Note: all prices are Australian dollars
This is not a comprehensive paper on remote power systems, but rather some notes on independent power systems I researched for when I go camping and other uses. You may be able to adapt my notes to your needs.
I've listed three exercises below that I've "recently" carried out in how I calculated my independent power needs. These are;
- CPAP #1 for Weekend Camping, and unreliable town power supplies.
- CPAP #2 for a weeks camping
- A Solar System For A Remote Laptop Computer
- Links to Further Information
CPAP #1 For Weekend Camping
Suddenly finding out that you have to spend every night sleep near a power source is at first an immense relief. Not because that is how it is presented, but rather because you suddenly found a accurate diagnosis as to why you were falling asleep all the time and you can now get back to a "normal" life. Rather, the realisation that nightly proximity to a power source is an ongoing future factor in your life dawns rather slowly. Eventually it becomes very frustrating, particularly when you lead a healthy outdoor life style that involved lots of bushwalking, bicycle touring and general camping. Eventually the frustration became such that I decided to investigate a way of overcoming this by locating a remote power source. This paper is about my investigations and how I ended up rolling my own solution.
Commercial Solutions exist. Generally they are orientated to installing additional batteries and solar panels into caravans and campers. Unfortunately there is not a clear category in theYellow Pages, so you might want to check battery suppliers and maybe caravan manufacturers. Either of these should be able to put you in touch with a local installer. Apparently there are quite a few retired people doing the great trek around Australia with modified campers and caravans. However, in 1999, I found this a tad expensive at around $3,000. The fact that we were considering the replacement of our van in a couple of years was also a consideration. The extra time allowed me to look further at a D-I-Y solution.
D-I-Y solutions usualy involve a choice of;
- Generator - noisy, smelly, long power cords, doens't run all night, expensive.
- Solar Power - errr, totally useless without batteries and it requires a lot of them, very expensive.
- Batteries - bing!, sounds great, how long will they last, how much, heavy blighters.
Okay, the above was a very simplistic evaluation, but by the time you've read the whole of this page, you will see I've covered the evaluation in greater details. For this example (going for a weekend camp), I've decided to aim at a battery based system that can be charged up at home, taken away and used, then brought back and recharged.
The Batteries No, you can not use car batteries. You have to buy proper deep discharge batteries. Warning, these are sometimes called "marine" batteries, but not all batteries sold as "marine" batteries are deep discharge. If you want to know why you need deep discharge batteries, then read the FAQ on Lead Accid Batteries.
Some users of CPAP ( and computers, etc) may be able to use lighter power sources, like SLA (Sealed Lead Acid), NiCads (Nickel-Cadium) or NiMH (Nickel-Metal Hybrids), but these are generally far more expensive.
The bottom line is that deep discharge batteries are designed to be run down and re-charged, can handle the discharge rates and are the cheapest engery storage system available. The other side is that they are rather heavy. I currently have a Trojan .
Power Requirements The first thing to work out is exactly how much power you really needs. The jargon is Amp Hours, which means how many Amps does your machinery require for how many Hours. In my case, it was 40 AmpHrs at 12Volts, but first I will show you how I worked that out.
The machine I needed to run was a Sullican 2 CPAP and the power requirements plate said that this was a 110-240V AC 80VA system. The 110-240V just tells me that it will run on voltages between 110 V and 240V as it has a switchmode power supply. In Australia, our supply is 240V. It is the 80VA that is important. VA is an abbreviation for VoltAmps. So at 240 Volts, this machine requires1 /3 Amp (or 333mA, Note 1,000 milliamps equals 1 Amp).
However, our remote power supply is a 12 Volt battery, so 80/12 = 6 &2/ 3 Amps. So, if I sleep for 8 hours per night, I require 54 AmpHours of power per night.
Looking at the available deep discharge batteries, there is the Trojan 130AmpHr for $265(2001)($290 2002) or the Century 120AmpHr for $200 (2002). Either of these batteries would give me two nights power. So I could leave Friday afternoon/evening and camp out for Friday and Saturday nights before they needed a recharge.
