts. A kettle is 2000Watts (which is why boaters use gas kettles) . A hairdrier and washing machine are about 2000Watts. TVs are about 100Watts. Some folk run their boat engine....which causes the engine alternator to supply current...and then run larger items through their invertor. This has the benefit of not drawing from the batteries. I personally use a generator to supply 220V when we run the clothes washer.

Edited August 10, 2013 by DeanS

[Graham Bowers]

Try googling for "electricity water analogy" (without the quotes), for example

http://science.howstuffworks.com/environmental/energy/question501.htm

Graham

 

[patty-ann]

Thanks for above comments, I also struggle to understand but intend having a file of information so can check up as I need to. I read and listen but it does not sink into my brain cells, this forum is great for helping people such as myself learn.

[smileypete]

Also have a look at the excellent course notes on the Tony Brooks Training website and consider doing the course if the other stuff will help, RCR organise them these days I think.

 

http://www.tb-training.co.uk/

 

cheers, Pete.

~smpt~

[steelaway]

I don't always describe myself as an idiot but when it comes to electricity, I am. I've spent hours reading through posts on here but still struggle to grasp the basics.

 

What do I know: that boat batteries are 12 volt and are charged either through an alternator or via a generator and charger. That an invertor can convert 12 volt to 230 volt. Boats often have 12 volt and 230 volt circuits.

 

What don't I know: Just about everything else. I hear of amps and amp hours, watts. The more I read the more confused I get.

 

Could anyone take the trouble to explain the basics to me, or perhaps point me in the direction of some reading that will clear this in my mind?

 

Thanks.

 

I don't always describe myself as an idiot but when it comes to electricity, I am. I've spent hours reading through posts on here but still struggle to grasp the basics.

 

What do I know: that boat batteries are 12 volt and are charged either through an alternator or via a generator and charger. That an invertor can convert 12 volt to 230 volt. Boats often have 12 volt and 230 volt circuits.

 

What don't I know: Just about everything else. I hear of amps and amp hours, watts. The more I read the more confused I get.

 

Could anyone take the trouble to explain the basics to me, or perhaps point me in the direction of some reading that will clear this in my mind?

 

Thanks.

 

Filling a bucket of water

Try thinking as Volts as the pressure (in a hose) Amps is the amount of flow (through the hose) Watts as the amount of water needed to fill the bucket.

High volts and low amps will fill the bucked or Low volts and high amps will required to fill the bucket.

Therefore the formula is -

Watts (water in the bucket) equals Volts times Amps

Amps is Watts divided by Volts

Volts is Watts divided by Amps

 

Its easier to remember the formula by drawing a triangle with a horizontal line across it, above the line write a W below the line write AxV the horizontal line is the divide by line.

 

Alex

[bizzard]

It'll be the volt of the Am-eater when its at ohm cos it'll gobble up all the wattsanames.

[Tony Brooks]

Much simplified:-

 

AMPS

 

Amps is how many electrons are flowing around a circuit - the would be form the battery or alternator, through the wires, through an appliance where the electricity does some work for you, and back to where it started via some more wires. Don't let anyone confuse you with "electron flow" of electricity because you will not be messing about with electronics. AS far as you are concerned electricity flows for the battery or alternator on a positive wire from the positive (+) terminal and back again down the negative wire (-).

 

 

The more electrons and thus Amps that are flowing the more "work" the electricity can do for you. An LED bulb will not need many electrons so only draws a fraction of an Amp. A water pump motor needs far more amps to flow because it is doing more "work".

 

The number of electrons equating to an Amp is so great there is absolutely no point in considering the number - stick to Amps - Amp = the quantity of electricity flowing.

 

Amps relate to real physical things so Amps are never used up when electricity works for you.

 

 

VOLTS

 

If those amps are to flow through a wire then something has to give them a shove and that thing is Volts. Volts is the electrical pressure pushing Amps around a circuit. When electricity does any work for you its the Volts that get "used up". If you push a barrow of sand up a hill then as you tire it is your push (the volts) that gets used up. You still have the sand that is like the Amps and that is as large a pile as it ever was.

 

 

 

OHMS

 

Anything electricity is flowing through tries to stop the electricity. An insulator like the plastic around a wire stops it completely while a copper cable hardly stops it at all. The degree to which parts of a circuit tries to stop the current flowing is the OHMS or resistance. With a bit of luck you will only ever find resistance of any significance in an appliance that is working for you. The volts get "used up" pushing electricity through a resistance.

 

 

Just to confuse: If you have (say) a 12V bulb it will allow a certain amount of electricity through it when 12V is pushing it. Reduce the voltage like when you have a flat battery then less electricity (amps) can be pushed through the bulb because of the reduced voltage a "push" so the bulb is dimm. Double the voltage and twice the electricity can be pushed through the bulb. The bulb will be very bright and probably blow.

