12v and inverter system

[Calbradtay]

Hi there
My current caravan currently only has ehu as a power source, but I've been thinking about adding a 12v system powered by a 120ah leisure battery (charged by a 250w solar panel) and then fitting a 3000w inverter with a fly lead attached to the ehu input on the caravan. Is this a reasonable idea or not? Anyone tried this before? Obviously I'd have the correct solar charger unit fitted and the battery or batteries would be located in a sealed unit away from the living areas. Any tips much appreciated. Cheers ......Rik

 

[Woodentop]

Ah, perpetual motion again eh?

If the inverter is taking power - and it is nothing like 100% efficient - then it will not be able to supply the needs of the caravan mains unit to recharge the battery.

A 3KW inverter will pull 3000/12 or something like 300A (allowing for inefficiency) from the battery which will kill it in about 15 mins. Now if you mean 300W then it will last a bit longer.

The inverter will only draw the current that its load requires - 300W (or whatever) is just its maximum capability. You would be well advised to get a pure sine wave inverter as the waveform shape produced by the cheaper 'modified sine wave' type will damage some kit.

 

[Damian]

To even think about your plan is silly.
For a start the cable size to take what could be a very large current flow would have to be huge.
Secondly a 120ah battery is very heavy on its own, never mind where you could put it.

A 250w solar panel will not produce anything like its stated output in this country even in the height of summer.

Lastly, there is nowhere "away from the living areas" in a caravan !!!

 

[ProfJohnL]

Hello Rik,

What you have suggested is not all bad, the idea of plugging the EHU into the inverter is fine, but don't forget to turn the battery charger off otherwise as Woodentop implies the you'll be trying the impossible. I agree with Wppdentop about going for a pure Sinewave inverter,

I think it might be wiser to look at the problem from the other side, - Work out what power you actually need, then see if its feasible to cover it with solar batteries and inverter.

It may help to think about being quite frugal with mains usage when on the inverter as Damian points out heavey mains usage will quickly deplete the charge in the battery.

 

[Calbradtay]

Thanks for the replies guys, it's just in the planning stages at the minute, just wanted opinions of people who may have experienced this set up. There will be no battery charger apart from the solar panels which would go through the relevant charge/controller. When I said away from the living area, I meant in a sealed locker so no gases from charging the battery would enter the living area. I thought theoretically this could be a good idea when no hook up was available ( which wouldn't be very often and certainly only for a day or two ) the figures of wattage and battery amps where only as a guide and I would welcome any suggestions as to what size inverter/battery etc would be needed. Cheers......Rik

 

[WoodlandsCamper]

Go back to first principles. What 240v appliances are you trying to power from a 250 watt solar panel (which will have losses in the system)?

 

[Calbradtay]

Probably just the lighting(I think it's all 12v and low wattage) maybe a small tv and charging of phones, possibly using the water pump and the fridge. Apart from the fridge which will be permanently on, the other appliances would be occasional and short term usage. Cheers......Rik

 

[WoodlandsCamper]

I don't think the fridge will run on 240v originated from a 250W source. It will need to run on gas.

 

[ProfJohnL]

Having read you more detailed requirements, I believe there is some realistic possibility of achieving what you're thinking of:-

If you are just running low wattage (preferably LED) lights, water pump, and occasional TV, then previously we have worked out that a good 50W solar panel coupled to fairly standard caravan 110Ah battery should be able to sustain the caravan quite well over several days, so a 250W panel should ace that part of the job. I can't stress how much you need to use a charge controller for the panel to protect the battery from overcharging.

The fridge poses a different set of issues. As has already been suggested you have the option of running it on gas. Don't under estimate this as it can be very effective and cost efficient, but you could also consider the 230Vac option:

Most caravan fridges are rated at about 125W of ac power. However and becasue the 230V element is thermostatically controlled, that means it will not be using that power 100% of the time if it were using power all the time that would be 3.0kWh. Fortunately Dometic have done some power testing and suggest the Energy consumption (230Vac) of typical touring caravan fridges is in the region of 2.0-2.2 kWh/24h. It's important to understand that the element has to work harder, when the ambient temperature is high, when the door is opened and how much heat items have that are put into the fridge, so the bulk of the power is required during daylight hours - when the 250W solar panel will be working and will take most if not all of the of the fridges electrical load. Its only at night when the ambient temperature are lower and the fridge will be opened less and kept shut for longer when the battery may be required to power it. Its only a rough estimate but the average power consumption during night time may be down to 50% or 60W/h

With such reduced night time power demand, coupled with a large solar panel it might be feasible to run the fridge on your rechargeable system.