The Inverter My CPAP is a 240Volts AC machine. To get 240 Volts AC from a 12 Volt DC battery, I need something called an Inverter. The first thing is that these come in various sizes, so get one that will cover your needs (just). In this case, you want one that just exceeds the Wattage (the VA figure - not strictly true but good enough). Do not buy a massively oversized inverter as the inverter itself uses a set amount of electricity, which increases with capacity. Hint, only connect your inverter to the battery when in use. Connect your inverter first, switch it on, then switch on your machine last. Reverse this proceedure for turning off. Hint 2; good inverters come with fuses, make sure you have spares.
Pure Sine Wave Inverters The second point to watch is whether your machine requires a pure sine wave power source. Your mains power supply is a pure sine wave and some generators are a pure sine wave, but most inverters are not. The cheap inverters are just a square wave. The better quality inverters are a stepped square wave. If in doubt as to what sort of inverter you need, ask your equipment supplier and ask the supplier of the inverter as well about what sort of machinery they will run. Most will run light motors, such as in a CPAP without any trouble.
For a CPAP, most of these do not require pure sine wave inverters, unless they have an inbuilt humidifier. It is the triacs that control the heaters in the humidifier that can catch fire. The upshot of this is that I can use my Sullivan 2 CPAP whilst camping, but not my Sullivan HC100 humidifier (which doesn't have a switch mode power supply and can not run on 110V supplies).
My First Inverter was a Powertech 60 watt model fromJaycar. It cost $88 (Nov 2001) and performed well on it one and only used. Okay, by now some of you will have twigged that I was talking about needing 80Watts/VA above, but only brought a 60 watt inverter. I interpret the 80 Watts to be the maximum power requirements of the CPAP, which would be at a pressure setting of 20. However, my setting is below that and I was able to measure the actual power consumption of my CPAP at my pressure setting. This turned out to be below 50 watts on the 240 volt supply. So I ordered a 60 watt inverter. When it arrived, the first thing I did was to plug it all together and take the current measurement off the battery. This gave me a new nightly power requirement of less then 40amphours per night, which means I can sleep for three nights (Friday, Saturday and Sunday) off my battery before needing a recharge.
Recharging The Battery As my set up is a mobile set up and I wanted it to be independant of a vehicle, I did not investigate vehicle recharging of the battery(ies). Most of my camping tends to be base camping, rather than travelling around, but the commercially installers can install your batteries and isolator into your caravan or camper and make sure that the vehicle alternator is sufficent to supply the recharging current during a days travel. Hint - make sure you discuss this point with them on installation.
For my purposes, the battery is brought home and placed onto a specially selected battery recharger. In my case, the charger was an economical brand and model (about $60). The seller of my deep discharge battery put me onto the source (Big W). It was a standard car battery charger with an AUTOMATIC mode. This means it charges the battery until it detects that the battery is fully charged, then switches to trickle mode which stops the battery being over charged. Standard car battery chargers have normal or boost modes, bothof which continue charging the battery until switched off. Overcharging can destroy your battery.
At least this was the theory. As I was later to find out, if you leave a battery connected to the charger in standby mode, it eventually boils off all the water in the battery and destroys your battery. So do not be tempted to leave your attery permanently connected to the charger.
Commercial Battery Chargers When I first started asking about deep discharge battery systems, I was being quoted figures around $500 for a battery charger. The reason these are so expensive is tha they are often FAST rechargers designed to pump power back into the battery as soon as possible. In applications like golf buggies (what an oxymoron), fork lift (every home should have one) and similar, these are quiet possibly essential. However, fast recharging eventually ruins the battery and I found it hard to justify paying double for the recharger. A safe battery charging rate is less than C/10, where C is the amphour capacity of your cattery.
Battery Recharge Time The catch with using this economical battery charger is that it takes longer to recharge the battery (at 4 amps) than I get use the battery. My inverter uses 5 amps. This is not a problem as at the most I would be going camping is every weekend, but then have the working week (Monday to Friday) to recharge my battery.
My Total Cost My total cost for this has been $405 (battery; $265, charger; $60 and inverter; $80)
Unreliable Town Power At the moment, we currently experience a major power outage (overnight) about once a year, so my battery sits on its charger in automatic mode most of the time, unless camping of course, and I only need to carry it into the house occassionally. Hint, buy a little cheap trolley for this. If your town supply is so unreliable that you need the battery and charger beside your bed regularly, make sure your room is well ventilated to remove battery charge and discharge gases (really only a likely problem at heavy rates and in closed up rooms).
