Charge controller buffer when overpaneled?

[WalkaboutTed]

I've read that under the right conditions, solar panels will produce more than their rated capacity, in particular with cold temps and altitude. I always said to myself that I never, in my right mind, would voluntarily be there, given that I'm a wussy as far as cold is concerned.  

Well today is one of those days. We've had moments of snow interspersed with clouds and then brilliant sun.  It's almost noon and it's only 40°F and we're at 7500'. My overhead panels are ostensibly 360 watts, but they've been going above that (10-15 watts) on and off all morning, according to my charge controller (Victron 100/30 with Bluetooth).   I have 30a fuses both inside the charge controllers and on the wiring. Normally I would be tempted to lower the voltage to reduce the input, but I've made 5 cups of coffee on the Keurig, run the microwave for 7+ minutes and boiled a litre of water in my electric hot pot, on top of normal use. So I need every watt to refill the batteries today.

My question is, what is the normal percentage buffer that a charge controller can take before I fry it? I can't find the numbers anywhere online.  I hope that isn't a problem today as the numbers are bouncing all over the place, that there isn't sustained sun, but what if? 
Thanks, 
Ted

 

[bullfrog]

I believe most electronic devices have a fairly high tolerance but I would call Flagstaff Wind and Solar as I have a 305 watt panel with the same Victron controller as you. They should be able to give you an answer as they are dealers for the brand.

 

[Ticklebellly]

Solar panels are constant voltage devices in the sense that they offer voltage to the controller. They are not constant current devices in the sense that they do not output current in direct proportion to the solar energy collected. The controller draws current from the panels as a function of the demand put on the controller by the battery to be charged. Essentially, you cannot over-panel a controller. You can under-spec a controller if it is used to charge a deeply discharged and large battery bank.

You can apply the thinking to the situation where you have a dual input DC to DC charger. The alternator input side may be connected to an alternator that is capable of putting out 200 Amps but the charger output spec is 20 Amps. Same same with large solar panels input to the controller. The combined wattage of the panel may suggest the panels could supply well in excess of the controller panel output spec but the current drawn from the panel combination will be determined by the needs of the controller.

 

[Seminole Wind]

you cant make a blanket statement like that as to over powering a controller. some controllers will try to pass all available power and then trip or melt if too much goes through. for example the bluesky 3024 is rated for 30 amps. when going into a 12 volt battery that is right around 400 watts. i have had 405 watts of kyocera panels trip the over current protection on it many times. sometimes it would take a day or 2 to reset. once i figured out what was happening i put less panel wattage on it and have been fine.

it is not just high altitude and cold that will bring out the big watts from a panel. last year at the oregon gtg at tillimook in july i had some 250 watt LG panels that were putting out 265-270 for a few hours during the peak. these were just laid flat on a 90 degree day at sealevel.

how much a charge controller can handle is different with each controller. i have mostly victron controllers. both 100/30 and 100/50 they are self limiting to a point. in theory they can just maintain their rated amps by runnng up the voltage on the panel to reduce current just like when they do it during absorb.

in practice, it is not a perfect world. right now, i have 1180 watts going through a single 100/50 into a 12 volt bank. i am in the coastal pacific north west and during the winter it was not an issue. here now in april we are seeing some sunny days that i can see max out the charge controller at 50 amps. most i have seen while watching is like 50.5 and it seams to handle it just fine. i plan to do an A/B/A test next time the rain stops to see how much they are capable of putting out so i know just how over powered they are. i did have 1 time where i had seriously discharged my battery bank. and then the batteries were real hungry for charge and the controller was running full tilt for a while. at some point long before full charge was reached, pretty sure it was still in bulk. the charger faulted with an error code for "over current" at that point it dropped into float at 13.5 volts and the batteries were still accepting 18 amps at that low voltage. i didnt see how to clear the error or convince the controller to jump back into bulk so i initiated an equalization cycle and it jumped right up to 50 amps and around 700 watts. the victron eq cycle is only an hour so i did one more of those before the sun got low. managed to put 4.7 KWH into the batteries that day from the 50 amp controller.

