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MPPT Charge Controllers - FAQ

Frequently Asked Questions:
Maximum Power Point Tracking Charge Controllers


The following is courtesy of, and copyrighted by
Solar Converters, Inc.

What is the algorithm the unit uses to find the MPP?
The unit is set-up to the average voltage that the MPPT will be found at, experience
has found this to be the nameplate MPP less 10%. Typically this is 16 V for a 17 V MPPT
(normal) panel or 17 V if the 18 V class MPPT panel is expected.
To switch between panel voltage class, the units average MPPT voltage is changed and
this is the voltage that is specified by the P XX suffix to the part number if the expected
panel is different from normal.
Once set, the unit will track +/- 20 % in voltage to find the MPP point.
On a normal panel this is 14 V to 18.5 V. The algorithm is best described with a radio analogy -
with a radio the only way you can tell you have the best reception is to move the knob slightly
and see if your reception gets better or worse. You also know where the signal should be.
Our Power Trackerô MPPT controller is the same. Its panel setting is where it expects the
panel MPPT to be. Then continuously it moves slightly off station and sees if it gets more or less power.
If it gets more - it moves further "off station" if it gets less it moves back to where it was,
and tries the other direction to see if it can get a better match.
The only way it can know it is on station, is to move slightly off station and see if it gets a better
signal or more power from the panel.
What is the charge method?
Dual float: the Dual float charge control method has gone through several names over the years.
Basically it is similar to the bulk/overcharge three state charging technique used by others with
one very important difference - The control algorithm changes as a function of battery acceptance
current not some arbitrary time.
What does this mean? If your battery only wants to accept @ 14.4 V only a certain amount of charge,
this is all it will get.
How does this compare with a three stage charge controller?
It is much the same. There is a constant current phase where basically every amp of power the
panel is producing is going to the battery. It is constant current only because the panel is current limited.
It has a bulk absorption phase, and a float phase.
The only real difference is the switch from bulk absorption to float charge occurs not after an arbritrary
time but after the acceptance charge of the battery has dropped under 3 amps. Using a 12 V example,
with 14.4 V on the battery, if you watched the current it will go 30, 20, 10, 9 ,7 ,5 3, 2, 1 amps. as it
tapers back the current to maintain the voltage. This taper back is for one reason - THE BATTERY
CANNOT HOLD ANY MORE. As the acceptance charge drops under 3 amps, the PWM controller
cuts back the voltage to float voltage and maintains the battery at its float value.
To maintain 14.4 V for longer on the battery is only powering unwanted reactions. Simply put, the
biggest advantage of the dual float technique is the battery itself, through its acceptance current is
telling you when it is full. Rather than choosing an arbitrary time that may be good today, but what if
the battery ages, it changes its temperature, its sulphication changes, its PH changes - No matter
how new, old, strained, abused the battery is, the acceptance current rather than an arbitrary fixed
time will tell the controller to cut back.
What is the effect of panelís temperature?
Heat is the nemesis of a MPPT controller. As the panels get hot, the MPP voltage droops, and you
no longer get great gains in charge current - indeed on some panels and very hot temperature, I have
actually seen a loss of power. However, even in these situations, great gains are made in off-hours
where the temperature is reduced and the sun is not directly on the panels.
Overall gain is reduced but is still present.
Will it MPPT into a full battery?
Full Battery will cause the charge current to taper back to only load current. This is not MPPT, this is
the PWM controller preventing overcharge of your batteries. If it applied full current to your
batteries - you would soon be buying new batteries. This is not a problem - it is supposed to do this.
How do you calculate the current gain?
The gain in current of a MPPT controller is roughly the same proportion as the percentage difference
between the panel voltage and the battery voltage. If the panel is 17 volts and the battery 12 V the
gain is 17/12 = 1.41 or 40%. This is under ideal conditions and never happens in real life.
The above equation assumes that the current at 17 volts and 12 V is identical. In real life it is not.
The nameplate panel ratings are at bright sun and 25 deg panels. If you have bright sun, it heats
your panels and you no longer have 25 deg panels. I find a good rule of thumb is take the
nameplate MPP rating and multiply by 0.9 - 0.95.
Heat affects the equation by decreasing the 17 V MPPT and of course lowering the gain.
As the battery fills, the 12 is increasing to say 13 or 14, hence the proportion increase in charge
current decreases.
Note however that the highest increase occurs where you need it the most - into discharged batteries.
What does the current limit do?
