Blocking Diode help ??

2Guido

Ok Guys, go easy...my first post:
My solar array consists of a combination of 144watt flex panels as well as a bunch of fixed glass models (all rated similar voltages, different amps). These wire into a standard breaker box. The two hot feeds from both sides of the electrical feeds in the junction box go into large amp blocking diodes (which get very hot during sunlight), then into a Coleman 440Amp charge controller (set at 24 v) From there, the banks of AGM's get the feed to the inverters. First question, do I need the blocking diodes in this scenario, or can all panels go straight into the controller??
Second,
the batteries are 6v AGM's banked in 4's..What is the minimum discharge voltage so my AGM's don't get ruined. Right now, the inverters will shut down at 22volts. In other words, what is the danger voltage on the combined battery outputs where I need to return to the grid for charging ??
Thank You !!!

Cariboocoot

Re: Blocking Diode help ??

Welcome to the forum.

There is no need for the blocking diodes. They're just eating up Voltage.

As for the minimum Voltage on a 24 Volt system, you can take it as low as 21 but that is pretty much "dead". Personally I prefer 24 Volts under load as that assures the batteries will not go below 50% SOC.

waynefromnscanada

Re: Blocking Diode help ??

Totally agree with Cariboocoot. Leaving it to the inverter to shut down on low voltage to protect the batteries is a definite no go! The inverters shut down on low voltage, yes, but it's to protect the inverter itself, not the battery, and by then, as Coot said, the batteries are basically drained. And unfortunately well on their way to getting ruined
Aim to stop discharging your batteries while they still have 80% of their power left in them, for longest life, but you can OCCASIONALLY drain them down to the 50% level. To take them lower than that is a good indication one should be saving up for new batteries a lot sooner than initially expected.
Unfortunately, the term "Deep Cycle" leads many to think it quite OK to regularly drain dead. Definitely not so if they are expected to last.
Good luck
EDIT:
OOPS! Think I need new glasses
Misread your original post, thought your Coleman was an inverter, not a charge controller.

BB.

Re: Blocking Diode help ??

In general, blocking diodes are not needed for 12 volt systems, are a 50/50 need on a 24 volt system, should be installed on a 24 volt system where the panels are connected directly to the battery bank. This prevents your battery bank from back feeding "significant" current back into the solar array at night.

With the standard charge controllers setup for series regulation (typical for solar panel based systems), the charge controller blocks any reverse current flow back to the solar array--So blocking diodes are redundant and cost power (around 0.5 to 1.0 volt or so drop per diode--which, as you see, generates lots of heat when there is current flow).

The reason for the details is I am not sure about the Coleman 440 amp charge controller you are using. Ignoring the fact that I am not sure that it can really handle 440 amps without melting down internally, there is the question of how the Coleman is configured.

I have seen these setup as diversion (aka dump, or shunt) controllers for Wind Turbines (wind turbines usually are connected directly to the battery bank to provide a permanent load to the turbine--and prevent over-speed in high winds and no electrical loads). A diversion controller connects to the battery bank and "turns on" when the battery is charged and typically feeds power to a resistor (load) bank.

So--it is possible in your system that your panels are directly connected to the battery bank and could be using the blocking function of the diodes at night.

There is also another need for blocking diodes that can make the system a bit "safer" (but not really "legally" safer). If you have direct connect to battery panels and/or lots of panel strings in parallel (typically 3 or more parallel strings), the battery bank can back feed a shorted panel, or the other panels can back feed a shorted panel--And blocking diodes can prevent a short circuit in panel wiring from turning into a fire. (note: for NEC/UL/NRTL, a single diode is not "safe" for use to prevent back current flow, you need another diode or fuse/breaker in combination for "real" safety rules).

If your solar array is configured with a series (as opposed to diversion) charge controller (typical PWM or MPPT solar charge controller)--Then you would normally forget the blocking diodes and add a series protection fuse or circuit breaker per string (typically 3 or more parallel strings) using a combiner box (with protection).

