MPPt vs PWM

[Sailsam]

I have six solar panels on my boat, a total rated capacity of 420 watts. I had a Xantrex C35 controlling them. After 3 years of flawless performance it failed and fried the batteries. Now there is MPPT and PWM. The Solar City website gives the following information:
"Maximum power point tracking (MPPT) charge controllers will convert a higher voltage DC solar panel output down to a lower voltage needed for charging batteries. Constantly monitoring both the solar panels and the battery, solar MPPT charger controllers help you optimize your energy collection."
"Pulse width modulators (PWM) are a type of solar charge controller that slowly reduces the charging current to avoid overheating the battery after it has reached the regulation setpoint. At the same time, the system continues to send the highest amount of energy over the shortest period of time , which results in rapid charge and high efficiency. Essentially, a PWM charge controller helps to increase charge acceptance of the battery while maintaining high battery capacity for a longer period of time."
I've two sets of questions:
Which charges faster? Approximately how much?
It sounds as if maybe the PWM would protect the battery from overcharging better? Is it a :fail safe" technology, one that wouldn't charge at all if the controller failed rather than to allow an uncontrolled voltage to boil the batteries?

[Sunking]

I've two sets of questions:
Which charges faster? Approximately how much?
It sounds as if maybe the PWM would protect the battery from overcharging better? Is it a :fail safe" technology, one that wouldn't charge at all if the controller failed rather than to allow an uncontrolled voltage to boil the batteries?

Forget about what you read because it is mostly false, and I would not buy anything or take advice from this company because they do not even know the basics.

A MPPT charge controller is the fastest because it is the most efficient. A MPPT controller is just as safe if not more so than a PWM controller. If a PWM controller fails it can fail either shorted or open. If it fails shorted will likely destroy the batteries.

A PWM controller is in a nut shell a on/off switch that modulates the on/off pulse width to regulate current and voltage. When it turns on it connects the panels directly to the battery. Its huge flaw is input current = output current. So lets say you have a standard 100 watt 12 volt battery panel with a Vmp = 18 volts and Imp = 5.55 amps. Assuming the battery is demanding full charge power from the panels can recieve up to 5.55 amps charge current. Th epanel voltage will = the battery votage of 12 volts. You 100 watt panel is delivering 66 watts to the battery. Use say a 100 watt 24 volt panel rated Vmp = 36 volts Imp = 2.77 amps on a 12 volt battery and you get 33 watts from a 100 watt panel.

A MPPT is a true dc-to-dc converter. It can use a standard 12 volt panel if you wish, or a 100 volt panel, and any 100 watt panel will deliver 7.91 amps to a battery or 95 watts.

[inetdog]

It sounds as if maybe the PWM would protect the battery from overcharging better? Is it a :fail safe" technology, one that wouldn't charge at all if the controller failed rather than to allow an uncontrolled voltage to boil the batteries?

Expanding a bit on Sunking's excellent response:

If anything, MPPT might be a better failsafe for the following reason (admittedly very improbable and artificial):

If you put four panels, designed to charge 12 volt batteries, in parallel, then a failure of the PWM controller in the closed switch position would try to send the maximum panel current to the batteries as long as the battery voltage stays below Vmp and/or Voc of the panels. That would be a lot of current and will boil the batteries quickly. And as long as the current stayed below the expected CC output, no fuse or circuit breaker could safeguard the batteries.

If you put the panels in series and use an MPPT controller, the panel output will only be the current of a single panel but at a higher voltage. If the MPPT controller fails in a way that connects the panels directly to the battery, the uncontrolled current will be only 1/4 of what it would be in the first case, and would boil you batteries more slowly.

That said, that sort of failure inside an MPPT controller is really very improbable, and would be more likely to happen as the result of a wiring error. If the MPPT controller is still working almost normally but has lost its output voltage sensing or control, it will could have even more current to provide to the batteries than the PWM controller would.

