MPPT vs PWM

Ako

Have a general question as to the different method of operation of PWM and MPPT . I read the MPPT provides more current during Bulk charging by lowering the Array voltage and therefore increasing the Amps going into batteries .

Read the blurb of a Morningstar PWM which said that while charging it maintained a voltage slightly higher then the batteries in order to charge them , surly that's exactly what the MPPT does , i have had Morningstar PWM controllers for 20 years and for the last 4 years monitored thenm online and i can confirm they work that way , during Bulk charging the voltage is always just slightly higher then battery voltage , once in absorption the voltages immediately rises and the current decreases which would indicate that the PWM was also increasing the current by holding back the unnecessary voltage and converting it to current while it was in Bulk , the selling point of MPPY chargers . Obviously once in Absorption theres no difference between the to types pf charger , the batteries are thengoverning the current it will allow in and both will be supplying more and throttled .

What am i missing .

Photowhit

Actually, neither maintain "a voltage slightly higher then the batteries in order to charge them". Lead acid batteries electrochemical nature do the maintaining... 

I guess MPPT do pick the best point that transfers the most current (Maximum Power Point Tracking) but the battery doesn't allow the voltage to go above a point transferring current. The voltage does slowly rise during charging until it reaches about 80% full then the current will 'run away', this is the point that is set by you on either charge controller (or preset on the simplest) that the charge controller steps in and holds the voltage. It is the point the battery charging switches from 'bulk' to 'absorb'. At this point the because the voltage is held constant the current flowing is reduced gradually, until it reaches 1-3% then switched to float.

PWM charge controllers do little until absorb is reached the pulse/cycle the amount of time current is passed to the battery. MPPT do change the power coming into it by tracking the point at which the most current flows into the battery and converter the power from a higher voltage to the lower voltage efficiently preserving the power. It has been suggested by the creator of some of the best charge controllers that they need 30% 'headroom' above the charging voltage to work efficiently.

Interestingly MPPT when holding voltage once the absorb voltage is met, deliver the current PWM.

Basic charging of a lead acid battery;

During charging, there are basically 3 stages of charging, Bulk, Absorb, and Float.

BULK;

First thing when charging starts you will be in bulk, the voltage rises from what ever the system voltage was to a set point, around 14.5 volts. At that point the Charge controller stops the voltage from rising. Higher voltage can damage sealed batteries.

ABSORB;

Once the battery hits the preset point the charge controller keeps it at that point. Your batteries are roughly 80% full. Flooded batteries will start accepting less current at 80-85% full AGM/Sealed may go a little longer before accepting less current.

On many controllers you can set this point, Some will have different presets for Flooded, and sealed batteries, or flooded, AGM, and sealed batteries. 

The charge controller has a couple ways to know when to switch to float, Most inexpensive Charge controller are just timed for 1.5-2 hours. Some will also see less current flowing through the charge controller and shut it down when minimal current is flowing through the controller. On more expensive charge controller. You can set battery capacity to give the Controller a better idea of when to stop. you can also set a longer Absorb time. Or set 'end amps' a amount of amps flowing through the charge controller to stop Absorb and switch to the final stage.

FLOAT;

Once the Controller has determined the battery is fully charged it reduces the voltage to a point where very little current is flowing to the battery. This will prevent the battery from over charging and heating up.

While in 'Float' the charge controller watch for voltage drop, which would indicate a load. If the voltage begins to drop the charge controller will allow as much current to flow from the panels/array to compensate and maintain the voltage. If the voltage can be maintained, the load will in essence be running directly off the array/solar. If the voltage drops below the preset float voltage, the controller may start a whole new cycle if it stays there for a period of time.

mike_s

PWM simply connects the panels to the battery until the desired voltage is reached. It then quickly switches (PWM) the connection on and off to maintain that voltage. You lose some efficiency because the panels aren't usually operated where they make the most power. But the controller itself is very efficient, basically just a switch. How much lost in panel inefficiency depends on the panels. You can look at the IV curve to get a good idea, but 25% loss is in the ballpark for a common "12 V 100 W" panel.

MPPT calculates (Tracks) where the panels create the most power, operates them at that point (the Maximum Power Point), then electrically converts that to maximize the current going to the battery. So, they operate the panels very efficiently. But, the controller's conversion process has some inefficiency. That's typically less than a 10% loss.

Is that gain in efficiency worth the added cost of MPPT? For small (a few hundred watt) systems, it may be better to just add another panel to a PWM system. If the panels are located any distance away from the controller, MPPT can allow less power loss in the wiring (voltage drop). For larger systems, MPPT can easily pay for itself.

>during Bulk charging the voltage is always just slightly higher then battery voltage

That's the case with either. If there's current flowing, the resistance of the wiring means there will always be a voltage difference between the controller and the battery. Ohm's Law.

NANOcontrol

A lot of parsing of words. Voltage is always higher or current doesn't flow.  With PWM, charging current is always Isc.  What can be controlled is the average current.  If desulfators worked that well, the PWM's pulses would make a lead battery work forever. I question whether PWM is that good for Li chemistry and whether BMS react well to it. I had some Morningstar PWM controllers till lightning took them out and was happy with them. The software of many of the lower cost MPPT controllers is appalling in their ability to track.  You need multiples of battery voltage as an input before MPPT is useful.

littleharbor2

 A big bonus with using MPPT controllers is the fact that your wiring is much simplified. There are more and more high voltage limit controllers  (450 - 500 volts ) that allow you to wire your entire array in series, send that high voltage a long distance if needed, on small gauge wire in some cases eliminating the need for a combiner box.
 The "All in one" inverter/charger/controllers are really making off grid solar easier and simpler with their high voltage MPPT chargers.

Photowhit

littleharbor2 said:

The "All in one" inverter/charger/controllers are really making off grid solar easier and simpler with their high voltage MPPT chargers.

It will be interesting to see how long they last, first one to 20 years wins... Of course, my high frequency controller is still going strong after 20 years with a replaced cooling fan...

706jim

Not really the same thing but I have to put out a word for my adorable Trace DR series inverter. Still going strong since 1994!

Yes 20 years plus 8.

Ako

Photwhit , bet yours is a quality make , my Mornigstar 30 amp PWM is also going well after 20 years between myself and the friend i passed it onto .

706 , just upgraded from a Solener Pure sinewave after 20 years continued use .

Paying for quality is cheaper long term not to mention convivence , where we live in Spain if something packs in you have a couple of days minimum before it can be replaced .

Photowhit

Ako said:

Paying for quality is cheaper long term not to mention convivence , where we live in Spain if something packs in you have a couple of days minimum before it can be replaced .

Crap (...or quality) usable spares can be awesome in such a situation!