Electrical confusion

[Dave_P]

I've never properly understood electrics.

 

I have 3 x 100 amp hour leisure batteries.

 

When I need to work out my consumption, surely I need to know how much power is coming out of them, in amps per hour, from using elctrical stuff and how much power I'm putting in via my battery charger or alternator.

 

Certain things like my fridge are easy since the manufacturer states that it uses 1.5 amps per hour. But what about other things? How do I work out how many amps per hour my t.v. uses? Or my inverter? Or my lights? Everything is given in watts.

 

I have to confess (and I suspect I'm not alone in this) I don't really understand what volts, watts and amps actually are???

 

I assume that a 70 amp alternator will put 70 amps per hour into my batteries, but I suspect it's more complicated that that with diminishing returns as my batteries approach full charge.

 

I have no idea how many amps my battery charger puts in.

 

In practice, through experience and looking at my voltmeter (I do not have, and I can't afford a fancy smart meter) I can judge how much engine or genny running I need to do according to how much I've been using electrical devices, so i suppose it doesn't matter. Except for this:- I'm thinking of buying a solar panel (something like this My link which is listed as 80 watts and 4.55 amps. The trouble is, that's meaningless to me. I have a general suspicion that it won't be enough to run my fridge, except maybe on a very sunny day! Does 80 watts equal 4.55 amps? Is there an equation I can use to convert watts to amps so I could figure out how many amps a 20 watt bulb uses? Obviously the panel won't be producing 80 watts for most of the time, so how could I work out how many watts (or amps) it will produce on the spring equinox (12 hours of daylight) assuming I'm moored in an open space, have the panel angled towards the sun and the weather for the day was 50% sun, 50% light cloud? I realise that it's impossible to give an exact figure but surely I can get a ball-park figure?

 

Ultimately what I want to figure out is how long will the panel take to pay for itself when compared to charging with my engine or genny which incur fuel costs.

 

Sorry if my ramblings make no sense to you or if I've made some elementary error.

[Chris Pink]

Basics first: Watts = Volts x Amps, ie at 12V a 24W light bulb will use 2A.

 

Your solar panel figures will confuse, so for an 80W panel expect a peak of around 6A

 

You'll never know when it pays for itself because the figures are too complex to be worth computing. All you will know is that in the summer you use engine charging less, how much less is one of those great explorations and unique to you.

 

You will need an ammeter, which is a fairly cheap piece of kit if you want to know any more. Your multimeter can be used in this way but is a pain if you don't know exactly waht you doing.

[Dave_P]

an ammeter. ok. i'll get one, thanks.

 

and thanks for the w = a x v.

 

I knew I should have paid attention in physics more.

Edited February 5, 2013 by Dave_P

[Chris Pink]

Your alternator will, as you rightly assume put a diminishing current into your batteries, but won't start at 70A, more likely 40A. You are better judging how charged your batteries are by reading the voltage, when the alternator voltage stops rising, at around 14.4V (if you're lucky enough to have a modern alternator) your battery is 80% charged, this will then take hours to reach 100% and is best left to a solar panel to finish off.

 

If you take a battery reading in the morning, This will give you a rough indication of state of charge 12.2V = 50% 12.8V = 100%

 

Voltage is only a good indicator of SOC if the battery has rested before taking the reading.

Edited February 5, 2013 by Chris Pink

[Dave_P]

 

 

Your solar panel figures will confuse, so for an 80W panel expect a peak of around 6A

 

 

 

Stupid question alert!

 

Does that mean it will put 6A per hour into my batteries at it's peak? I want to assume it does but i'm aware of the dangers of assuming.

Edited February 5, 2013 by Dave_P

[Chris Pink]

you mean 6Ah per hour. Units are important to clear confusion.

 

Yes, that's right.

 

6A for one hour = 6 Ah

3A for two hours = 6Ah

 

6A is a peak figure and will normally be a lot less.

