Advice on solar panel size please

Folks, I am about to enter the world of solar powered camping and I would be grateful for some sage advice from those who know better. What is the "ideal" wattage for a folding solar panel supplying the following load:

Waeco CF50 fridge. (I am told to allow 30 Ah)
12V fluro camp light.
75W CPAP machine supplied from a 300W full sine wave inverter via a 5M 240V extension lead. The CPAP machine has no DC capability.

My 4WD has a 120Ah deep cycle lead acid battery installed in the rear. I am able to mount the regulator and inverter adjacent to the battery and I am able to access the battery readily. The battery is currently managed by an Opposite Lock dual battery system. I understand that the solar contribution is subject to the variations in solar exposure and I am prepared to "sun track" the panels without being obsessive about it.
Thanks for the advice in advance.

Russell
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Reply By: Member - Rosco from way back - Friday, Mar 07, 2014 at 12:27

Friday, Mar 07, 2014 at 12:27
G'day Russell

Seems to me your problem goes way beyond panel size. Without doing the sums I think you'll find the battery won't go the distance running both the fridge and CPAP at night. Added together we're looking at somewhere around 80W after making allowance for losses etc. You'll likely be in strife after a couple of hours I think.

That CPAP is quite power hungry.
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Follow Up By: Charlie B2 - Friday, Mar 07, 2014 at 13:49

Friday, Mar 07, 2014 at 13:49
Hi Russell,

Have to agree with Rosco, although I make no claim to be an expert.

It looks to me that if you were only running the CPAP for 8 hours a night, that seems like 600 watts (50 amps at 12 volts) you'll be dragging out of the battery bank overnight, even without allowing for losses through the system and inverter! How big did you say your battery was?

You'd need a battery system that will allow you to draw out those 50+ amps, plus whatever else you use for the other electrical items over the 24 hour period and still not fall below, say, a 50% state of charge (OK, experts, preferably even less) - and a solar system (or some other form of battery charging) that will let you replenish the power you've used, ready for the next round of use! Don't think running out of power during the night would be your preferred choice!

Regards,


Charlie
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Follow Up By: Member - John and Val - Friday, Mar 07, 2014 at 13:59

Friday, Mar 07, 2014 at 13:59
Err..... Charlie....

The 75W CPAP will require about 6 or 7 amps at 12 volts (i.e.75/12), not 50 amps! Add in an allowance for inefficiencies and it is still quite a heavy load, but not as bad as you suggest!

Cheers

John
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Follow Up By: Charlie B2 - Tuesday, Mar 11, 2014 at 07:41

Tuesday, Mar 11, 2014 at 07:41
Hi John & Val,

Please don't think I'm arguing with the technical experts on this site - you guys know more about 12 volt stuff than I ever will - but, while I might have used "amps" instead of ah, doesn't your post 2 of 8 below support the actual number I used ? - ie 50 amps drawn from the battery bank over the 8 hour period of using the CPAP?

I most certainly wasn't trying to suggest the CPAP would be drawing 50 amps for the whole period of use! A lot of older alternators would struggle to keep up with putting that back into a battery!

All I was trying to do was make the same point you've made so much more eloquently, below - the OP's batteries are a bit on the short side for capacity.

Regards,

Charlie
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Follow Up By: Member - John and Val - Tuesday, Mar 11, 2014 at 08:40

Tuesday, Mar 11, 2014 at 08:40
Hi Charlie,

I wasn't throwing stones! Far too often electrical posts bring out the more aggressive types to be found on this forum and they will start a brawl that leads others who could contribute to simply keep their heads down - I do not participate in brawls and tread carefully so as not to start one!

You used incorrect units and suggested that the CPAP would draw 50A. No! Amps are a measure of current flow, the RATE at which energy is being delivered. The total delivered is this rate multiplied by time, hence Ah. The machine will draw about 6A, not 50A, and will do so for say 8 hours, which leads us to 6 x 8 = about 50 Ah.

Yes you got the right number and drew the right conclusion about battery capacity but by my lighthearted response I was hoping to correct the path you took to get there!

Cheers

John
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Reply By: Member - John and Val - Friday, Mar 07, 2014 at 13:52

Friday, Mar 07, 2014 at 13:52
Hi Russel,

As Rosco suggests, I think you have a battery capacity problem.

