# Confused by all things electrical......

Discussion in 'The Beginner' started by Phil56, Feb 12, 2014.

1. ### Phil56

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Thus far, all my numpty questions have been answered with good humour and kindness. This one may really test your patience!

I am trying to get my head round batteries and power consumption. As I understand it (and that's probably overstating things) you look at all the things you are running and add up the combined wattage. Then you divide that by the voltage you are running on (12 or 230) and that will give you the amps you are burning. The batteries are rated in Ah, telling you for how many hours they will produce 1 amp. So if you are burning 5 amps, then divide the figure by 5 and that will tell you how long the battery will last. IF (big if) I have got that right, then three questions, please:

1. We have been advised to buy a TV for the MH that runs of 12 or 230v. If the formula above is right, then running something off a 12v battery (eg the leisure battery) will draw a lot more amps, than running it off 230. That sounds wrong to me - but that's what the figures suggest. Is that right?
2. If you were running an inverter off the leisure battery, converting 12v to 230v, what effect does that have on battery consumption. I'm sure it gets worse, but how do you calculate the Amps being consumed?
3. So - having done all this I then read somewhere that you should only run a leisure battery down to 50% capacity before recharging. If that is so it suggests to me that a 100Ah battery is, in reality, a 50Ah battery. Can that be right?

Forgive me if these questions have been answered before. I have trawled the forum, but couldn't find anything.

Thanks all,

Phil

2. ### gozomikeFunster Life MemberLife Member

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Hi.

Keeping it simple, you seem to have the idea.

1) Correct 19.166 times more

2) Yes, as you worked out 1,000 watts = 83.33 Amps at 12 Volts and at 230 volts = 4.347826 amps

3) It is true.

Mike

3. ### Zains PopsFunster

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Remember that when you convert 12v to 230v through an inverter you have additional losses caused by the conversion so the load on your 12 v battery will be higher running a 230v TV than using a 12v unit.

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4. ### cmcardle75

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And there will be an annoying fan running, too.

5. ### TerryFunsterLife Member

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I am deffo no expert but do understand the theory's
Yes only ever run batteries down to 50% cap or better still no where near that (or you will kill the battery)Try to run everything on 12v and forget about inverters if possible :thumb:You may have a switch on your control panel allowing you to choose which battery to charge while on hook up --if not your on board charger will probably charge the LB not engine one Your fridge should only be on 12v while engine is running :thumb:
terry

6. ### PhilandMenaFunster

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7. ### DBKFunster

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1 and 2. Yes, as already said. The explanation is the TV is going to consume the same amount of power (watts) whatever voltage it runs off. There may be a very, very slight difference (less than 1%) between the two but nothing you would notice.

So if for example the power consumption of the TV is 50W, which it is unlikely to be more than, on 12 volts it will take 50/12* = 4.17 Amps. On 230 volts it will take 50/230 = 0.22 Amps (both to 2 decimal places).

That it only takes less than quarter of an amp on 230 volts does not mean it is taking less power and it will run for longer on your leisure battery. If you are getting the 230 volts from an inverter it is the current going into the inverter which matters, not the current going into the TV and the losses in the inverter will mean more than 4.17 amps will be drawn from the leisure batteries. I would expect the actual consumption to be around 5 amps. The peak efficiency of one unit I looked at was 88% so in broad terms using an inverter to produce 230 volts will use about 10% more current than if you could connect the same device directly to the 12 volt supply.

In summary, I think you only need a 12 volt TV. It may well come with a lead and transformer so you can plug it into 230 volts but this would only be of any use if you used the TV at home. On an EHU your batteries are being charged so there is no problem running the TV or anything else off 12 volts.

I think typical experience of people is if you have two leisure batteries of say at least 85 Ah you should be able to last 4 or 5 days off hook-up if you are not silly with using electricity.

3. As already mentioned, yes, but it is not a cast-in-concrete rule, just a sensible guideline. You may well be able to discharge some batteries down to as low as 20% but they won't last very long. The 50% is a guide based on cost/lifespan. Your batteries will last even longer if you only discharged them to 80% but you would need at least twice as many of them to provide the same power over the same period compared to discharging a smaller number of batteries to 50%.

*Amps (A) times Volts (V) = Power (Watts) (W)
or A * V = W

Thus (as they say in the maths textbooks)

W divided by V = A

Which can be written as W/V = A

Power consumption considerations can get a bit more complex when comparing the 12 volt DC with the 230 volts AC. This is why power consumption in DC is always considered in Watts but in AC because of phase shift the term VA or Volts times Amps is also used. This measures apparent power and may not actually be the same as the power consumed in Watts - but I don't think these considerations matter for a TV. They are of more relevance when considering say large AC motors.

Last edited: Feb 12, 2014
8. ### Phil56

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Thanks everyone. Additional thanks for not pointing out that there was a similar thread on a similar topic (I missed it - my wife found it!)

I think I am going to enjoy this motor homing lark......learning lots of new stuff (Well, revising lots of long-forgotten school stuff, anyway!)

Regards,

Phil