If the loss of your batteries would be critical (like when on an extended sailing leg), then I would recommend that you install a totally separate monitor which will alarm when the battery voltage exceeds a preset threshold. (It might actually have to monitor both voltage and current to do the job in a failsafe way.)

[Sailsam]

At present I have 6 panels, 2-60 watt and 4 - 75 watt, all in parallel. It sounds as if you're recommending that I wire the panels in series rather than in parallel as they now are and use an MPPT controller. Is that you recommendation?
Can you recommend any particular controllers?
There is little problem from controller failure boiling the batteries on a passage. The problem occurs when I leave the boat for a few days with food in the refrigerator. I'd rather lose the food if the controller failed than lose the batteries AND the food, which is what happened.

[inetdog]

At present I have 6 panels, 2-60 watt and 4 - 75 watt, all in parallel. It sounds as if you're recommending that I wire the panels in series rather than in parallel as they now are and use an MPPT controller. Is that you recommendation?
Can you recommend any particular controllers?
There is little problem from controller failure boiling the batteries on a passage. The problem occurs when I leave the boat for a few days with food in the refrigerator. I'd rather lose the food if the controller failed than lose the batteries AND the food, which is what happened.

I understand and sympathize!

Since the panels are not all the same wattage, you would have to look at the voltage and current specifications of each of them to see whether you could put them in series. For that to work the Imp (maximum power current) for each of them would have to be within 5% of the others. And you might have more problems from unequal shading of the panels than if you keep them in parallel.

Whether you should keep them in parallel and change to an MPPT charge controller depends on what the Vmp (maximum power voltage) for each of the panels is. If they are designed to charge a 12 volt battery, you will still get some additional battery charging capacity from the panels by using an MPPT controller, but you will need to keep the large size wiring from panels to CC to avoid voltage drops.

[Sunking]

You have no choice but to run all the panels in parallel.

[Mike90250]

if a sailboat, all panels must be parallel, 12V charger panels (19V output). Reason is, shadows from masts and stays will disable a series panel array. If the C35 lasted you a good many years, a simple replacement should do as well. When a component fails, sometimes it takes others along with it, too bad it was the expensive batteries.

[Sailsam]

Mike90250 - yes, it is a sailboat. So I guess you've answered about series vs parallel. The C35 lasted almost 3 years, then fried $400+ of batteries. I don't like that performance.
In parallel the panels total published rating is 420W or 28A. In the Sea of Cortez in the beginning of July 2008 I got, according to my Link 10, 27A. Right now I am using only 2 panels in parallel, the others are not connected, and the Xantrex C12 that I had kept when I went from 2 - 60W panels to the full 6 panels and C35 in 2006.
What do you think of the Blue Sky Solar Boost 50 MPPT Charge Controller?

[Mike90250]

What do you think of the Blue Sky Solar Boost 50 MPPT Charge Controller?

I've not used one, but have heard they are OK.

But, there is not much reason to use MPPT with parallel panels - unless you have the high voltage grid tie panels, that are in the 50VDC range, then you would see a benefit, reducing the interconnect wire gauge, saving weight and cost of copper. The MPPT converts the high voltage parallel panels to battery charging voltage, and you are set.
BUT the MPPT controllers are much more complex, and have more things and ways to go wrong, than the PWM. Generally, the trace C series have been pretty robust. In a marine environment, many things do not last as long.

Just how did the controller kill the batteries? Did it boil them dry, or fail to charge at all ?

[Sailsam]

MIKE - It boiled them dry.

I've not used one, but have heard they are OK.

But, there is not much reason to use MPPT with parallel panels - unless you have the high voltage grid tie panels, that are in the 50VDC range, then you would see a benefit, reducing the interconnect wire gauge, saving weight and cost of copper. The MPPT converts the high voltage parallel panels to battery charging voltage, and you are set.
BUT the MPPT controllers are much more complex, and have more things and ways to go wrong, than the PWM. Generally, the trace C series have been pretty robust. In a marine environment, many things do not last as long.

Just how did the controller kill the batteries? Did it boil them dry, or fail to charge at all ?