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CPAP #2 For a Weeks Camp
When researching the requirements of taking CPAP Number One camping, it was decided to update and buy a second CPAP should things go wrong whilst camping (i.e the power system dies and the CPAP becomes non functional). Unfortunately, the second CPAP was purchased and in use before the first was tested in the camping power situation (a tent in the back yard actually). The edict was handed down to develop a system to take the quieter, second CPAP camping and keep the noisy first CPAP in reserve.
We were also considering the requirements of going camping for an annual holiday of some length. So the requirements of this system were expanded from the above system. The description here will be simpler as I will assume you have read example one. The requirements were to be able to camp for a week using the new CPAP.
CPAP #2's Power Requirements At this stage, CPAP #2's Power requirements are massive because it is not possible to obtain a definite figure from either the manufacturer (i.e. they have no idea) or to measure it in use (as it is actually an APAP and power requirements vary). Resmed, the manufacturer says that my Autoset Spirit requires a maximum of 120 watts, without the internal humidifer. Note, they specifically warn against using the internal humidifier whilst using an inverter, so you need to remove this from the Autoset when using it on an inverter.
CPAP #2's Battery Requirments 120 watts is 10 amps from a 12 volts battery, which means 80 amphours per night. This means that I can get only one night from a battery (130Amphours capacity), or to put it another way, two nights of 6.5 hours sleep per night. Obviously I was going to need another battery at least. Warning, if you intend to connect multiple batteries together, make sure they are the same brand, same capacity and same age and have had similar history of use otherwise you will not get best performance.
Inverter #2 Accordingly, I ordered another inverter. This time it was a 140 watt inverter, which retails at $110. A few tests on arrival revealed that it consumed 0.5 amps in standby mode, which would rob me of an hour of sleep if it was left connected all day. So remember to only connect your inverter to the battery when needed, then switch the inverter on, then switch your machine on. Do not switch machinery off and on by connecting or disconnecting the inverter.
The good news was that power consumption on open mask was well below the expected 120 watts and was only slightly above CPAP#1. Given that it was newer machinery, it could be expected to require less, but this machine has internal processor (computer in other words) and it's motor is driven in surges, so being just slightly above was good as this means I could have two full nights of sleep on one battery. That was a big sigh of relief.
As I write this, I've misplaced the figures, but will plug them in when I find them again (filed too safely {:-).
Todate - we have recorded one session of three short nights sleep on the Tojan 130 amphour battery with inverter number 2. They were shortish nights as we were helping with a platypus survey and had to be up at 3:30am to bush walk to the observation place in time for dawn.
Keep a History of Your Battery(ies) It is important to point out that you should keep a written log of the use of your batteries and clearly indentify all equipment. This will allow you to pin point faulty equipment, such as chargers, or missused equipment. It will also allow you to know when to replace your batteries, which can be important if your health depends on them.
Buy a Multimeter A digital one to give battery voltage starts at $10 and by using one to measure the battery voltage each time you use it, you will understand better the state of health of your battery. If you read the deep cycle battery FAQ, you will learn that your battery is considered 100% discharged when the voltage reads 11.89 volts. With a little bit of experience, you will be able to determine how full, (or empty) are your batteries and whether you need to recharge or not. If you watch the figures, you will be able to detech when your battery is starting to die and needs to be replaced.
Do Not Loan Your Power Supply A word of advice, do not loan any part of your power supply to anyone. That means family, friends, relations, neighbours, etc. This means not offering your nicely topped up deep discharge battery to a neighbour with a flat car battery. Also, do not use your battery charger to charge any other battery. In my case, my health and well being depends on this system being in good working condition in a disaster situation. Using deep discharge batteries in a car means that they will be discharged above a safe rate and thus have their life shortened (at best). I actually have a spare car battery on a third charger to loan to neighbours and for use when I connect a trailer to my vehicle (its electrics will not handle the trailer electrics).
Interim Setup After our trial, we thought the likely final setup for a weeks camping will be two Trojan 130Amphr batteries, two battery chargers and one generator. Each battery can be relied upon to give two full nights of sleep, perhaps five full nights if I gang both batteries together (requires a pair of heavy cables). For a longer period, we have decided upon having a generator and I will discuss this below.
After four nights, the plan is to fill up the generator, start it up, then connect the two battery chargers with each doing its own battery. I should thus need to run the generator during the day, say 10 hours maximum (you need to put more in than you get back - it is in the FAQ linked above). This means we have one noisy day, with peace and quiet for the other three. Depending on the particular generator, I may need to refill it about midday.
