Mark—
This is probably a much more thorough answer than you wanted, but here it is.
BTW, a more complete article on solar panels and charge controllers is on my
blog.
Chances are more than likely that you’d need a charge controller. The basic rule of thumb is that if the panels daily output is more than 1/60 of the bank’s rated amp-hour capacity, then a charge controller is probably necessary so that the panel doesn’t boil off the batteries. A 25 Watt panel puts out about 1.39 amps. The daily output is about 6.95 amps. So, unless your battery bank is larger than 417 amp-hours in size...you’l need a charge controller. This is especially the case if you’re using AGM or Gel type batteries. However, using a charge controller makes a lot of sense for two major reasons.
First, they will protect the batteries from discharging back into the solar panels at night. Solar panels, unless connected using a blocking diode—which drops their effective voltage output about a volt—will discharge the batteries at night. Using a charge controller basically prevents this from happening, and not needing to use a blocking diode increases the effective output of the panel.
Second, a charge controller will also help protect and condition the batteries as they charge them. A battery goes through three phases of charging—bulk, absorption and float—as the charge level of the battery changes. An intelligent charge controller—like the
FlexCharge NC25a, will manage the panels output, which can be as high as 20 volts on a shade-resistant panel, and drop it down to the voltage levels required by the battery—depending on what the charge state the battery is at. Many of these can also help condition the batteries. Some charge controllers even have the ability to "equalize" the batteries. Some are pulse-width modulation based. Others, like the NC25a, are not.
There are three types of charge controllers out there.
First, there are dumb charge controllers. Don’t waste your money on these—they’re just not worth it. These usually use just a relay or shunt transistors to vary the voltage levels, and most only can do one or two-stage charging. They basically short the panel out when the batteries are “fully charged”.
Second, there are three-or-four stage smart charge controllers, like the NC25A I mentioned above. It is a non-PWM (pulse width modulation)-type three-stage intelligent charge controller and it looks like this.
Companies, like Morningstar, make PWM-based three-stage charge controllers that are comparably priced. There are debates as to which charging method is better. I have used and like the NC25a, and keep the one I own on the boat as a backup if my MPPT charge controller fails.
Third, are the MPPT-type charge controllers. These are the best of the breed. Morningstar, BlueSky, and Outback make versions that are suitable for marine use. The major difference between an MPPT-type charge controller and a PWM-type three-stage charge controller is intelligence, as most of the MPPT-type charge controllers are based on a three-stage PWM-type controller core. Using an MPPT charge controller will often yield a 15-30% increase in efficiency for your solar panel charging system. The FlexCharge NC25A was replaced by a BlueSky SB2000E MPPT-type charge controller. You’ll see why in a second.
MPPT charge controllers do this by trading voltage for amperage, or amperage for voltage.
Please note: the following is a simplified example that assumes a few things, like no charging or MPPT-controller losses, but will give you a good idea of what is going on.
An example: Normally a 25 Watt panel will be outputting 1.39 amps at 18 volts. 18 volts is too high for the batteries to charge with, since bulk charging requires only 14.4 volts. With a normal three-stage controller, the excess voltage is shed as heat—that’s why the charge controllers heat up so much. 18-14.4=3.6 volts—3.6 volts * 1.39 amp = 5 watts. So you’re losing about 5 watts out of 25 watts to heat.
With a MPPT-type controller, it uses a high-frequency DC-to-DC converter to drop the voltage down to 14.4 volts, but increases the amperage to 1.74 amps at the same time. So, instead of getting 6.95 amp-hours from the panel for the day, assuming about five hours of full output, you get 8.70 amp-hours, or a
25% effective increase in amp-hours to the batteries.
This makes a lot of sense, since you’re basically recovering the 5 watts that was being lost to heating the three-stage, non-MPPT, charge controller.
Another thing most MPPT-type controllers will do is trade amps for volts, if necessary. For instance, if the panel is partially shaded, the output voltage on it will drop. Say the output drops to only 12 volts, at 1.39 amps, because a third of the panel is shaded. Normally, this would just be lost as the three-stage controller can’t use it to charge the battery, which requires 14.4 volts to charge. Some MPPT-type controller will take the 12 volts at 1.39 amps and convert it to 14.4 volts at 1.16 amps. Now, instead of getting
ZERO amp-hours, due to insufficient voltage to charge the batteries, you’re now
getting 5.8 amp-hours for the day.
Not too bad, is it??
Mark G wrote:My electrical system is stock - two batteries. If I were to install this panel to keep the batteries topped up, would I need to install one of the controllers or could I simply hook they to the batteries? Absolute ignorance here.
