What is the price difference in the two types of controllers?
The Flex Charge is about $60.00 and the Genasun ranges from about $124.00 for up to 50W panel to $167.00 for a 150W panel.
This however is a small part of the story. For a more fair comparison you'd need to compare a PWM controller with three stages to the MPPT controller. The price difference becomes even smaller.
A shunting controller is not one I would recommend.
The Flex Charge controllers are inexpensive and among the best of the shunting controllers but still not good for many reasons. Flex Charge does makes very reliable products. That being said, your total time to "full charge" will be compromised with this type of controller. I would still suggest a PWM or MPPT controller.
Please be careful with shunting controllers. These devices begin switching on/off once the bank hits absorption voltage and they can really cut your "time to full" by days, not minutes.
In the case of some of the el-cheapo Sunforce controllers, often sold at West Marine, they switch OFF at around 14.2 volts and they do not switch back on again until the battery bank voltage falls back to 13.0V. This may be fine for off grid where you often have a load that sucks the bank back to 13.0V very quickly, but, with a boat on a mooring, with no loads, it can take a healthy bank a LONG time to drop the surface charge back to 13.0V.
I have some customers banks of AGM & deep cycle wet cells that will hang out at over 13.0V for over an hour. That is an hour of lost charging time waiting for a cheap shunting controller to switch back on and provide maybe 60 seconds of charging before raising the bank to 14.2V and then shutting off again. Sometimes these cheap controllers will never restore the bank to full, no matter how big the array, in the time you need them to.
Below is a video of one of the "better quality" shunting controllers, a Flex Charge PV-7. It shuts OFF at about 14.4v and does not come back on until the bank voltage has dropped to about 13.6V. The first video is showing the "current" but you can see it turn on and off. I applied a load of 0.1A just so it would go to -0.1A when OFF for the video. When you get close to full your OFF time can be as much as 10-20+ times longer than your on time with some cheap controllers.
This second video is showing the voltage behavior.
In this case the "ON" time is about 5-6 seconds and the OFF time close to a minute to get the bank back down to 13.6v before it can turn back on again. This bank still had 20 Ah's to go and it would take 5-7+ days for this bank to eventually get "full" from an 80-85% state of charge with 5-6 seconds on and 60 seconds off, remove the 0.1A load and it takes this even longer..
It should be noted that before I took these videos the banks were charged to full using a shore charger until they were accepting less than .5% of C (capacity) at 13.6 volts. At this point, after being on float for two days, the battery monitor was re-calibrated manually to full. I then applied loads and removed approx 30 Ah's from the bank. I then monitored the time it took to replace the 30Ah's removed.
Contrast that Flex Charge controller, or a Sunforce, with a quality PWM or MPPT like the Genasun's and the difference in the last 5-15% of charge, and the time it takes, can be quite dramatic.
Last summer I replaced both a Sunforce and a Flex Charge shunting controller with Genasun MPPT controllers. On one boat "FULL" was cut from averaging 7-8 days to about 2 days and on the other boat "FULL" was cut from roughly 5-6 days to about 1.5 days. Same boat, panels, wiring, batteries, just a much better quality controller that does not turn ON/OFF like a shunter will. A lot of the controllers you buy from "discount" solar houses & eBay sellers are "shunters".
If you stick with Morningstar, starting at the Pro-Star series, Blue Sky, Outback, Genasun and a couple other reputable controller manufacturers, you'll be doing well. I've found the eBay stuff can very often be a scam, so be careful.
I have replaced a number of eBay, "so called", MPPT controllers that were not MPPT at all. The were complete scams. The harsh reality is that behind the "MPPT" sticker and box was nothing more than a simple shunting ON/OFF controller. MPPT & PWM controllers do not shut off for long periods of time like shunters do. The average Joe does not know how they should work so these eBay scammers often get away with it.
The Genasun MPPT's are a very good value for smaller for panels. Under 150W, that is pretty much all I install now. When I have a larger array I generally use Blue Sky or Morningstar MPPT controllers though I still use some Morningstar PWM controllers too. I find the Morningstar PWM controllers to be one of the most reliable.
Please be careful with "cheap" controllers as you often get what you pay for, and how much did the batteries cost..?.
