Solar works very well when it is sunny out. Seems pretty straight forward but it is a very key thing to understand. While you will get some degree of charge to the battery in the clouds and rain, it isn’t nearly enough power to keep a battery healthy in the long run.
Solar panels start around 17 volts collecting energy from the sun. This energy goes from the panel itself and then passes through what is called a charge controller. This controls the raw voltage down to around a controlled 14 volts which is suitable for charging a battery.
The solar panel itself has two wires (positive and negative) that go into the charge controller. Then two different wires (positive and negative) come out of the charge controller to the battery itself. The voltage has now been controlled to the appropriate voltage for charging.
Some charge controllers do a better job than others (more energy is passed through and less is wasted) It generally depends on how much you pay for them, but they all work to some degree.
Try not to let the volts of the battery, the amps of the charge ratings, and the watts of the solar get complicated. It’s just the industry trying to make you crazy. They all convert and the formula is relatively simple once you get the hang of it.
(Pacific Batteries in Surrey and Maple Ridge can help you with this)
The 12 volt battery you want to charge has an amp hour rating. That’s how much energy it can store. The watts of the solar convert to how many amps the solar panel can put back in. You take out amps, you put in amps.
Example A – You have a deep cycle battery the size of a jug of milk that you want to use and recharge. It has a rating of 80 amp hours. You are only supposed to use half of it for long term health of the battery. So you have an effective 40 amp hours available to use. (They also use reserve minutes capacity but this also translates to amp hours so we will ignore it for this blog)
The rule of thumb is to have your charge rate equal to about 10% of the batteries amp hour rating to keep your battery healthy and last the longest. In this case your battery has an 80 amp hour rating.
So you realize the best amount of charge is 8 amps. To get that from a solar panel, the panel needs to be rated at about 160 watts. This is because each 100 watts of solar panel gives you about 5 amps of charge in the bright sun. So now you decide to get a 160 watt solar panel. This is going to give you about 8 amps of charge per hour in the bright sun. This equals about 10% of the battery AH rating. OK, now we’re talking.
So now when you drain your battery to half way down (from 80AH down to 40AH) you need to put the 40AH back in.
Your 160 watt panel is putting back in 8 amps per hour, and this means it will take about 5 hours to recharge your battery
(8 amps x 5 hours = 40AH)
That’s awesome. It doesn’t take too long and the battery chemistry is happy. If you are only taking 40AH out of your battery each day, and your solar can put back 40AH per day, it’s perpetual (in the sun) or for as long as your battery life is. (The battery lifespan depends on how much you spend on it – it can be anywhere from 3 years to 10 years)
Well done. You now have a great matched system for when it’s sunny.
Buuuuut now it’s raining. And now your solar panel can only provide about 1 amp her hour. This is simply not enough charge rate to keep the battery healthy even if you had the 40 hours to put it back in. The chemistry in the battery will start to fail. For a ball park figure in less than a year your battery will say ok, even with the trickle charge, without the higher amperage charge rate, I can’t function any more.
You would have to go up to 800 watts of solar to ensure you are getting the 8 amps per hour you need in the rain. 800 watts = 8 amps of charge in the rain)
Now 800 watts of solar panels is a lot for one battery. It can be done. Buy four 200 watt panels. But wait. In the sun these 800 watts of panels are now putting out 40 amps an hour. The heat produced by such a large charge on this little battery would cook it. It wants 8 !
Yes some manufacture’s say their panels work in all weather. And some do. Some will give you more than 1 amp. Maybe even 2. But you will be hard pressed to find one that can reach that magic 8 number. And even if you did, it’s back to cooking the battery when it’s sunny.
You could disconnect most of the solar for the sunny times and reconnect for the rain, and if that’s what you have to do, that’s what you have to do.
So what can you power per day with your 80AH battery and your 160 watt solar panel in the sun?
A laptop through an inverter will use it all in 5 hours
A strip of LED lights will use it all in 24 hours
A microwave will use it all in 25 minutes
A fridge will use it all in 12 hours
There are a lot of variables but you can see one battery can’t store a ton of energy. Van dwellers know this and only use it for things they HAVE to. Typically LED lights, a radio, maybe a small fan, a cell phone recharge, and that’s about it.
Example B: Now lets say you wanted to use ALL of the above each day (24 hours) AND have the solar put it back.
The laptop (via inverter) will use 40AH for it’s 5 hours of use
The LED light strip will use 10AH for it’s 6 hours of use
The fridge will use 80AH for it’s 24 hours of use
The microwave will use 40AH for it’s 25 minutes of use.
That totals 170AH per 24 hours.
So now you need a battery that has a 340AH rating. (we recall you should only drain lead acid batteries 50% down)
For bonus points AGM batteries can go more like 75% down- so if you buy AGM batteries the formula changes – to more like a 225AH rating required.
One common size lead acid 12 volt battery is rated up to 110AH. So you would need 3 of them and they would reach 330AH. Half of that is usable leaving 165AH. Close enough. This is awesome. 170AH out, 170AH in. Now you can use all your stuff and the batteries can store enough energy for you to do so each day.
Now we need to charge them back up with solar. Following the 10% charge rate rule we want 33 amps of charge. With solar being 5 amps per 100 watts, we need 7 x 100 watt panels to reach 35 amps. We have to put that 170AH back. At 35 amps an hour we are looking at about 5 hours of direct sun from 700 watts of solar panels. (the important part is the charge rate, not the amount of time it takes to put it back in, other than it has to be done while the sun is out, and in time to use it all again the next day.)
Sweet. Now we have a bigger set up that let’s you use all your stuff, and the solar puts it back, the charge rate is good, the time needed to do it is good, everything is good.
Annnnd then it rains.
Just the way it is.
Typically with a camper that you take out twice a year you would charge your batteries right up by plugging in before you leave. You can go about 5 days with 110AH if you are being moderate in your usage. No heaters, no AC, no fridges (or use a propane fridge) avoid the microwave, avoid the Keurig, avoid anything you can. The solar you do have even in the rain helps top them up and lets you stay out a little longer.
By the way, your solar doesn’t care if it is two 12 volt 110AH batteries, or two 6 volt 220Ah batteries. (We do, the two x 6 volts will last longer than two x 12 volts of the same AH rating in years for about the same price)
The magic system is:
1. BLAST (plug in – built in chargers usually have up to 40 amps of charge rate available)
3. TOP UP (solar, genny, drive)
Plugging in is always the best way to charge. Solar helps. Using the genny helps. Driving helps. Anything helps. But you HAVE to get that 10% rule of charge rate back into it at least once a month or your batteries will go south fast and typically none of these methods can do that very well year round.
For the record driving and using the alternator to charge the batteries puts a small charge rate into the batteries. Say maybe 4 amps. So yes they get charged, but no they don’t reach the correct charge rating for long term health.
So now that we are aware of the limitation’s of solar, let’s look at some basic rules of thumb as to why it IS a good idea to use solar:
- Any amount of charge going into a battery helps keeps them from freezing.
- Any amount of charge going into a battery helps you stay out longer
- A well matched system lets you go all summer long
- Sizing solar to battery banks is relatively easy – Pacific Batteries Surrey and Maple Ridge can help you with this.
- Once you have bought the solar package they provide free energy and last a very very long time
- If you can’t plug in, it’s one of your few options
* All of the figures used here are approximate. They have to be. Every device used varies on how much power it uses. All time frames depend on the type of the battery, the type of the battery charger, the type of the solar brain box, the type of the solar panels, and the type of the inverter used.
* This is all ok. If it’s off grid survival it needs to be dead on. If it’s camping these figure are close enough. Happy Camping