Edited February 5, 2013 by Chris Pink

[Dave_P]

Your alternator will, as you rightly assume put a diminishing current into your batteries, but won't start at 70A, more likely 40A. You are better judging how charged your batteries are by reading the voltage, when the alternator voltage stops rising, at around 14.4V (if you're lucky enough to have a modern alternator) your battery is 80% charged, this will then take hours to reach 100% and is best left to a solar panel to finish off.

 

If you take a battery reading in the morning, This will give you a rough indication of state of charge 12.2V = 50% 12.8V = 100%

 

Voltage is only a good indicator of SOC if the battery has rested before taking the reading.

 

Thanks again.

[MIKE P]

an ammeter. ok. i'll get one, thanks.

 

and thanks for the w = a x v.

 

I knew I should have paid attention in physics more.

 

This could be helpful

Current /Amps = Power/Watts divided by Voltage

Power/Watts = Voltage multiplide Current/Amps

Voltage = Power/Watts divided by Current/Amps

[Chris Pink]

This could be helpful

Current /Amps = Power/Watts divided by Voltage

Power/Watts = Voltage multiplide Current/Amps

Voltage = Power/Watts divided by Current/Amps

 

 

Easy mnemonic (easier than spelling mnemonic) is

 

W

I V

 

cover the one up you want and you get the sum for it, ie cover W and W= I x V cover V and V = W / I

 

also works for

 

V

I R

 

where R = resistance

Edited February 5, 2013 by Chris Pink

[Justme]

 

 

Your solar panel figures will confuse, so for an 80W panel expect a peak of around 6A

 

 

 

 

No it wont. The voltage that the 80 watts is produced at will be more like 18v not 12v. If connected with PWM controller the panel will only be able to produce 4.55 amps x 12.8v (14.4v) = 58 watts ( 65 watts) & not the 80. Thats why MPPT controllers are better as they allow the panel to produce max power at the best voltage.

[Ex- Member]

I've never properly understood electrics.

 

I have 3 x 100 amp hour leisure batteries.

 

When I need to work out my consumption, surely I need to know how much power is coming out of them, in amps per hour, from using elctrical stuff and how much power I'm putting in via my battery charger or alternator.

 

Certain things like my fridge are easy since the manufacturer states that it uses 1.5 amps per hour. But what about other things? How do I work out how many amps per hour my t.v. uses? Or my inverter? Or my lights? Everything is given in watts.

 

I have to confess (and I suspect I'm not alone in this) I don't really understand what volts, watts and amps actually are???

 

I assume that a 70 amp alternator will put 70 amps per hour into my batteries, but I suspect it's more complicated that that with diminishing returns as my batteries approach full charge.

 

I have no idea how many amps my battery charger puts in.

 

In practice, through experience and looking at my voltmeter (I do not have, and I can't afford a fancy smart meter) I can judge how much engine or genny running I need to do according to how much I've been using electrical devices, so i suppose it doesn't matter. Except for this:- I'm thinking of buying a solar panel (something like this My link which is listed as 80 watts and 4.55 amps. The trouble is, that's meaningless to me. I have a general suspicion that it won't be enough to run my fridge, except maybe on a very sunny day! Does 80 watts equal 4.55 amps? Is there an equation I can use to convert watts to amps so I could figure out how many amps a 20 watt bulb uses? Obviously the panel won't be producing 80 watts for most of the time, so how could I work out how many watts (or amps) it will produce on the spring equinox (12 hours of daylight) assuming I'm moored in an open space, have the panel angled towards the sun and the weather for the day was 50% sun, 50% light cloud? I realise that it's impossible to give an exact figure but surely I can get a ball-park figure?

 

Ultimately what I want to figure out is how long will the panel take to pay for itself when compared to charging with my engine or genny which incur fuel costs.

 

Sorry if my ramblings make no sense to you or if I've made some elementary error.