sofar the victron has withstood this abuse with just the one over current fault which did reset it's self over night and worked as normal the next day

a 3rd mppt controller i have is a 30 amp Chinese PowMr i have run 600 watts through it down at the rtr this year with the panels angle spot on at mid day. on the sunny days i was seeing 37-38 amps going into the battery, from a 30 amp rated charger. no melt downs or faults. a couple time the cooling fan did come on

your millage might vary, but this is what i have seen on my systems

 

[jimindenver]

There is more than one way to "over panel" a MPPT charge controller. The first is the one everyone thinks about, watts that relate to available amps. The program in the controller manipulates the voltage of the panels to stay within its limits. Morningstar was the first to advocate it because it allowed you to get more power early and late in the day when the panels were not producing their full potential. What I do not like about it was before Morningstar said this we had the 20% buffer rule of thumb to keep from stressing the controllers. A controller producing its peak output is going to also produce the maximum amount of heat and a over paneled controller does it daily.

While the first type of over paneling may affect the controller over time, the second kind can cause a more instant result. MPPT controllers have Volts open current (Voc) limits and many will die if you surpass them. Bob tells of a person that traveled over a pass at 12,000 ft on a cool day and it did fry his controller. I have seen a freezing panel spike at 4 volts higher than it was rated for and I always expect a boost from high altitude. The combination of both, especially if your panels are in series can easily push you over the controllers limits if you are already pushing it.

A few manufactures have found ways to protect the controller in the event of being over volts. They simply shut off when the voltage goes to high which is great at protecting the controller but it means that when you expect the most power that you will get nothing instead. So it is still better to get a controller that has sufficient Voc capacity

 

[Weight]

The best advice? Get a controller that is specified to your system. A controller will have specifications for maximum voltage. No controller should put more amps into a battery that is charged. Maybe a cheap controller or one that has failed.

 

[jimindenver]

Over paneling has nothing to do with putting more amps into a charged battery. A charged battery will simply not accept any more but you can still cook it with too high of a voltage. Over paneling has to do with what is available for use and charging in off peak times.

Personally I would think that if you have room for the panel then get a controller big enough to take advantage of it That way you get more power at both the off peak and peak times of the day.

 

[Weight]

The higher the charging voltage the more amps go into the battery, until the battery cooks dry. Then the fun starts.

 

[jimindenver]

This is true. However it does not take over paneling to bring a fully charged bank to over voltage. It takes only a few watts to take a fully charged bank from absorption voltage to say equalization. Even a 100 watt panel can do it as it can provide the handful of amps needed. You are confusing over paneling which is pre controller to over voltage that is post controller. Also there is a difference between over voltage at the controller vs at the bank.

Over paneling is adding more watts at the correct voltage to the MPPT controller. The controller will not over voltage the bank as long as it is set properly.

 

[Weight]

Exactly. Get the controller sized for the voltage of the panels. Don't look for the cheapest controller to save a few bucks.

 

[Seminole Wind]

Exactly. Get the controller sized for the voltage of the panels. Don't look for the cheapest controller to save a few bucks.

it is a bit more than that, as was mentioned earlier. over volting a controller is often fatal to the controller. but most decent controllers can handle some over powering with out damage. (big difference between over volt, and over power) some of those can just keep right on going, others will shut down, to the point of needing to be reset.

there really is not realistic rule of thumb to use. you really need to read the specs for any contoller you plan to use, then read the specs for any panel you might use and make sure they are going to be compatible. and when comparing the 2, remember to give your self a buffer for when the panels might put out more than they are rated for. some regulation require an additional safety margin, it may be wise to do the same if you have expensive equipment or mission critical applications

 

[Weight]

A bit more? Didn't you just repeat what I have been saying? Get a controller sized to your panels.