The series protection fuse is usually called out in the solar panel data sheet--And usually runs around 1.56 to 2.0 x Isc of the solar panel (older panels may not list the value to use for a series protection fuse).

Note if you mix smaller and larger panels into a common solar array--You may need a mix of fuse/breaker values for your array (a smaller wattage array may need a 7 amp fuse, and a larger array may need a 15 amp fuse).

Sorry, simple question with a complex answer.

-Bill

2Guido

Re: Blocking Diode help ??

You are awesome for this info. I removed the jumbo blocking diodes from my breaker panel. Now my charge controller is showing a small negative feed in the amp meter (-0.03 amps) which I assume is a small current travelling back to the panels. (Of course it is nightime here) Is this something I should be concerned about ???

BB.

Re: Blocking Diode help ??

If this is a 24 volt battery bank--then:

Power = Volts * Amps = 24 volts * 0.03 amps * 12 hours of dark = 8.64 Watt*Hours per night

That is not very much power loss for a larger system. Note--many current meters are not really accurate at low current ranges--So, the 0.03 amp flow may be real or not (or larger/smaller). Did the meter read "zero" when the array was disconnected?

But you should replace the diode pack with a combiner box+circuit protection.

Longer term, you may want to review your Coleman controller and how everything is working together. There is the general issue that series charge controllers tend to do a better job of "properly" recharging a battery bank vs a shunt controller.

If you have a wind turbine--You could connect a Series Solar Charge controller between the panels and the battery bank. And use the Coleman (shunt?) controller for controlling excess wind turbine power. Set the shunt controller voltage just a bit higher than the set-points for the series/solar charge controller.

Note there are many series solar charge controllers that can be setup for shunt/diversion/dump load control too.

-Bill

niel

Re: Blocking Diode help ??

bill is right that this isn't a large loss, but it can get bigger at the time of the winter solstice and adding cloud cover for many days could up the total back feeding losses you see. you will probably find that the controller itself will also draw power from the batteries. if all of these losses are not acceptable you could disconnect the cc from the batteries with disconnecting the pvs first so as to stop all losses at night. to turn on in the morning start with the cc first and then turn on the pvs to the cc.

other controllers may eliminate the back fed power at night and may have lower operating requirements to leave the controller on all of the time. i wasn't aware coleman had a 440 amp controller and to my knowledge nobody does.

Rybren

Re: Blocking Diode help ??

He might have one of these.

440A 12/24/48V Wind/Solar Diversion-Charge Controller with Meter

2Guido

Re: Blocking Diode help ??

Rybren wrote: »

He might have one of these.

440A 12/24/48V Wind/Solar Diversion-Charge Controller with Meter

YES...this is my controller...and to answer both posts together, the amp meter is at 0 with the breakers on disconnect with a 24volt system. The batteries are re-charged back from the inverters (3 pro-sine 3K inverters DC paralleled wired). Right now I am reading 28.4volts with negative 0.03 amps. The mains utility auto-disconnects each morning at 5:00 am, then the inverters go online until 6pm and auto-cut back to the grid and begin the charging process....I am bit concerned over discharging the 6v AGM's beyond acceptable limits....Any help to improve the system would be gretly appreciated..
thanks again

BB.

Re: Blocking Diode help ??

I am a big proponent for Battery Monitors (Victron is another good brand). And I would suggest they are just about mandatory for monitoring sealed batteries.

They are not perfect (calibration/setup -- Read the instructions)--But do make day to day management of sealed batteries (and even flooded cell batteries) much easier.

-Bill

Rybren

Re: Blocking Diode help ??

The wiring diagram for this controller calls for a blocking diode between the panels and the controller - NONE are built in to the controller.

There is a note that states that the VOC of the panels should not exceed twice the system voltage.

niel

Re: Blocking Diode help ??

am i seeing things or is that a multivbrator type setup? it says relay and there's no way the components in that circuit board could possibly handle high currents like they advertize. i think they are regulating the on time of the relay to hold the voltage. primitive pwm.:roll: how quaint.:p now i've seen everything.

if you wish to use that (cough) controller then take my suggestion and just have a switch on the pv input and the battery input to the cc. my gut says there won't be any concern with not switching out the pvs, but just in case you should do it too with the priority being a switch on the battery input to the cc to prevent losses. this will not be automatic and a different cc would've made life allot easier on you.