If your only 230V requirement is the fridge, then as the fridge element is a resistive component, then you could use a 150W modified sine wave output inverter, which are cheaper, and more efficient than pure sine wave converters, BUT, if there is any possibility that the 230V power may be used for other items, then the pure sine wave output inverters are a must.

 

[Calbradtay]

Once again thanks for the replies, it is an education this caravanning !!! Originally I was going to fit a solar panel with charge controller and a 120 battery just to power lighting really, (which will be 12v led strip lights) but then I thought if I got a big enough inverter then I could run the ehu from it short term when rough caravanning with no hook up. I'll have to do some calculations and find out exactly what power I'm likely to need before I purchase the system. Once again thanks .......Rik

 

[rcpilot]

Look up the wattage of your fridge on 240v then if you run it from an inverter it will pull 20 times more amps out of your batteries (20 x 12v = 240v), traction batteries are the way to go I have 2 - 6volt Trojans coupled in series to give 12v they each have a capacity of 240A/H but the capacity is not doubled as they are connected in series, however the battery compartments have had to be modified as the batteries are higher than standard batteries.
So if you have a 120 A/h battery the fridge (on an inverter) will flatten this battery in no time, but you must remember never to run a leisure battery down to more than 50% of it's capacity.

 

[ProfJohnL]

Look up the wattage of your fridge on 240v then if you run it from an inverter it will pull 20 times more amps out of your batteries (20 x 12v = 240v), traction batteries are the way to go I have 2 - 6volt Trojans coupled in series to give 12v they each have a capacity of 240A/H but the capacity is not doubled as they are connected in series, however the battery compartments have had to be modified as the batteries are higher than standard batteries.
So if you have a 120 A/h battery the fridge (on an inverter) will flatten this battery in no time, but you must remember never to run a leisure battery down to more than 50% of it's capacity.

Four issues from your post:

Mains appliances are now specified at 230V not 240.

Inverters also require some power to make them work. Modified sine wave devices are more efficient at converting power, but the output wave form is full of nasty harmonics which can affect some appliances, so it's safer to use pure sine wave output, but this does impairs the efficiency, so a full sine wave inverter couple use a couple of amps of 12V over the powers needed by the load just to operate.

Leisure batteries were originally derived from traction batteries because of the likelihood of deep discharging cycles.

And finally can you define your "in no time" bearing in mind the 230V fridge element is thermostatically controlled?

 

[rcpilot]

I was ignoring internal requirements for simplicity, I have just looked at the power requirements for my fridge and running it from an inverter would use up aprox 10amps/hour (0.5amps on hookup) running a leisure battery down repeatedly to less than 50% of its capacity damages the battery(that's fact) so that battery would only last aprox 6 hours, so if you went out for the day you would return to a flat battery, the only consolation is that the inverter ( if it was a good one) would switch itself off before any damage to the battery occurred.
Also it is common knowledge that many companies were claiming their batteries were leisure batteries when in fact they were just ordinary starting batteries that has been proved and you cannot tell the difference just by looking at them, so if you get traction batteries from a good reliable source you are on to a winner and have just answered your own point 3

 

[ProfJohnL]

Hello RC,

With the latest Consumer Rights Act, if a battery is marked up as a leisure battery and it fails to perform, you would have a claim against the retailer.

The point about fridges running on 230V supplies is that the element is controlled through a thermal switch, so it doesn't pull power 100% of the time. It most likely only using power 50 to 70% of the time, so the ability of a battery to supply the necessary power to run a fridge will be extended.

It generally is not viable to run a caravan fridge on battery power, but given a big enough solar array and a battery of adequate size and performance it could be done. But its the size of the array and battery which is probably a compromise too far for most touring caravans.

 

[rcpilot]

I do believe it was one of the caravan clubs that brought to light about starter batteries being marked up as leisure batteries, greedy manufacturers, also 230v -240v is here nor there in fact a 230v appliance pulls more power than a 240v one unless it is one of these all singing all dancing models that conserve energy. The arrays as you stated would not be viable to run a fridge just imagine the size of the roof to fit them on not taking into consideration the weight and also where would you put the battery bank I only have space for 2 and again the weight, the towing vehicle, if the weight was allowed, would have to be a tractor unit off an articulated lorry.