 

WATTS

 

Watts is a measurement of the amount of power an appliance will use or the amount of power something like an inverter can supply. An ordinary boater really can pretty much ignore Watts (power) except in one instance:- Power is calculated by multiplying the Amps by the Volts. Many appliances are only rated in Watts so to find out how much electricity you need to store, how large a cable has to be or how well you can charge a battery you need to convert Watts to Amps so:

 

Divide the watts by the battery voltage to give you amps.

 

To convert the Watts of a MAINS appliance to find out how many amps the 12V battery would have to supply an inverter to drive that appliance divide the mains wattage by 10, not 12, because that will build in an allowance for losses in the inverter.

 

 

AMP HOURS

 

This is a sort of artificial unit that has little to do with the basic electrical units of Amps and Volts. Its sole use is related to the charging and discharging of batteries. A battery can only hold a finite number of electrons so it can only hold a certain number of Amps. However the number of electrons being sent around a circuit depends upon

how many the appliance in use needs. So a small bulb will only need a fraction of an Amp to make it work while a bow-thruster will needs lost and lot of amps. Clearely a battery will take longer to discharge with just that bulb on that if the bow-thruster was running so boaters need a way of knowing how many amps the battery can supply and for how long.

 

A 100 Amp hour battery (100Ah) can in theory supply 100 amps to an inverter for one hour or 1 amp to a bulb for 100 hours. It does not quiet work like that because the inverter would flatten the battery in less than an hour while the bulb.

 

Come back if you need more explanations.

[arbutus]

OK. My turn to try an explanation!

 

Using the analogy of water flowing through a pipe,

Voltage (measured in Volts) is equivalent to water pressure, Current (measured in Amps) is equivalent to the flow of water through a pipe.

 

Electrical power is measured in watts and is calculated by multiplying the voltage by the current flowing (Volts by Amps).

Voltage affects the amount of electrical insulation a cable requires. Current affects the electrical cable size that needs to be installed. The more current, the thicker the cable required. So to transmit a certain amount of power (Volts x Amps), it can either be done by using low voltage (say, 12v) and high current ( Amps) with thick cables, or using high voltage (say 230 volts) and low current with thinner cables.

One other thing to consider is the resistance of the cable. Resistance causes the voltage to drop along the cable. This is more noticeable on a 12 volt system, so you may start with a healthy 13 volts or so in the engine room at the back of the boat but the voltage right at the front of the boat may only just be adequate to run your 12volt equipment. Thicker cable reduces the resistance of the cable and hence reduces the voltage drop. Some boats use a 24 volt DC system rather than 12 volt to reduce voltage drop problems. Trucks and buses also use 24v DC.

 

12 volt systems use a battery and are DC (Direct Current) systems. The voltage is constant at 12 volts (or actually about 13.5 volts).

230 volt systems are AC or Alternating Current systems that in the UK run at 50 Hertz, or 50 cycles per second.

This means that 50 times a second the voltage starts at zero, then in a graceful curve increases to about 290 volts before curving back to zero, then going negative in a graceful curve to minus 290 volts before curving back to zero.

It’s called a Sine wave.

(Just as an aside, the 230 volts quoted as mains voltage is the RMS or Root Mean Squared value of the voltage, this is meant to give a comparison with a DC system).

This rising and falling of the AC Sine wave voltage is very important. Many electronic devices monitor the rising of the sine wave voltage to trigger their various functions, for example a light dimmer switch works this way, adjusting a knob on the dimmer alters at what voltage on the sine wave curve the electronics “fire” and therefore how much of the cycle gets passed through to the light to illuminate the lamp.

On a boat a pure sine wave inverter reproduces a smooth 230 volt sine wave from the 12 volt DC batteries. A Modified sine wave inverter, although cheaper, will not produce such a smooth waveform, producing a stepped waveform, like something made out of Lego bricks, some electronics, even in things like washing machines, can have problems with these jerky waveforms.

Although nor relevant to boats, the constantly changing AC sine wave is critical to how things like transformers work. Pass an Alternating current through a coil of wire and the sine wave creates a magnetic field that is constantly building and collapsing. This change in the magnetic field can induce an electrical current in another coil wound on the same former. If the second coil has twice as many turns of wire, then the voltage induced will be twice that of the original coil (although the current flow will be halved).

 

Batteries are chemical devices and are rated in Ampere Hours, this is a measure of the total energy they can store. (Ampere is the full name for Amps).

So a battery rated at 132 AH can produce 1 Amp for 132 hours, or 4 Amps for 33 hours etc.. However, generally batteries can only produce about 50% of their rated capacity before going flat (I think the last sentence is correct!).

 

Generally, heating appliances take a lot of power (immersion heater, electric kettle, hair dryer, cold fill washing machine with a water heater). It’s not a good idea to use these without a shore line or at least with the engine running as the current pulled from the batteries can be large. For example, an electric kettle can be rated at 3000 watts.