 

 

 

Hi

 

It's not actually that difficult, there's even a special calculator for doing this I recall. Basically most appliances will have a watts usage, if your system is 12v then divide the watts by 12.

 

Ie 120 wattts = 10 amps. You then of course calculate the time each divice is used.

 

Alternators are rated at their highest output and most alternators put outt that maximum output usually between 5 and 7 thousand revs. If your boat engine idles at 1000 revs and your engine pulley to alternattor pulley is 2 to 1 hen the alternatttor will run at 2000 rpm which will unlikely put out anything near its full output of 50 amps. Some boat engines have increased pulley size to make the alternattor spin faster, so at 4 to 1 ratio is 4000 rpm at idle but 6000 rpm at 1500 rpm say cruising. This should have the alternator putting out it's maximum amps. However i will only put out those amps if the batteries will accept them. There are many stages of charging and it's quite complicated, but basically the state of charge of your batteries determines how many amps it acceps from a charger/alternator.

 

The amps you quote for your fridge are very good, what fridge? but 80watts does not equal 4.55 amps it should be 6.66 amps.

 

On a good summers day it would average around 4 amps from 8am to 8pm so nearly 50 amps. Solar panels though are good for your batteries as slow trickle charging aids longevity of battery life.

 

Solar panels are so cheap now that fitted on a boat your investment will be covered easily in 2 years, probably a year following a very hot summer.

[nicknorman]

I've never properly understood electrics.

 

I have 3 x 100 amp hour leisure batteries.

 

When I need to work out my consumption, surely I need to know how much power is coming out of them, in amps per hour, from using elctrical stuff and how much power I'm putting in via my battery charger or alternator.

 

Certain things like my fridge are easy since the manufacturer states that it uses 1.5 amps per hour. But what about other things? How do I work out how many amps per hour my t.v. uses? Or my inverter? Or my lights? Everything is given in watts.

 

I have to confess (and I suspect I'm not alone in this) I don't really understand what volts, watts and amps actually are???

 

I assume that a 70 amp alternator will put 70 amps per hour into my batteries, but I suspect it's more complicated that that with diminishing returns as my batteries approach full charge.

 

I have no idea how many amps my battery charger puts in.

 

In practice, through experience and looking at my voltmeter (I do not have, and I can't afford a fancy smart meter) I can judge how much engine or genny running I need to do according to how much I've been using electrical devices, so i suppose it doesn't matter. Except for this:- I'm thinking of buying a solar panel (something like this My link which is listed as 80 watts and 4.55 amps. The trouble is, that's meaningless to me. I have a general suspicion that it won't be enough to run my fridge, except maybe on a very sunny day! Does 80 watts equal 4.55 amps? Is there an equation I can use to convert watts to amps so I could figure out how many amps a 20 watt bulb uses? Obviously the panel won't be producing 80 watts for most of the time, so how could I work out how many watts (or amps) it will produce on the spring equinox (12 hours of daylight) assuming I'm moored in an open space, have the panel angled towards the sun and the weather for the day was 50% sun, 50% light cloud? I realise that it's impossible to give an exact figure but surely I can get a ball-park figure?

 

Ultimately what I want to figure out is how long will the panel take to pay for itself when compared to charging with my engine or genny which incur fuel costs.

 

Sorry if my ramblings make no sense to you or if I've made some elementary error.

Firstly, it is helpful to get the terminology right otherwise it stores up problems for later! So current flow, eg the output from the alternator, is measured in amps, not amps per hour (there is no such thing as the latter). When current flows into something eg a battery, it is stored. So if you put 1A into a battery for 1 hr, it will store (roughly) 1 amphour. That is why battery capacities are quoted in amphours, whereas alternators flow rates are quoted in amps.

 

Take the water analogy. Volts is like the pressure in a water pipe. Current is like the rate of water flow in the pipe. If the tap is off, there is still pressure but no flow. So in electrical terms if the circuit is switched off there is voltage in the wiring but no current. Nothing useful is happening and no water is being consumed.

