Tuesday, May 07, 2013 at 15:46
Joel,
re battery sizing - your figuring looks right, BUT appears to assume that your 240W panels will deliver 240W and your 100 Ah battery will deliver 100 Ah. Unfortunately, those critical assumptions are wrong.
240W of panels will not deliver 240W to your batteries, which is what you'd need to get 100 Ah in 4-5 hours.
1)Panels are rated at their maximum capacity (usually at 25 deg C) under full sunlight (actually 1kW/square metre) at normal incidence, and when delivering current at their optimum voltage. These conditions are rarely achieved due to sub-optimal temperature and orientation, shading, atmospheric dust, clouds, latitude........ Averaged over a year 240W (nominal) will probably supply the equivalent of about 80 Ah per day. Depending on the controller, 5 to 25% of this will be lost before it reaches the battery. Depending on usage patterns, the battery itself may be 85-95% efficient.
2) One should avoid running batteries down much past their 30% charge level, as going down much further will seriously reduce their life-span. 2/3 of rated capacity is about as much as it's wise to draw from a battery, and it should be recharged asap to minimise deterioration.
"our biggest draw will be the fridge which is 1ah(288w/h) after that is the tv 240w/h, which leaves us to my estimate around 300w/h for everything else we get together" mmmmm....!!
Joel - it's important to get the units right here (I'm not just being pedantic or an academic 'expert' !)
The fridge - first - If it's a 3 way fridge, forget running it from batteries as it will draw far more than they can sensibly supply. Assuming it's a compressor type I'd expect the fridge to draw an average of at least 1 amp (NOT 1 Ah!!), probably nearer 1.5 amps. If we go with an average of 1 amp at 12V then it requires 24 amp hours (Ah) per 24 hours. If we assume our battery delivers 12 volts (which it doesn't!) then that comes to 12x24= 288 watt hours (NOT watts per hour or w/h). (BTW best to think in terms of amphours rather than watthours with this stuff. )
TV - depending on size, I'd expect this to draw perhaps 2 or 3 amps for say 2 or 3 (?) hours per day, so that's say 5 to 10 Ah. Charging phones, laptops, torches,..... maybe another 10Ah to give a total requirement of about 45-50 Ah per day. (This sounds about right - Our rig uses about 35-40 Ah per day.)
With this level of demand, I'd aim for 200 Ah of battery storage. Depending on your likely traveling time per day I'd seriously consider a dc-dc charger rather than aiming for lots of solar. A 30 amp dc-dc charger will push close to 30A into the battery/s while the engine is running at more than a fast idle, and meet your daily electrical demand in less than a couple of hours. With 200 Ah of battery you will get by for 2 or 3 days without any charging. If you are stationary for more than a day or two, 160-200W of solar panels should handle your requirements provided the sky is clear. (I note that you are aware of the need to consider our alternator's capacity if you go for a dc-dc charger.) There are several quality dc-dc chargers available, some such as the Ctek, Redarc, Projector include an MPPT solar controller.
In regard to some of the other contributions - There's been interest in using the solar input of an MPPT controller to accept input from a dc power supply so as to simulate a multistage mains charger. The solar input needs to be at 18 volts or more. To pretend to be a solar panel, the power supply needs to be at least 18V, which is an inconvenient voltage and not much use for anything else. Not too sure how the controller will respond to a steady voltage supply, as it's expecting a solar panel which is pretty close to a current source, rather than a voltage source. (These controllers monitor their source voltage/current while periodically loading and unloading the source (panel) to see how it's travelling.) Certainly, to avoid confusing the controller, the power supply and the solar panels should not be connected simultaneously to its input unless diodes (preferably Schottky diodes so as to minimise losses) are included to isolate the two sources from each other. (Current flow from one to the other will be negligible in any case, but isolation is necessary to prevent the filtering in the power supply from upsetting the operation of the controller.) Better in my view would be to employ a dc-dc charger and feed it (and NOT the solar input) with a 12-15V power supply that can be set to say 13.8V. This will act as a multistage charger, with the further advantage that the power supply can power the whole rig from the mains if need arises.
Ken
asks about my 13.8V supply. I picked one up cheap years ago on the grounds it "might be useful". Variable from 5 to 20V and capable of 40 amps. It has turned out to be very useful! 13.8V supplies are readily available as a standard voltage for "12V" battery systems - see ebay for lots.
Your 250S is already a smart charger, and if you add another smart charger, both will become confused. That's one reason I prefer to simply add a mains power supply to the front end of my multistage dc-dc charger. Better in my opinion to avoid interfering with the solar input, which has it's own higher voltage requirements.
Guys,..... please have a look at
Electricity for Camping. It's the most popular blog on ExplorOz. I wrote it to answer most of the questions that come up here, and it saves a lot of typing!!!!!
Cheers
John | J and V
"Not everything that can be counted counts, and not everything that counts can be counted."
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