 

[Seminole Wind]

A bit more?  Didn't you just repeat what I have been saying? Get a controller sized to your panels.

you just mentioned voltage. volts and power, either in watts or amps, both need to be paid attention to.

if the controller is rated for the voltage (plus safety margins) but is way short on amperage capacity, you will at least be limiting you collection and at worst toasting the controller. on the other hand if you run a couple panels in series and your controller is not capable of the higher voltage, you may toast the controller even though it is rated for more than enough amps. so yes, it is a bit more than getting...


"the controller sized for the voltage of the panels"

i imagine you knew what you meant, but some newbie next year reading that could be misled

 

[Weight]

Amps come into the equation on the battery side, not the panels. The controller must be sized for the panel maximum voltage.

 

[Seminole Wind]

Amps come into the equation on the battery side, not the panels. The controller must be sized for the panel maximum voltage.

it must also be rated to handle the amps. both are important when picking a controller. why do you think all the charge controllers come with amperage ratings?

 

[B and C]

I have a 350 Ah battery bank and a 30 amp charger. Do you mean I need to get a 350 amp charger with only 400 watts (200 fixed, 200 portable) total of solar?

 

[Ticklebellly]

why do you think all the charge controllers come with amperage ratings?

In my experience, charge controllers are rated for input voltage and a maximum output capability in amps.    Having a panel array that could potentially supply more amps than the controller is rated to output is nothing to worry about.    The voltage being applied to the controller is also rated while connected to the controller, not open circuit panel voltage.   The little twist to that last bit is the general rule to connect a controller to a battery before connecting the panels to the controller.

 

[Weight]

Yes. Solar Charge Controllers are rated by the amps they can supply to the battery during bulk charging. Nothing to do with the panels. They are restricted by their voltage rating for the maximum voltage they can handle from the panels. That is why controllers must be sized for the panels.

More for the controversy-ists. The reason to connect the controller to the battery first. Some controllers need to recognize the battery voltage and set themselves. Quality controllers have switches that can be set. Panels can have lots of voltage available and then there is a risk of lots of sparks and shock if your drawers are wet. Connecting the battery first gives those sparks somewhere to go. Many solar operators I know and myself, switch the battery and panels with a STDP switch and disconnect both at the same time.

 

[jimindenver]

The voltage limits on MPPT charge controllers are the Volts open current, (Voc) not the working voltage. The Amps a controller is listed at is how much it can pass to the battery and means different amounts of panel wattage depending on the voltage of the bank. With a 24 volt bank a controller can accept twice the wattage as it can with a 12 volt bank. Exceeding that wattage is called over paneling which not all MPPT controllers can handle.

A example of a controller that these numbers affect is the Eco-worthy 20 amp MPPT. It's Voc limit started at 42 volts which was high enough for most of the larger house panels of the time up to 275 watts but was too low to handle two 12 volt panels in series. On the other hand they were over heating if loaded up to the rated 300 watts mostly due to poor installation and undersized cables to the battery. Now the Voc limit is 50 volts and the wattage to a 12 volt bank has been reduced to 275 watts. In other words they changed the wattage to include a buffer and over paneling is a no no.

The ability to connect the panels when the battery is not connected is not universal. The above mentioned Eco-worthy could be moved from battery to battery once set up and running, even provide power to run a load without the battery as long as the sun was shining. As soon as the sun was interrupted the controller would shut down and have to be reconnected to the battery, then set to the proper voltage again before it would work again.

Some PWM controllers like those used on portable panels do not need a battery at all. You can set them in the sun and use the USB ports.

I have seen Victron controllers survive being disconnected from the bank while the panels were connected. In one case the controller would turn on as long as the sun was up and even acted like it was charging the batteries.

Morningstar MPPT controllers on the other hand do not like being disconnected from the batteries while connected to the panels and will not be covered under warranty if it happens and they die. They need to be connected to the batteries and allowed to get running before being connected to the panels, so a simple on/off of the controller and panels at the same time will not work.

So what will or will not work for someone else mat not work for you. Follow the instructions for your controller.

 

[Weight]

Please provide a reference for that information about Morningstar controllers. I am surprised to find out I have ruined my controller, Not.