 

Now open the tap. Because the water is pressurised (volts) it flows out of the tap (current) and if you conntected it to a turbine, water wheel etc it could move something (power, a combination of pressure and flow or volts and current. Electrical power is measured in watts.

 

Now connect the tap to a tank and turn it on. The water flows (current) into the tank (battery). If you fill it at 100 litres per hour (100 amps), after an hour you would have 100 litres in the tank (100 amp hours) which you could use later by draining it out.

 

The slight confusion there is that amps is a flow rate - it is really charge per second - but amps is so much easier! - whereas litres is a volume (charge) and there is no convenient unit such as amps to represent water flow rate, so we have to call it litres per hour. So you need to read the previous para carefully to get it right!

Edited February 5, 2013 by nicknorman

[Dave_P]

No it wont. The voltage that the 80 watts is produced at will be more like 18v not 12v. If connected with PWM controller the panel will only be able to produce 4.55 amps x 12.8v (14.4v) = 58 watts ( 65 watts) & not the 80. Thats why MPPT controllers are better as they allow the panel to produce max power at the best voltage.

 

Thanks but you've lost me completely. What do PWM and MPPT mean? and what do you mean by "they allow the panel to produce max power at the best voltage". i.e. what do you mean by "best voltage" and "max power"?

 

Hi

 

It's not actually that difficult, there's even a special calculator for doing this I recall. Basically most appliances will have a watts usage, if your system is 12v then divide the watts by 12.

 

Ie 120 wattts = 10 amps. You then of course calculate the time each divice is used.

 

Alternators are rated at their highest output and most alternators put outt that maximum output usually between 5 and 7 thousand revs. If your boat engine idles at 1000 revs and your engine pulley to alternattor pulley is 2 to 1 hen the alternatttor will run at 2000 rpm which will unlikely put out anything near its full output of 50 amps. Some boat engines have increased pulley size to make the alternattor spin faster, so at 4 to 1 ratio is 4000 rpm at idle but 6000 rpm at 1500 rpm say cruising. This should have the alternator putting out it's maximum amps. However i will only put out those amps if the batteries will accept them. There are many stages of charging and it's quite complicated, but basically the state of charge of your batteries determines how many amps it acceps from a charger/alternator.

 

The amps you quote for your fridge are very good, what fridge? but 80watts does not equal 4.55 amps it should be 6.66 amps.

 

On a good summers day it would average around 4 amps from 8am to 8pm so nearly 50 amps. Solar panels though are good for your batteries as slow trickle charging aids longevity of battery life.

 

Solar panels are so cheap now that fitted on a boat your investment will be covered easily in 2 years, probably a year following a very hot summer.

 

Thanks for all this. My fridge is a shoreline and it looks like an older version of the RK130 which is stated at 1.59 amps. My figure of 1.5 is rough but it was the principle I was confused over.

Edited February 5, 2013 by Dave_P

[nicknorman]

Thanks but you've lost me completely. What do PWM and MPPT mean? and what do you mean by "they allow the panel to produce max power at the best voltage". i.e. what do you mean by "best voltage" and "max power"?

 

Ah - knowledge is power! Jargon is used to sort the "in the know"s from the "not in the know"s. Well that would be a harsh way of looking at it!

 

PWM is pulse width modulation. In other words, it regulates the solar panels by switching them on and off very rapidly, with varying timings of on vs off. They are OK and cheap but not terribly efficient.

 

MPPT is maximum power point tracking. It is a way of extracting the most power out of your solar panels. Clever, but more expensive and not always as good as its made out to be (depends on the model)

 

The nature of solar panels is that they will give their best power when loaded so as they are producing a specific voltage. That voltage probably won't match the batteries. So the MPPT keeps the voltage at the panels at the optimum, whilst converting it to the correct voltage to charge the batteries. That way, you squeeze a bit more current out of the panels than if you used a PWM controller.

