Muppet electrical question re Watts Amp hours etc....

[Colin Smith]

OK, I know the calcs Amps = Watts/Volts etc but am getting a bit woolly when it comes to understanding what it will actually mean to my battery bank – or drain on it to be precise - for various bits of equipment. For example: I’m thinking of getting a TV. The unit I’m looking at is rated at 40W max when ‘on’. The power supply is a 240v to 12v adapter and the bumf says it’s 12v 5A.

 

I have 5 x 110Ah batteries which are not new. I don’t have a Smartygauge as yet so tend to go off the voltage reading from the inverter controller. When fully charged this shows 13V for a short time than drops to 12.9V and stays there until I start to use something. I never let the voltage drop below about 12.3V and charge them ‘til they reach ‘float’.

 

So, for example what will the TV actually take from my batteries? Does the 40W mean it’s drawing 40W over an hour? If so, does that mean it will draw 120W over 3 hours or is there some sort of electrickery thing that buggers up the calcs? If I connect it directly to the 12V supply does that mean it’s drawing 3.3 Amps? If I connect it via the 240V inverter, what will it be drawing in Amps over an hour? (I know I need to add something for the inverter ‘losses’)? Am I going mad?

 

Thank you in advance…

Colin

[dor]

Your 40W TV will probably draw a bit less than the rated figure. Allow about 3 amps draw from your batteries. The power (watts) will be much the same whether you run it from 12V or 230V, but in the latter case you have to allow for some loss in the inverter due to overhead and conversion efficiency.

[Cupboard]

Colin - Lets keep it really simple. You have worked out that 40W at 12V is about 3.3A Well done! The bumf says 5A at 12V so take that 5A to allow for losses and then assume roughly you will take 5A from your battery every hour the TV is running. This 5A will be about the right figure if it run direct from the batteries or the inverter.

 

You have 550A of batteries and every hour the TV is running will take 5A out of the batteries IN THEORY.

 

In practice you wont get 550A out of one charge of the batteries but it is a rule of thumb to start with.

[alan_fincher]

I would say that wording on the 240 volt to 12 volt adaptor means it is capable of supplying 60 watts if it needs to (12 volts x 5 amps), but that the 40 watt rating of the TV means it will not in normal use draw more than those 3.3 amps you have calculated, (40 watts / 12 volts).

 

If you power via an inverter and then the mains "brick", you are effectively going 12 volts -> 240 volts -> 12 volts, and are introducing some losses because of both conversion processes, and it is probably realistic to add at least 10% and possibly 20% at worst to those 3.3 amps to cover this. (I'm assuming your inverter is left switched on all the time, so the amount of power it consumes when doing nothing will not come in to it, because that is being used whether you have the TV on or not....)

 

Worst case scenario, I would say, is that you are drawing 4 amps from your batteries - less if you power direct from "12 volts", but you then run that risk (that some think is real, and some think imaginary!), of putting well over 12 actual volts into the TV, (particularly if you run your engine whilst it is on), and possibly doing it some damage.

 

I'd go with 4 amps - 1 hours use you will take 4 Ah from your batteries, 2 hours use 8 Ah - and so on..... Obviously normal rules of charging mean that charging at 4 amps for an hour doesn't actually put 4 amp hours back, but that applies just the same as to all your other electrical usage.

[ChrisPy]

for clarity:

the TV consumes 40W of elecrrical power when it is turned on, that is not related to time.

if it consumes 40W then at 12V it is drawing a current of 3.3A which again is not related to time.

the current drawn from your batteries will be about 4A (allowing for losses) which again is not related to time.

the drain on your batteries in one hour will be 4Amp.Hours (Amps x Hours).

if you left the TV on for 10 hours the drain would be 40Amp.Hours out of you nominal battery capacity of 550Amp.Hours.

 

hope thast helps explain the different units of measurement.

[WotEver]

Because it's not been mentioned I'll also add that your 550Ah bank only has a usable 275Ah when new if we're going to observe the 50% rule. The bank isn't new, so let's take say 40% off that, leaving a usable 165Ah. Obviously that last figure could be higher or lower, but it's a good guide to show how what you think you may have suddenly disappears when you start doing the maths.