Why A Generator? The short answer is because it is the most economical and most versatile.
Solar panels are expensive, being around $1,000 each. My intial calculations showed that I would need the biggest panel. A stationary solar panels only returns half the capacity (on average), so we would need a second panel. Solar panels work well on bright sunny days, but our camping would not be restricted to bright sunny days, much as we would like them to all be that. So we would probably need to have a third panel for overcast days. In addition, all the good guides for solar power systems say that you should have up a weeks power reserve in the batteries. Hmm, three panels at $1,000 each means we could purchase another eight batteries (and chargers) with the same money and this would mean twenty days capacity anyway. Plus, solar panels are large, which means special handling. If the idea of having a special trailer for camping (it holds all the gear you take away) comes off, we will probably build a couple of panels into the roof as an experiment in any case.
Which Generator Having arrived at the decision to buy a generator, the next question was which brand and model?
Here, I would suggest you use the Yellow Pages again and start to ask questions. The first piece of sound advice I was given was to avoid two stroke generators as too noisy. Some brands have small models especially quietened for camping and any of these would probably suffice. Generators are like inverters, that is best run at close to full load as possible for maximum efficency. Unfortunately, most start at 750 Watts, which is way over what I need (two chargers outputting 60 watts max each). They also start around $1,000 and rapidly climb.
Some generators have a 12 volt DC output for charging batteries. This may or may not be useful. If you plan to stay around camp and keep an eye on the generator and battery state whilst using this option, them it may be useful. Just make sure that you do not exceed the C/10 charging rate recommendation, otherwise you will shortern the life of your battery. Read the deep cycle battery FAQ if you do not know what "C/10" means.
However, as I do not plan to sit around attending to the generator whilst it does its work, this isn't an important option for me. To my way of thinking it is equivalent to using a standard battery charger that will go on pumping 10 amps into the battery even when it is full. It would be a nice thing to have for an emergency (flat car battery even), but not a reason to buy a generator. I've also yet to sort out if I was the victim of some sales FUD here about how these generators really work as it was suggested that generators that give 12V DC out all contain their own inverters to give the 240VAC.
Alternate Fuels As I dislike petrol for its explosive qualities and my wife is upset by the fumes, I wanted to consider alternate fuels. Somethinking about bio-diesel, etc. Unfortunately the big generators are diesel and diesel generators don't come at anything less that that suitable for powering a small town, or similar.
LPG was the next consideration, as we have four large cylinders in our camping gear already. Unfortunately, no one makes a LPG generator, except I understand they were a fad sometime ago in the USA, but that is a bit far for shipping. The best I could do was to buy a petrol generator and pay $450 to have it converted to LPG.
So, at the moment, we will buy a standard petrol generator, with the long term plan of hacking a smaller petrol/lpg motor to a small generator.
As far as brands go, the the obvious decisions was a Dunlite, which is a well known, Australian brand, but they are relatively expensive and located in South Australia. In their favour, they do have a model with a big fuel tank that would not require topping up at midday, which also means that I could run it all night if I really needed to.
The next consideration was Advanced Power, who are located nearby in Sydney. Again, they were of the same price range as Dunlite and the only one that offered LPG conversion. I have also found one model that isn't listed in most catalogues that that is reasonably priced and looks to have a large fuel tank, but has a Briggs and Stratton engine. Unfortunately B&S engines have a reputation from my experience of being rather tempremental. I'm still considering it.
There are various models of overseas extraction brought in by various importers. Whilst some of these have brand names that are considered reliable, quite a few also make rubbish power tools, so I'm a bit dubious about spending any money on one of these. They are also typically one or two hundred dollars cheaper that the Australian brands. My main concern is servicing should they have a fault.
Last consideration is the cheapie chinese Tiger brand. At $460, almost half price some others. Dare I take the risk?.
I will come back and update this page with experiences.
How Long To Recharge My Batteries At this point, I experienced a big "oh-oh". Just how long will my little Arlec-6 battery chargers take to recharge the batteries? On automatic mode, they seem to put back 5 Amps into the battery. This means to recharge a 130AmpHour battery, they will take 26 hours, which is obvioulsy not going to work because there is no way we are going to run a generator that long.