Edited February 5, 2013 by nicknorman

[Dave_P]

I just looked up PWM on wikipedia it made my head hurt so I turned it off.

 

Ah - knowledge is power! Jargon is used to sort the "in the know"s from the "not in the know"s. Well that would be a harsh way of looking at it!

 

PWM is pulse width modulation. In other words, it regulates the solar panels by switching them on and off very rapidly, with varying timings of on vs off. They are OK and cheap but not terribly efficient.

 

MPPT is maximum power point tracking. It is a way of extracting the most power out of your solar panels. Clever, but more expensive and not always as good as its made out to be (depends on the model)

 

The nature of solar panels is that they will give their best power when loaded so as they are producing a specific voltage. That voltage probably won't match the batteries. So the MPPT keeps the voltage at the panels at the optimum, whilst converting it to the correct voltage to charge the batteries. That way, you squeeze a bit more current out of the panels than if you used a PWM controller.

 

I prostrate myself before the superior electrical intellects who have replied!

 

I have learnt that I should get a solar panel for when I leave my luxurious shore-line powered mooring in a month or so.

[MoominPapa]

MPPT vs PWM.

 

We've already established the basics, power is volts multiplied by amps.

 

Now think about a solar panel: it's producing a current (amps), and a certain voltage. The two are related, as the voltage decreases, the current increases. The exact values depend on how sunny it is.

Also think about a battery: for any state-of-charge, the voltage is determined by the battery, and the charge current. If the battery is discharged, even a large charge current won't increase the voltage.

 

Now recall that power is volts times amps. As the panel voltage changes, the current changes as well, and product of the two (power) also changes. The relationship between current and voltage isn't linear, but it's always in one direction: as the current increases the voltage decreases, and vice versa. The relationship between power (current times voltage) isn't like this: instead it's hump shape. The power increases to maximum at one particular voltage/current value, and then decreases again.

 

Now, a PWM controller. This controls the current flowing into battery. The voltage at the battery has to equal the voltage at the panel. Back to our discharged battery: the voltage at the battery is controlled by the current and state of charge. This is also the voltage at the panel. If (as is likely) the voltage at the panel isn't the voltage at which the panel power is at the top of the "hump" then we're not getting the maximum energy out of the panel.

 

To fix this we need a MPPT controller. This has two bits, a DC-DC converter and tracker. The DC-DC converter removes the constraint that the panel voltage equals the battery voltage. Now, they can be different. Instead battery power (current times volts) must be equal to (in practise a bit less than) panel power (current times volts). The controller feeds as much current as it can into the battery, at the voltage the battery dictates. At the same time, the tracker independently alters the voltage and current of the panel to keep it the top of the power curve hump. The current and voltage corresponding to this hump vary as the light changes, so the tracker has to keep adjusting things over time.

 

So: PWM : panel voltage always equals battery voltage.

MPPT: panel voltage varies independently from battery voltage.

 

MP.

Edited February 5, 2013 by MoominPapa

[nicknorman]

MPPT vs PWM.

 

We've already established the basics, power is volts multiplied by amps.

 

Now think about a solar panel: it's producing a current (amps), and a certain voltage. The two are related, as the voltage decreases, the current increases. The exact values depend on how sunny it is.

Also think about a battery: for any state-of-charge, the voltage is determined by the battery, and the charge current. If the battery is discharged, even a large charge current won't increase the voltage.

 

Now recall that power is volts times amps. As the panel voltage changes, the current changes as well, and product of the two (power) also changes. The relationship between current and voltage isn't linear, but it's always in one direction: as the current increases the voltage decreases, and vice versa. The relationship between power (current times voltage) isn't like this: instead it's hump shape. The power increases to maximum at one particular voltage/current value, and then decreases again.

 

Now, a PWM controller. This controls the current flowing into battery. The voltage at the battery has to equal the voltage at the panel. Back to our discharged battery: the voltage at the battery is controlled by the current and state of charge. This is also the voltage at the panel. If (as is likely) the voltage at the panel isn't the voltage at which the panel power is at the top of the "hump" then we're not getting the maximum energy out of the panel.