 

Tony

[by'eck]

Think of watts as power required to run an electrical device & watthours as energy consumed over time in running that device. Generally this is only used to measure AC consumption.

 

With DC, convert the wattage of the device into current allowing for losses in adapters. Then multiply that current with the time the device is on. So if your TV draws an average of 3amps @ 12 volts, it will consume 9Ah from your batteries over 3 hours.

[Theo]

Colin - Lets keep it really simple. You have worked out that 40W at 12V is about 3.3A Well done! The bumf says 5A at 12V so take that 5A to allow for losses and then assume roughly you will take 5A from your battery every hour the TV is running. This 5A will be about the right figure if it run direct from the batteries or the inverter.

 

You have 550A of batteries and every hour the TV is running will take 5A out of the batteries IN THEORY.

 

In practice you wont get 550A out of one charge of the batteries but it is a rule of thumb to start with.

 

Oops there is some confusion here.

 

The ampere is the rate of flow of charge so the TV will use 5A all the time.

 

Charge is current x time so the TV uses 5ampere hours (Ah) of charge for each hour running so for 2 hours it will use 10Ah.

 

Nick

[steelaway]

Hi

 

You can see from the above that it is an exact science, but - in the real world you need to put in a bit more than you take out.

If you install an amp meter you will develop a feeling for it all and have some indication of what's going on.

 

Telly has been on for 5 hours and the amp meter is reading -5amps, you have used roughly -25 amps.

When you are charging the amp meter will read +25amps, keep it charging for an hour and a half and you wont be far wrong.

Some of the time it is more, when the fridge switches on or you use a pump, but you will see it on the amp meter and charge accordingly.

 

You can buy much more sophisticated gauges but this is all I use and I never have to worry.

Again in the real world and you move everyday the 4 hours you are running will usually top up the battery's for the evening

 

Alex

[smileypete]

Hi,

 

If you don't have a panel ammeter then probably you're making things hard for yourself.

 

If you do have one, this post may help you, (if not please say why!)

 

cheers,

Pete.

 

ETA: A TV with an LED backlight will draw the least power.

 

A normal TV with 12V adapter is risky to run straight off the batts, especially when the engine is starting/running.

Edited December 8, 2011 by smileypete

[nicknorman]

Think of watts as power required to run an electrical device & watthours as energy consumed over time in running that device. Generally this is only used to measure AC consumption.

Being picky about it I think the wattage is relevant to 12v systems that have switch mode power supplies (nearly all electrical goods these days, even LED lights), because the current varies inversely with the voltage (which on a boat can of course vary significantly even with the charging systems off) whilst the wattage remains the same.

[Colin Smith]

Wow, thank you all for your answers - the paranoia inherited from school days when the cane was liberally used for being a 'dunce' has subsided!!

 

It is pretty straight forward now that it has been kept simple for me - at least I can now make an educated assumption of what the effect of various devices will have on my battery capacity. It is an LED TV that I'm thinking of getting and will only probably be used in the evening to watch a film etc. So I'm comfortable running it directly from the 12V supply - could always switch it over to the inverter/240v brick if I was charging the batteries at the same time etc (don't have a fridge!).

 

Thanks again to everyone.

 

Colin

[Chalky]

OK, I know the calcs Amps = Watts/Volts etc but am getting a bit woolly when it comes to understanding what it will actually mean to my battery bank – or drain on it to be precise - for various bits of equipment. For example: I’m thinking of getting a TV. The unit I’m looking at is rated at 40W max when ‘on’. The power supply is a 240v to 12v adapter and the bumf says it’s 12v 5A.

 

 

The 5a may be a surge figure. Many electronic/electrical devices will draw a current higher than their normal running current when they're first turned on. This can be caused by the extra power needed to start a motor or charge up the capacitors in an input network. Often once past the surge the power requirement is a lot less.

[Mat B]

This Excel spreadsheet might be useful

 

 

m@