The current limit is an electronic maximum current that the charge controller will put out.
The old solar pros will tell you of all sorts of effects that increase for a while the sun's apparent power.
These include snow reflection, edge of cloud effect, high altitude lack of dispersion etc.
In the old days, the higher than expected current or voltage would burnout the controller.
This is one reason for the NEC imposed 25 % margin often designed into systems.
Solar Converters Power Trackerô charge controllers with MPPT will current limit, protecting
itself and your equipment from the high transient current or voltage until it passes.
What does the Auxiliary Drive Do?
The Auxiliary drive is a relay driver that comes on at about 95 % full charge voltage.
It has a number of uses.
It can be used to control a relay to switch in another charging source, i.e. generator or windmill
when the battery is low. The secondary charging source is then disconnected near full charge
while the Power Trackerô does its "finishing" PWM charge technique for the health of your
batteries.
It can run a small fan to come on just as you are near full charge and do the job of a battery
vent control.
It can run a small light to let you know your batteries are very near full charge.
It can switch in a dump load to heat your house, pool or water with the power of your
renewable energy system to more fully utilize every watt of power form your system.
What is an LVD?
All Solar Converter charge controllers except for the smallest have an LVD function or an LVD
driver to control an external relay. To prevent low battery voltage from destroying your battery,
the unit can be used to disconnect the load from your battery when it's out of charge to
prevent damaging it. Normal operation resumes once the battery has received some charge
either from the PV charge controller or auxiliary charger.
What is the remote control?
The remote control uses the on board LVD to act as a load control at the command of an
external signal.
The most usual use is for security systems when you want the lights to come on at specific times.
By using a simple timer connected to the remote control, your timer can turn on the load.
i.e. security lights, on and off at preset times.
It can also be used in battery backed pumping systems as a pump on/off control.
What is temperature compensation?
The ability of the battery to store and accept charge varies with temperature.
For optimum battery performance the charge controller needs to adjust its output voltage control
to correspond to the needs of the battery as the temperature changes.
A standard temperature co-efficient used by our charge controller is -4 mv/ Deg C/cell.
Other temperatures co-efficient are available.
Is lightning a problem?
Lightning is always a problem. After about 20 years of designing power electronics,
I have seen small signal MOS or Microprocessor related devices get fried by nearby strikes.
The cause is usually the parasitic SCR structure inherent in the MOS process gets triggered
by the EMP burst- and you buy a new one.
The industry has made tremendous jumps in improving these technologies, but I still shy away
from them.
Solar Converter Inc. Charge Controllers are all analog / transistor based.
These units have been tested to and passed ANSI 62.41 6KV- simulated direct lightning strike.
The test itself utilizes a very large capacitor and is charged to 6 KV (6000 volts),
then it is placed on the terminals of the unit. This is both across the PV input and
both PV input to earth ground. Personally speaking, without massive precautions,
I have yet to see or hear of a MOS or Micro device pass this test at the 500 V level
rather lone 6 KV (6000V). Forget about strikes in the area affecting this equipment -
it will take something a lot heftier and a lot closer to take out this equipment.
History / Reliability?
While not perfect, units have exhibited excellent reliability. Our returns are low.
Usually we find returns are from reverse battery voltage without a fuse or breaker,
connected to 120 V AC instead of PV , or just plain knocked around.
One unit came back looking like it grew fur from the copper strands that sometimes
happen when wire is poorly stripped with the wrong tool. To date we have fixed any
and all known problems.
Can the Power Trackerô run from a windmill?
Yes, but when the battery is full, the windmill voltage will rise tremendously.
I had one unit, a PT 48 -20 running from a 24 V windmill and the battery fuse was
inadvertently pulled. The windmill voltage hit 250 VDC on a "regulated" windmill.
Needless to say the unit was history.
Whenever a windmill or hydro source is used, a diversion load of some sort that is
not accidentally disconnected is a must. Our higher power units have built in load
diversion controllers to clamp the input voltage if it goes too high.
Remember to connect the load diversion resistor.
How do I get the Power Trackerô to equalize if I want to?
The dual float charge technique in theory needs no equalization.
In practise it's always good to give your battery a stir every now and then - talk to your
local expert. The simplest way is to use a switch to short the temperature compensation
input through a 4.7 K resistor. The batteries will charge to higher voltage.
When manually equalizing DO NOT FORGET ABOUT IT.
Excessive equalization will destroy your battery.