 

To fix this we need a MPPT controller. This has two bits, a DC-DC converter and tracker. The DC-DC converter removes the constraint that the panel voltage equals the battery voltage. Now, they can be different. Instead battery power (current times volts) must be equal to (in practise a bit less than) panel power (current times volts). The controller feeds as much current as it can into the battery, at the voltage the battery dictates. At the same time, the tracker independently alters the voltage and current of the panel to keep it the top of the power curve hump. The current and voltage corresponding to this hump vary as the light changes, so the tracker has to keep adjusting things over time.

 

So: PWM : panel voltage always equals battery voltage.

MPPT: panel voltage varies independently from battery voltage.

 

MP.

 

I would only say that its not quite true that the panel voltage must = the battery voltage for PWM. Firstly, it obviously must be greater than the open circuit battery voltage to get anything to happen, but more relevantly, whilst the PWM is switching off, chances are the panel voltage is much greater than the battery voltage, when it is switching on, it of course comes down to the charging voltage of the batts. In reality these fluctuations are smoothed out by the electronics, but if you were to put a meter on the panel, and again on the batts, you would find the panel voltage was above the battery voltage especially when the bats are well charged and the controller is shutting things down.

 

Also, the extra effectiveness derived from an MPPT is to some extent dependant on the best power voltage of the panel - if its reasonably close to the bats, there is not that much to be gained by an expensive controller (I appreciate that it does of course change with insolation), whereas if its much higher, there is more to be gained.

 

And finally, rumour has it that some (cheap) MPPT controllers are not much use. To get a good one, you have to spend quite a bit?

Edited February 5, 2013 by nicknorman

[MoominPapa]

I would only say that its not quite true that the panel voltage must = the battery voltage for PWM. Firstly, it obviously must be greater than the open circuit battery voltage to get anything to happen, but more relevantly, whilst the PWM is switching off, chances are the panel voltage is much greater than the battery voltage, when it is switching on, it of course comes down to the charging voltage of the batts. In reality these fluctuations are smoothed out by the electronics, but if you were to put a meter on the panel, and again on the batts, you would find the panel voltage was above the battery voltage especially when the bats are well charged and the controller is shutting things down.

That's a reasonable criticism of my explaination.

 

Also, the extra effectiveness derived from an MPPT is to some extent dependant on the best power voltage of the panel - if its reasonably close to the bats, there is not that much to be gained by an expensive controller (I appreciate that it does of course change with insolation), whereas if its much higher, there is more to be gained.

True. Of course a suitable MPPT controller allows you to put panels in series rather than parallel, saving in wiring costs and losses.

 

And finally, rumour has it that some (cheap) MPPT controllers are not much use. To get a good one, you have to spend quite a bit?

I think that was established some time ago, and caused me to avoid some other brands in favour of a "Juta" controller, which is cheap and is a proper MPPT controller.

 

MP.

[Chris Pink]

MPPT vs PWM.

 

 

There is another, non-technical way of looking at it.

 

if you use feed-in surplus panels (usually around 200W or more) then you need an MPPT controller to match the panels to your battery. If you pay less than £120 for a controller it's the wrong one.

 

if you use lower voltage panels, usually 120W or less, then, to quote one of my suppliers "people aren't buying MPPT controllers any more because it's cheaper and more effective to buy extra panel(s) at current prices to get the same improvement"

[nicknorman]

if you use lower voltage panels, usually 120W or less, then, to quote one of my suppliers "people aren't buying MPPT controllers any more because it's cheaper and more effective to buy extra panel(s) at current prices to get the same improvement"

That is true provided you don't mind the extra space taken up on the roof. Having a shiny boat I am not allowed to put any panels on the roof (= clutter), but if I were I am sure that minimising their area would be a critical factor!