One solution to test will be to run the battery charger in normal mode and see what current it puts back into the batteries. However, it is more likely that I will have to purchase a bigger battery charger to fast charge each battery in turn. Applying the C/10 rule, that means I can use a battery charger that will put up to 13Amps into the battery, but this is still going to take 10 hours plus and that only recharges one battery.
If you are ahead of me, you will have realised that I am basically back to recharging a battery every second day. So there is no need to have a second battery, but I will for backup purposes.
The Final Setup #2 At this stage, the final setup will consist of two X 130 Amphour deep discharge batteries, two or three battery chargers and a generator. Basically, there is two of everything, except for the generator, which if it failes, then we just have to packup and move to an alternate power source (cabin, caravan park, etc).
Final Cost #2 This is current estimate of the final cost for setup #2.
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Number Item Cost 2 Trojan 130AmpHr deep discharge batteries@$290 each. $580 2 Arlec-6 Battery Chargers@$90 each $180 1 Bigger Blighter Battery Charger $150? 1 Multimeter $20 1 Generator 2.2KVA $Max $1,250 Total Cost $2,180
A Solar System For A Remote Laptop Computer
To not put too fine a point on it, this exercise came from a proposal to power an old computer that was going to be located on a roof top as part of a Community Computer Wireless Network. The proposed suggested that all that was needed was a little solar panel and a nicad battery. Yep, right, pull the other ones, it has bells on it.
My response was to post an explanation as to why that was so much fair floss {:-) by running through similar calculations to the above.
Using An Old Computer is definitely not on. Most old computers have a 250 watt power supply for very good reasons. Even if you were to replace the 240V part and run them directly off a 12V source, 250 watts is still 20+ amps, which involves some very hefty battery banks before we get into solar panel consideration. Looking at something more realistic.
Using An Old Laptop Computer is feasable, but still requires some hefty gear. For this example, I am going to use an old Toshiba 2400CT. The bottom panel says it needs 18 volts DC at 1.1 amps. However, if you pop the battery, you can see that it is a 12 volt battery. This makes it easier as it is a lot easier to run directly from a 12 volt battery, than to have two 12 volt batteries in series to provide 24 volts, which you then regulate down to 18 volts and waste a lot of power doing so.
One Day's Power is 1.1 * 24 = 26.4 amp hours at 12 volts. So, you need a solar panel system that can generate 29 amphours per day (26.6 * 110% as you need to put more in than you get out).
Allowing For Short Days means that calculations have to be based on the shortest day of the year, which is June 22nd in Australia, for which you have sunrise in Sydney is at 6:58am and Sunset in Sydney is at 4:56pm, so we have 9 hours and 58minutes to generate 29 amphours, which is 2.91 amps at 12 volts, or 35 watts.
Allow 50% efficency for a stationary panel. Unless your panel is on a solar tracker, the rough rule of thumb is that it is only 50% efficent. So you are going to have to buy a 80 watt solar panel, which costs $800.
Surviving During Bad Weather The above gives you a very basic system, one battery and one solar panel with regulator which will carry you for four days (130Amphr/29 amphr/day) in the event of bad weather. This is not enough capacity. The normal recommendation is to have capacity in the battery bank for at least a week. So we are going to need two batteries, whcih will give us eight days capacity.
Recovering From Bad Weather The next problem is how fast the system recovers from a run of bad weather. In the above system, there is only the capacity to recover 4.1 Amphours per day, which would take 49 days to recover after a week of bad weather. Not good. So you are going to need a second solar panel, which would take about 8.1 days to recover from a week of bad weather to recover. Would you risk it? or would you consider a third panel? That is matter that only experience can tell you, or perhaps some weather pattern research.
System Costing For a system with three solar panels (at $800 each), plus panel regulators (at $90 each), plus two deep discharge batteries (at $295 each), your basic cost is $3,260.
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Links to Further Information
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- CAR BATTERY & DEEP CYCLE BATTERY FAQs
- Battery Operation of CPAP Devices for Sleep Apnea Treatment, Part 1--Why bother with a back-up system?
- Battery Operation of CPAP Devices for Sleep Apnea Treatment, Part 2: A battery backup system for use with CPAP treatment
- Battery Operation of CPAP Devices for Sleep Apnea Treatment, Part 3--Very Technical Notes
- Battery Operation of CPAP Devices for Sleep Apnea Treatment, Part 4--Safety Warnings