I agree with your 30Ah per day for the fridge.

CPAP: If you can do without the humidifier (especially without a heated tube system) the demand will be much less, but running a nominal 75W CPAP through an inverter will probably call for up to 50Ah per night, more than half being for the humidifier.

Add in some lighting and charging cameras, ipods, torches etc, say a total of 5 to 10 Ah, and your daily energy requirement is 80 - 90 Ah. It isn't good for your 120Ah battery to have to deliver that amount - generally, batteries should not be repeatedly discharged down below their 1/3 of capacity point ( i.e. try to avoid taking more than 80 Ah from your 120 Ah battery.) I'd suggest, unless you can get by without the heated humidifier, that you consider doubling up on your battery.

How much solar do you need? Depends on your travel and usage patterns. To keep up with the above demand you need to be able to push close to 100 Ah per day into your battery/s. When travelling some of this could come from your alternator. It may be worth considering a dc-dc charger to assist in this. (Mentioning a dc-dc charger here may start the usual vitriolic argument. They do have their place though and your's may be one of them!) Regardless, it is very desirable that you use heavy cables running from front to back to minimise voltage loss.

Each 100W of solar capacity can deliver to the battery/s a maximum of about 6 amps if using a simple PWM controller, or 7 amps with a MPPT controller. If we assume good full strength sunlight for say 6 hours per day, and keep the panels reasonably close to normal incidence (its a sine function so no need to be too accurate) we could harvest a maximum of about 35-40 Ah per day from each 100W of solar capacity.

On this basis, on the perfect sunny day 200W of solar would go close to meeting your daily demand. Add some charge from the alternator, double your battery capacity to provide some flexibility and 200W looks a good size for your purposes. If you were to run the CPAP without the humidifier this would give you a handy buffer.

HTH

Cheers

John
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Follow Up By: Russell L - Friday, Mar 07, 2014 at 14:09

Friday, Mar 07, 2014 at 14:09
I feared as much..... as for CPAP consumption, the 75w figure I am going on is what the book says for load in flight mode, that is, without humidifier and without hose heating. Sigh......
I am currently setting up a trial run outside my bedroom. I have my 120Ah battery out of the car, I have my CF50 fridge turned on, I have my inverter set up, I have my CPAP (in flight mode) plugged in, I have a normal battery charger taking the place of solar panels on to fully charge my battery by 18:00 or thereabouts. My cunning plan is to turn the battery charger off at 18:00, measure battery volts, leave the fridge on about 2 and run my CPAP as normal tonight. So.....if you hear a gasping sound from N.E Victoria around midnight or so, you will know that it has been a total failure.
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Follow Up By: Member - John and Val - Friday, Mar 07, 2014 at 14:57

Friday, Mar 07, 2014 at 14:57
Hi Russel,

Worse than I thought. I'm currently going through the preCPAP process, trying machines and masks (and developing an intense hatred of all masks!)

The only realistic figures I've come across are for the Philips machine that uses a 60W power supply to cover compressor and humidifier, and a 90W one with heated tube. (This one is also native 12V which will solve problems.) On this basis I suspect that the compressor probably doesn't draw more than 25-30 watts. Hopefully your 75W is a conservative overstatement.

I'll listen for you tonight - I'm bound to be awake adjusting a mask to overcome leakage! If you hear cussing from near Canberra......

Good luck with your experiment. Suggest taking a voltage reading a few minutes after starting CPAP and again just before stopping the machine. (This will ensure that voltages are stabilised.) Unfortunately the relationship between battery voltage and state of charge isn't well defined, but if your "after" reading is much less than 11 volts it will indicate the battery state of charge is well down. Anything less than 10.5V will indicate end of the line and recharging is urgent.

Please let us know how you get on. (Also, for my sake, which machine are you using?)

