Solar charge controller questions. Is this an option???

I'm not currently putting a solar system together, just trying to educate myself.

I've been digging around online and looking at prices for various components. Some of the price variations are surprising, and the manufacturers claims tend to be confusing. Specifically, my question[s] are about the charge controller. I've noticed that a controller for a small system [~500w @ 20A or less] can be purchased for as low as $10. But once you start getting into controllers large enough to handle a whole home's electrical needs, the priced go thru the roof, ranging from $500 to over $1000!!!

I was thinking it "might" be possible to put together a system where a single [or pair] of solar panels feeds into a single low cost 20A controller and that charges a single battery, but then have several of these low cost setups side by side. The batteries [plural] could then all feed into a 120V converter, but have a battery isolator on each one of them so the adjoining batteries/solar panels dont back-feed into their neighboring systems.

Naturally this system would involve slightly more complex wiring, and I'd guess the isolator[s] would have a slight parasitic power loss... But at the same time, I could also see some hefty benefits to setting up a system like this. Not only in a significant cost savings, but also by having the cells/batteries/controllers on individual "mini systems", trouble shooting problems with individual components would be much easier compared to a large system where EVERYTHING is ties together.

In my mind, I see this system working perfectly and offering a huge savings to someone setting up a solar system on a budget, but I just dont know enough about these things to be sure. Anyone want to take a shot at answering these questions?

Oh, and please answer in a way that an idiot such as myself could understand

Darryl Roederer wrote:
I was thinking it "might" be possible to put together a system where a single [or pair] of solar panels feeds into a single low cost 20A controller and that charges a single battery, but then have several of these low cost setups side by side. The batteries [plural] could then all feed into a 120V converter, but have a battery isolator on each one of them so the adjoining batteries/solar panels dont back-feed into their neighboring systems. ....
Oh, and please answer in a way that an idiot such as myself could understand

I'm a little rusty on this but I don't think your set up makes sense. Batteries need to be set up in series and/or in parallel. So we have 8 6v batteries in 4 banks. The purpose of the putting 2 together in series is to add the volts, thus making a 12 volt battery. When joining in parallel you're adding amps.

All the batteries need to be joined to feed power into an inverter so separate charge controllers wouldn't make sense.

Cheaping out vs doing it properly rarely pans out. As the saying goes, you get what you pay for. Why spend money on good solar panels only to have them ineffective due to improper charge control? Or damage batteries by over or under charging?

Charge controllers are generally broken into two styles these days: PWM (Pulse Width Modulated) or MPPT (Multi Point Power Tracking).

PWM are older, more basic, lots cheaper, as they are simpler. MPPT will deliver about 25% more power from the same solar due to it's improved charge methods. Which you can google for full explanation.

The current capacity - Amps is where the money goes. An 80A controller will cost lots more than a 20A. And that's where higher DC voltage helps, as with a 48VDC system you can have 4x the amps than a 12V system. One common mistake is presuming 12VDC. It's a real pain to deal with.

Using a mess of cheap controllers and batteries and loss-inducing diodes will fail. And try pricing out the high current diodes needed to isolate batteries! Save yourself that hassle.

Kevin

Kevin,
I took your advice and searched Amazon for MPPT charge controllers as well as battery isolators. I'm beginning to understand that for a "whole house" solar setup it would make a lot more sense to spend the big bucks and "do it right", but I'm also seeing that for a small/medium sized off grid cabin, my idea might have some merit.

When I searched for a 20A MPPT controller, I found this one for $11.98 click here

Searching for an 80 amp controller netted me several hits all priced between $500 to $1100

Battery isolators could be found as well. I'm a little confused as to the power draw [in amps] that a 12v to 120v inverter requires, but by reading the manufacturers specs, I'm guessing that 2000 watts will draw about 80 amps. To that end here's a 90 amp isolator that can be had for $26.27 click here

So by my thinking, if you set up a 1000 watt solar system running on 4 of these "mini systems", you'd need 4 250W solar panels, 4 small charge controllers, 4 batteries, 4 isolators, and one 12v to 120v inverter. The controllers @ $11.98 each works out to $48. The 4 isolators @ $26.27 each works out to $105. That's $150 you'd spend instead of spending $500 on an 80A charge controller. By my math, it would be more economical to build ANY system under 3000 watts using this "mini parallel system" instead of a large "single line" system using a big controller.

Yes, yes, yes... I'm an un-educated solar virgin and I dont know what I'm talking about. But I'm trying to learn here. Obviously there's a market for controllers that cost $500 to $1100 or they wouldn't exist. If my idea was so good, others would already be doing it this way... Please help me understand why my idea isn't a good one.

I'm including an illustration of how my "mini parallel system" would work to clarify what I'm talking about.

It doesn't make sense to isolate batteries that have to be connected together to before they can go to the inverter.

Your drawing doesn't show how the individual batteries connect to the inverter.

The isolators allow current to only flow in one direction. They're used on RV's and motor homes to allow you to use 12V lights and appliances while camping without draining power from the battery used to start the vehicle. Once you re-start it, the juice from the alternator can flow into the second battery to recharge it, but it cant flow backwards.

By using isolator[s] between the mini systems [as shown above] it would isolate each system, allowing you to use the much less expensive 20A charge controllers on each system, but still allow all the system[s] to pour full power into the inverter.

Hi Darryl,

Firstly, the $12 controller is not an MPPT controller. False advertising. Read the reviews. Be best to buy several spares so that WHEN they fail you have new ones there on hand. At that price they are disposable-grade. Good controllers fail "open" so that they don't damage battereries. Check what the cheaop failure mode is - if it fails closed then the solar could cook the batteries real fast by over-voltage. And finally, does it allow the input voltage range that the solar panels produce? PV modules and controllers really need to be designed together. 250W panels these days are commonly in the 35-40 VoC (open circuit voltage).

Second, the battery isolator you suggest is not designed for this purpose. It is obsolete technology designed to charge 2 batteries from 1 alternator. They were common years back in boats / RV's etc with a dual house/start battery system. But always problematic. We now use electronic devices do do the same thing, without the losses. There is significant voltage drop associated with those isolators, so that means the inverter will be shutting down due to low voltage when the batteries are ok. And they fail constantly.

Your configuration _might_ "work" in the loosest sense of working, for a few months or so, with lots of fiddling. A good way to turn people off solar is having haywire systems that are unreliable and require constant fiddling and fixing. But you would find the batteries would get out of balance, needing to charge some and not others. And not to mention all the heavy cables needed to run that config. A 2 kW 12V inverter needs 4/0 wire. And that stuff is expensive.

A question I sometimes ask customers is "do you want power, or do you want a science project?"

http://www.blueskyenergyinc.com/
These folks make quality, low amp MPPT controllers. I have 100's out there in industrial projects.

http://www.midnitesolar.com/products.php?menuItem=products&productCat_ID=21&productCatName=Charge%20Controllers%20-%20Classics
This is the best MPPT controller on the market today. $800

http://www.midnitesolar.com/products.php?menuItem=products&productCat_ID=43&productCatName=Charge%20Controllers%20-%20KID
They have also just released a smaller version - 30A kid. $400. It will control 3 x 250W panels @ 12VDC, or 6 x 250W @ 24VDC.

Sticking to 12V means you will pay more for controller. Going to 24 or 48V reduces current (amps) and thus can get better value.

Hope this is useful.

Kevin

Yes Kevin, it's all useful. I do very much appreciate you taking the time to [attempt to] educate a block head like myself. LOL