Even if an inverter is 100% efficient, the current (Amps) drawn from a 12 volt battery bank would be 3000 watts divided by 12volts = 250 Amps.

[Ricco1]

Thanks so much for all the replies! I've had a quick skim through for now. I'm going to read each one more thoroughly, and the suggested links. Hopefully then I'll just describe myself as a fool rather than an idiot! Seriously though many thanks, your replies are really appreciated.

 

If I could ask one specific question. I thought I knew the answer to this but now I'm not too sure:

 

Scenario: I want to run various 12v and 230v appliances with an inverter switch on. I'm aware that my batteries are getting low. I know that I can run the boat engine thereby charging the batteries with the alternator. But: as an alternative, can I use a generator to charge the batteries, or is a generator just to provide a live feed (for want of a better term) to the 230v circuit?

[DeanS]

You can run a battery charger from a generator...and connect it to the batteries to recharge them.

[ditchcrawler]

Thanks so much for all the replies! I've had a quick skim through for now. I'm going to read each one more thoroughly, and the suggested links. Hopefully then I'll just describe myself as a fool rather than an idiot! Seriously though many thanks, your replies are really appreciated.

 

If I could ask one specific question. I thought I knew the answer to this but now I'm not too sure:

 

Scenario: I want to run various 12v and 230v appliances with an inverter switch on. I'm aware that my batteries are getting low. I know that I can run the boat engine thereby charging the batteries with the alternator. But: as an alternative, can I use a generator to charge the batteries, or is a generator just to provide a live feed (for want of a better term) to the 230v circuit?

If your inverter is what they call a "combi" it will have a charger built in. If you feed 230 volts into the shore supply plug it will also charge your batteries. Not all invertors are like this.

[bottle]

I know that I can run the boat engine thereby charging the batteries with the alternator. But: as an alternative, can I use a generator to charge the batteries, or is a generator just to provide a live feed (for want of a better term) to the 230v circuit?

 

 

See answers #12 and #13

 

plus:

 

The alternator/charger will only charge the batteries if the load on the batteries is lower than the output of the alternator/charger.

 

The alternator/charger will supply the load and any excess will charge the batteries.

Edited August 11, 2013 by bottle

[nb Innisfree]

To understand anything you have to pursue it relentlessly, no good picking bits up here and there in a disjointed manner, eat sleep and drink the subject and eventually it starts to make sense as your brain learns how to learn.

 

Warning, thinking is very hard work, it takes determination to get there.

[MtB]

To understand anything you have to pursue it relentlessly, no good picking bits up here and there in a disjointed manner, eat sleep and drink the subject and eventually it starts to make sense as your brain learns how to learn.

 

Warning, thinking is very hard work, it takes determination to get there.

 

Quite right. The concepts are easier to grasp in my view than double entry book keeping, but still require a fair bit of thinking to figure out which analogies here are accurate and which are cock-eyed....

 

MtB

[nebulae]

If you dont understand electrics,do not mess with 240volt circuits. Get a qualified sparks to do any work.240volt in a boat is potentialy lethal. Although legal to work on your own property,you are supposed to be what the law calls a "competent person" Simple 12 volt circuits should be O.K.so long as everthing is properly fused.

[ditchcrawler]

. Although legal to work on your own property,you are supposed to be what the law calls a "competent person"

I think that only applies to houses.

[Ally Charlton]

I don't know if this helps. but I was wondering about similar things. My question was how can I calculate how much electricity I need to generate in order to run the stuff I want to run on a boat (without a mains supply)

 

Again I apologize if this doesn't answer your question but the following generally cleared a lot of things up for me, electrical wise!

 

I asked a lot of people but in the end my friend gave me this (and relate this to Tony Brooks definitions of earlier)

 

power (WATTS)(W) =VOLTS(V) x AMPS(I)

 

therefore

 

W = VI

 

e.g. you have a 50W appliance (labelled on the thing itself)

 

then, approx.:

 

50= 250 x 0.2

 

I = W/V = 50/250 = 1/5A

 

A battery of 50Ah for example, that means will run 50amps per hour

 

I don't know if that helps or not, but for me understanding the relationship between the volts and the watts and ampy things really helped!!!

 

good luck!

[Tony Brooks]

 

I don't know if this helps. but I was wondering about similar things. My question was how can I calculate how much electricity I need to generate in order to run the stuff I want to run on a boat (without a mains supply)

 

Again I apologize if this doesn't answer your question but the following generally cleared a lot of things up for me, electrical wise!