Cheers

John

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Follow Up By: Russell L - Friday, Mar 07, 2014 at 16:29

Friday, Mar 07, 2014 at 16:29
Thanks for your support John. My machine (grrrr grrrrr grrr) is a Fisher and Paykel Nova auto so it has all the bells and whistles which is fine until you want to go camping and use solar power. Oh don't get me wrong, it's a great machine. I just hate having to rely on CPAP to keep me alive at night. My CPAP dude has told me about a product designed for portability. Transcend. take a look but at $1200 plus for the basic starter package it's a bit rich for my blood.
WRT the after measurement, I fully understand what you are saying. The big test will be if my beer is still cold in the morning and the fridge cycles normally when the CPAP machine is turned off. My next phase of the experiment is to get my 120Ah 3 or 4 year old battery tested. Perhaps it needs replacing. Look, I'm talking about a failed test already.
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Follow Up By: Russell L - Thursday, Mar 13, 2014 at 20:06

Thursday, Mar 13, 2014 at 20:06
Well folks, I haven't been ignoring you. I have been very busy conducting tests using a friend's set of 150W panels and the math is in! It seems that the 75W figure as quoted in the CPAP manual for flight mode is, shall we say, not as accurate as it could be.

OK here goes.... Fisher and Paykel Nova machine set to "flight mode" (all humidifier and hose heating off)
150W inverter.
150W solar panels.
120 Ah battery
I measured the DC load going into the inverter with nothing plugged into the inverter but the switch ON. = 360mA @ 11.78 Vdc
Next test, inverter plus CPAP machine on in flight mode, CPAP pressure at 4cmH2O =1.5AmpsDC @11.7Vdc.
So, if my math is correct, this should give me a power consumption figure of 17.55W
A far cry from the 75W quoted in the manual.

So, I'm back to 30 Ah for my Waeco CF50, 11.7Ah for 8 hrs CPAP running plus 6Ah or so for camp lights etc. A tick under 50Ah.

Given an ideal solar gathering day and a healthy 120Ah battery, I reckon I should be able to get away with the 150W panel set?

Thoughts?


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Follow Up By: Member - John and Val - Saturday, Mar 15, 2014 at 10:35

Saturday, Mar 15, 2014 at 10:35
Hi Russel,

Good move, doing the actual test. Your numbers are much more what I'd expect than F&P's 75W. I have some lingering doubts though. I'm guessing that your test was with the machine running simply into the mask and the mask not in use. This static load, especially at minimum pressure, would not be typical of the dynamic load which applies when the little turbine is busy following your respiration pattern. If my guess about your test conditions is right, then I'd suggest doubling you calculated consumption figures to get a conservative maximum. (Are you really running at only 4 cm overnight? I'm 3 times that! Energy requirements will rise as roughly half the square of the pressure. Perhaps that's how F&P manage to get up to 75W at the machine's maximum pressure.)

Another concern with your numbers is the conversion to watts. Your calculation is correct of course, but from the battery's perspective it's the amps that count, and by using the reduced terminal voltage in your sums to convert to watts your result may be up to 10% low.

Putting all that together, I think you are looking at about 60Ah per day, rather than your 50Ah. You might almost get away with 150W of panels plus some help from the alternator, but my personal choice would be 200W (or pretty close to it) of panels plus twice the battery capacity.

Cheers

John
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Follow Up By: Russell L - Saturday, Mar 15, 2014 at 17:06

Saturday, Mar 15, 2014 at 17:06
Yes John, mask was on and yes, I'm 4cmH2o (that's the base value anyway, but I have looked at my trends and I'm around that most of the night except when I pop up above 6 very briefly during full apnoea events at the moment). Based on the tests and my propensity to err on the side of "better to have than want", I have ordered a set of 220w panels. I am coupling them with a CTEK 250S charger to my 120Ah battery. I actually ran my CPAP machine in flight mode for 2 nights last week directly from the battery for a total of 16 hrs and it held up pretty well, despite an overcast day and little effort on my part in sun tracking. Just 150w panels in a general Northerly aspect.

I have more hope that my holiday in April will be a success sleep wise anyway.
Thanks for the follow-up.

Russell.
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Reply By: Allan B (Member, SunCoast) - Friday, Mar 07, 2014 at 18:26

Friday, Mar 07, 2014 at 18:26
Hi Russell,

Thankfully, unlike yourself and John, I have no need for CPAP....... yet!
It is bad enough to have to support a CPAP in the 'field' without all the issues of battery supply and charging, what with overcast weather and wishing to 'stay-put' and lose the benefit of the car alternator charge.

So, have you considered the alternative of a small petrol driven 230v generator? I have encountered several caravaners who run their CPAP machines this way. It could eliminate a lot of hassles but I can understand if you have an aversion to motor gennies.

