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/diy/ - Do It Yourself

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Are there any solar wizards here?
I need to desing a system that will power a small fridge , my laptop and a few lights.
This is all going in my van.
I was thinking 300 watts of solar on the roof
MPPT charge controller (not sure what size)
Some sort of charger from the alternator
A trickle charger I can run off shore power
Two six volt costco golf cart batteries.

Any advice here?
Does this all seem sufficient for weeks of beach camping at a time?
Add up all the wattage used by everything when everything is turned on all at once. Then times that number by 3 to 5 and you will have the number of watts your system will need to be built around. The amount of panels and size of battery array will depend on that.
Oh and that's the bare minimum when you have 100% sunny days. Times when you have badly overcast days, you'll need a far larger battery array.
I will be in mexico on the desert beach so lots of sun , also will be driving a bit so some power there
I've been looking at building my 99 astro into a camper van for a while now. From what I've seen 300 watt solar is what most people go for. Also look into golf cart batteries, they are only 6 volt but have alot more mha.
If you have the roof space, go for 500w (2x250w). If your system will be in constant use, you will need it.

Remember that peak efficiency is for less than 6 hours a day.
You at least have 12 full hours of the panels just sitting there doing nothing, and the rest at greatly reduced efficiency.

I get combined winter/summer average of 700-800w a day from a 250w panel. Summer avg is around 1,250w.

You will also need to factor in ~10% inverter loss, and take into consideration that you cannot use the full capacity of the battery if you want to keep it in good shape.

Also, do not fall for the 12v fridge meme. Just get a small $90 110v fridge from Walmart.
If it dies every year (which it won't), you can get another one for ten years for the price of a single 12v one.
Since you have a battery+inverter, you will have no trouble starting the compressor.

In my opinion, if you can handle the humming it would be wise to get a small wind turbine.
If your on a beach get a small windmill. As well.
A VAWT would work well for a beach. Low profile too.
most solar panels pull in between 11-16 volts

run it into a diode pack and then to a charge controller,

then to the batteries (bridge the batteries with interconnecting leads, + to + , - to - )

if you want run additional charge to the batteries from the alternator in the van, just run some extra leads to it, put these leads in conduit.

im fit solar systems into caravans, motorhomes occassionaly, im a auto electrician.

you can find cheap charge controllers on ebay etc. we use voltech ones at my work, never has any problem.
When its windy at the beach i go inland.
I hate wind
He alway winding at playa, my nigga.
Guys I have a related question.

I've been looking at PV panels and some of them output 18v instead of 12v.

They're apparently designed to hook up to your car battery. Can this higher voltage actually cause damage?

Pic related. This fucker is supposed to be 30w 18v
You need a charge controller to prevent tour batteries from overvolting.
All panels that are 'nominally 12 volt' are actually higher, around 18 volt.

300 watts will almost certainly be too small.

You need to make sure your fridge is efficient, because you have limited space for panels on the roof of the vehicle.

Some ballpark numbers:
Take a standard(ish) mini fridge (which you shouldn't use, you should get a better, more efficient one): 650 Watt-hours a day.
Laptop (Assuming 5 hours of use): 100 Watt-hours a day.
Lights (Assuming 5 hours of use, and that they're all total 10 watts or less: 50 Watt-hours a day.

That's 800 Watt-hours a day before efficiency losses.

You can only count on about 4.5 hours of good sun on a good day, and your system will probably lose 40% in efficiency losses.

This means, you need 800 Watt-hours after the 40% loss, in other words you need about 1350 Watt-hours generated a day, so divide that by the 4.5 and you get 300 Watts of solar.

Now, you have to factor in that the panels will NOT produce that 300 watts all the time, probably (from personal experience) about 80% yield during your peak sun hours is a reasonable expectation.

So factoring that in, you'll need at least 375 watts of panels, so we can go with a good round 400 watts.

Moving on to battery storage, if you're using flooded golf cart batteries, you'll need to take your total power use before losses (1350 Watt-hours) and multiply that by how many days you need to run without sun.
Figure out how much you need as far as battery storage, and go from there.
>4.5 hours of good sun on a good day
This is the only thing I think is different.
In mexico I get way more than that and it baja it is NEVER cloudy.
I think you are still right about 400 watts though so I will shoot for that.
The other thing is I will also be charging the battery through the alternator.
So my thinking is that if I get low I can idle for a few hours? Does this sound reasonable ? I am hoping to get by with two big six volts batts rated at about 210 amp hours total at twelve volts

Your mini fridge numbers are spot on. The ones I am looking at are about 630whs
>More sun than 4.5 hours a day.
It's true you might get more sun, but I think you'll be really happy with the increase in array size from 300 to 400 watts. Heck, if you can fit 500 watts, you'll like it even better.
Kind of a piece of mind thing, plus the extra power for a few more fans or something when it's hot as balls will really save your bacon... er... save you from becoming bacon.

Series vs Parallel:
If you're wiring those batteries in series and need 210 ah total, make sure each battery is 6v 210 ah. (If they're each only 105 ah at 6v, when you put them in series you have 12v but still only 105 ah.)
Series adds voltage, parallel adds amp hours.

Now, regarding the capacity you'll need:

You can't really use more than 50% of your battery capacity (and want to avoid going that low even if possible), so if we go back to my earlier estimate that you want to factor your usage as 1350 Watt-hours a day, the 2520 Watt-hour total capacity is a bit light, as you'll only have 1260 Watt-hours of usable storage.

What I'd do, is double up on that. Use 2 sets of the 6v 210ah batteries, that way you'll have plenty of power.

2*6*200ah = 2520 Watt-hours

2520Wh*2 = 5040 Watt-hours total capacity, and of course that puts your 50% mark, the useable capacity, at 2520 Watt-hours.
And your expected use of 1350 Watt-hours each day will only drain those batteries to ~70%. They'll last a very long time like that.
This wouldn't fit but...

If you're using flooded batteries, which I assume you are if you're going to be using golf-cart batteries, you need to make sure you can water them!
Especially in a hot environment, you'll need to regularly (once a week or more!) check those batteries and make sure to top up the water level.

Flooded batteries are great because you CAN maintain them, but they are horrible if you forget.

Sealed batteries are great because you don't have to do anything to them, but they're horrible because you CAN'T do anything to them really, so if you overcharge and lose electrolyte or something, it's new battery time.

Make sure if you are using flooded batteries, you mount your batteries in a location that you can quickly pop the caps and take a peak at the water level. And make sure of course to keep them upright.
Keep some distilled water on hand to top them up with. :)
The batteries I am looking at are 210 each at 6v so the same for both at 12.
I cant really double that because of space and weight constraints so the next step up is expensive lithium . I hope I can avoid that with careful management and idling
You can probably avoid it, or consider looking into getting 3, 12-volt 100 ah batteries. Maybe you could fit those? That'd give you 'just enough' I'd say.
Oh, almost forgot:
Don't forget to put a battery isolator in the system so you don't accidentally drain your starter battery!

And with the 400 watts of solar and mostly sunny weather, hopefully you can get away with only idling for a few hours every day or so.

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