 

I asked a lot of people but in the end my friend gave me this (and relate this to Tony Brooks definitions of earlier)

 

power (WATTS)(W) =VOLTS(V) x AMPS(I)

 

therefore

 

W = VI

 

e.g. you have a 50W appliance (labelled on the thing itself)

 

then, approx.:

 

50= 250 x 0.2

 

I = W/V = 50/250 = 1/5A

 

A battery of 50Ah for example, that means will run 50amps per hour

 

I don't know if that helps or not, but for me understanding the relationship between the volts and the watts and ampy things really helped!!!

 

good luck!

 

Thats OK for a start but your comment about a 50Ah battery is wrong.

 

Gibbo used to go ballistic when people said things like 50 amps per hour.

 

A 50 Ah battery holds a given amount of electricity when fully charged. In this case 50Ah worth. This means (ignoring the complications caused by Mr Pukhart) That it can supply 1 amp for 50 hours, 2 amps for 25 Hours 10 amps for 5 hours, 50 amps for one hour, 100 amps for half an hour and so on.

 

In reality it will not do that. At a discharge at lower amps (current flow) it will last longer than suggested while at the high current flows it will last for less time.

 

However even this is too simple because to get the optimum battery life it is generally accepted that we should not discharge to below 50% charged on a regular basis. Add to that the fact than many boats only charge their batteries to about 80 to 90% of fully charged each day and you find that your 50 Ah battery can only be expected to supply 15 Ah (30% of capacity) on a day to day basis. That 30% is derived for a maximum discharge of 50% recharging to 80% = 30% usable capacity.

 

Working the other way to find out how long it will take to recharge for a given Ah discharge is again complicated by two major factors.

 

1. The recharging is far from 100% efficient but it is so variable and depends upon so many factors that it is impossible to give an accurate figure. I personally work on needing to put back 30% more Ah than has been taken out but some people work on 50%.

 

2. Alternator output only stays at the rated maximum for a comparative short period of time and again the time depends on many factors making it impossible to give a definitive figure for effective charging output. What we can say is that over two to four hours (variable) the charge will be about 50% of rated output and if charging takes longer then that 50% of rated output gradually falls lower and lower. For example you can assume a 70 Amp alternator will only deliver about 35 amps for the first two to four hours.

 

The key to getting a trouble free battery and charging system that works for you is working out your electrical use in Ah and then how long it takes to put that back in with the system(s) you have but it is not as simple as it looks at first sight.

[Iain_S]

(snip)

therefore

 

W = VI

 

e.g. you have a 50W appliance (labelled on the thing itself)

 

then, approx.:

 

50= 250 x 0.2

 

I = W/V = 50/250 = 1/5A

 

A battery of 50Ah for example, that means will run 50amps per hour

(snip)

But that's 0.2A at 250V (Just over that at 240V) If the battery is 12V, then the amps from the battery is 50/12= 4.2A. And that's in a perfect world, with no invertor losses. Guestimating these brings the 50W appliance to 5A or so. Assuming the 50Ah battey is discharged to 50%, you can run that 50W for 5 hours. (provided the invertor doesn't cut out on low voltage first )

 

Iain

[Ally Charlton]

I love this forum!!! look at the stuff you learn!!!

 

but why oh why did you (tony and ian) ruin my nice, clean, easy to understand sums and disturb my equilibrium thusly ;-) I thought I'd got my head around it now I'll have to spend the night on google.

[Tony Brooks]

 

I love this forum!!! look at the stuff you learn!!!

 

but why oh why did you (tony and ian) ruin my nice, clean, easy to understand sums and disturb my equilibrium thusly ;-) I thought I'd got my head around it now I'll have to spend the night on google.

The reason is that the amps - volts - watts bit is normally fairly easy to understand once you have a "model" in your head that works for you. Even Ah is not a difficult concept when taken at face value. However a look back through the years and you will see we regularly get posters who do just that - take the Ah bit at face value - and "prove" to themselves that their system is fine despite suffering endless problems. Then we have a few who use the "face value" concept to give others inappropriate advice.

 

Once you get into charging systems and batteries it all tends to fall apart and many simply can not believe there is no definitive way of calculating things.

 

I would suggest that you just accept that an inverter driven appliance draws about 20 times its mains amperage when run from batteries and if you divide the wattage by 10 for its 12V current consumption it will all work out OK. Once you have worked with that for a while and proved to yourself it works you can then start looking at the underlying science if you feel the need.

 

Anything to do with charging and discharging batteries may be best answered by asking questions about specific cases here but filtering the replies so you discard any doubtful ones. You can ignore the fact that low discharges can be supplied for longer than "theory" suggests because you end up with a battery better charged than you thought but you do need to be very suspicious of anything related to high discharges and assume their supply time will be dramatically

shortened.

 

As far as charging the batteries is concerned then the best you can probably do is to ignore theory and charge until the current (amps) is between 1 and 2% of the battery capacity. My bank is a split charge system n a single alternator and has a total capacity of 400 Ah so I tend to charge until the ammeter is reading less than 8 amps.