Cheers
Allan

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Follow Up By: Russell L - Friday, Mar 07, 2014 at 18:45

Friday, Mar 07, 2014 at 18:45
Allan, you understand right. If I have to do the generator thing, I will stay at home and watch all you good folk enjoying youself.
If it comes down to it, I will set up a 220w set of panels and a larger deep cycle battery that will be enough for JUST my CPAP. It will mean going back to ice and perhaps limiting my stay somewhere based on my refrigeration abilty with a good esky. It will be better than nothing.
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Follow Up By: Member - Rosco from way back - Friday, Mar 07, 2014 at 20:25

Friday, Mar 07, 2014 at 20:25
G'day Allan

In you travels have you also noticed the propensity for said caravaners to locate their gensets comfortably out of their hearing range (usually with the exhaust diametrically located in relation to their van), without much in the way of consideration to anyone else within 4 kilometres ........... grrrr, grrrrrrrrr

;o))
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Follow Up By: Allan B (Member, SunCoast) - Friday, Mar 07, 2014 at 22:19

Friday, Mar 07, 2014 at 22:19
Err yes Rosco, I have encountered that........ one kept Roz awake most of the night. I was very tempted to get the side cutters out!

But I was trying to remain positive in my reply to Russell. As it happens, he is not going that way, so we can breathe easy. LOL, pun intended. Sorry Russell and John.

Cheers
Allan

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Reply By: Member - Grundle (WA) - Friday, Mar 07, 2014 at 19:30

Friday, Mar 07, 2014 at 19:30
If i see you out there Russell and you need a cold beer i will give you one.

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Reply By: The Bantam - Friday, Mar 07, 2014 at 23:30

Friday, Mar 07, 2014 at 23:30
Yeh I think all considered, you need to get that battery capacity up....there realy is no way arround it......120 Ah just does not go far..especially if you expect decent service life from your batteries......ya gota be talking 200Ah pluss to be comfortable

Sticking my thumb in the air...your worst case demand could be arround 100Ah per night

as far as the charging capacity...well that depends on how far you plan on traveling in a day and how long you plan to be in one place off grid.

A generous solar array would be best for sure.....but even a solar array that wont keep up with the demand will considerably lengthen your stay time between drives or shorten the necessary drive time to get a full charge back.

If you want to keep up wth demand well you'd have to be talking 160watts of pannels to be even in the race....200 to 300 watts would be more comfortable...particularly traveling in winter at low latitudes.

OH that 12 volt fluro...ditch it in favour of LED.

cheers
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Reply By: Member - Nolo (Brisbane) - Saturday, Mar 08, 2014 at 08:36

Saturday, Mar 08, 2014 at 08:36
I am on to my second machine now. The first was a 240v only unit and when run via the inverter it was hungry on battery, necessitating a second battery install. However the latest (DeVilbliss brand) has 12v and 240v power input. Without the humidifier it is very frugal on 12v power and when tent camping I just use a 22ah sealed battery overnight in the tent with the unit. Alternatively I run an anderson plug extension lead from towbar 12v output plug to tent and draw from the vehicle setup. I have used the 22ah battery for two nights without charge on a couple of occasions as well and not discharged below 50% soc. Daily charge is usually via the vehicle solar setup or if travelling, just connected within the vehicle charging system.

Sorry, but in CPAP world, 12v is the go. Otherwise you need to compensate with extra battery / solar as others have covered.

Greg N
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Reply By: Echucan Bob - Saturday, Mar 08, 2014 at 09:47

Saturday, Mar 08, 2014 at 09:47
Russell, I estimate that to supply 75 W your inverter might consume 120 W so draw 10 A for 8 hrs. With a 110 Ah battery thats a big percentage discharge every night. Maybe you need another battery in parallel to deal with such a big load. I'd look at a 250W panel as they are cheap because of volume - just bought a 40 panel system with good inverter and installation for $10k so panels are less than $200.
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Reply By: Lazybugger - Saturday, Mar 08, 2014 at 18:22

Saturday, Mar 08, 2014 at 18:22
Hi Russell,

I've got a resmed S9 and am actually on a trip at present. I am using the 12v adaptor. According to the Arkpak I used less than 5% of the 95a/h agm battery last night.

I recommend you consider sourcing a cpap with 12v capabiliies from the US. You will find the net price better than Aussie prices less private health rebates. I used secondwindcpap.com for mine.
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