BB.

Re: Blocking Diode help ??

The Coleman uses a large 12 volt solenoid relay (contactor--metal can with a couple heavy + control wires connected to it in some pictures) to turn on/off load (or solar array if series connected). I think that is where they get the 440 Amp rating (from the contractor ratings).

-Bill

Cariboocoot

Re: Blocking Diode help ??

Given the specs of that controller and the lack of any wind turbine on the system I'll agree with Niel; best thing to do is change the charge controller. That one is not very good for PV.

2Guido

Re: Blocking Diode help ??

Ahhh...so my controller sucks!! how about a recommendation for a replacement. I am already up to around 2KW of PV and growing..hoping to achieve around double that within the new year so am looking at a fairly heavy duty MPPT..Thanks !!

Cariboocoot

Re: Blocking Diode help ??

Review time!

You have a 24 Volt system.
You have an assortment of panels presumably wired for 35 Vmp +/- strings totaling 2000 Watts. This means the current is likely to be just under 60 Amps maximum from the panels. An MPPT type controller might bring that over 60 Amps, depending on exactly what the panel Voltages are.

Depending on the actual panel specs and number of each type there may be some advantage in reconfiguring, but lets leave that aside for now.

So minimally you'd need a Morningstar TriStar 60 Amp PWM controller: http://www.solar-electric.com/tr60amp12244.html
Top end a MidNite Solar Classic 150: http://www.solar-electric.com/mnclassic.html

Inbetween the two there's the MidNite 'Lite' version, Outback FM60, Morningstar TriStar 60 MPPT, Xantrex XW 60 MPPT, Xantrex C60 and probably a host of others I'm not thinking of at the moment.

With that large of an array the MPPT type will probably show some advantage over PWM. Kind of have to see all the panel particulars though.

BB.

Re: Blocking Diode help ??

Before you spend money--Sort of need to know more about your needs...

PWM Solar Charge Controllers are very similar to the one you have, except they use FET (field effect transistors) instead to turn power on and off... This allows them to do it hundreds of times a second and use less power than a mechanical relay can... Of course, there are limits (around 60-90 amps max for a "large" FET based charge controller vs the "440 amp" relay).

If your array is pretty close to the battery bank (charge controller should always be next to battery bank and inverter). And it is not too hot in your area and you can get cost competitive solar panels with Vmp in the range of 17.5 to 18.6 volts or so--Then a "simple" PWM controller will be the most cost effective.

For MPPT controllers--Their big attraction is that you can:

1. Run much higher Vmp-array voltages (~100 to 250+ VDC depending on brand and model of charge controller). This will cut the current in the wiring from the array back to the charge controller (2x array voltage, 1/2 current solar array current--Allows you to send the power 4x farther or use 1/4 the gauge (6 awg size reduction) vs a "standard" PWM controller.
2. The new "low cost/high wattage" solar panels are typically in the Vmp of 30-40 volt range--That usually does not match the required most efficient Vmp-Array voltage for 12/24/48 volt battery bank (around 18/36/72 volt Vmp for standard battery bank voltages). So, instead of paying $2-$5 per watt for smaller "18 volt panels" in the ~140 watt maximum size, you can instead buy at less than $2 per watt for 250 watt panels instead. The draw back is you need a separate MPPT charge controller for every "different" Vmp panel rating (one for a 30 volt array, a second for a 40 volt array, etc.).

The glossy brochures will tell you that MPPT can give you 30% more power--Technically yes if you are in sub zero F envrionment--But for most installations you are probably looking at ~10% or so increase in power output (vs a well matched PWM + Vmp~18 volt array).

There are some other "nice things" about MPPT controller and higher volatge arrays such as high ambient temperatures where a 17.5 volt Vmp panel may have problems equalizing a cool 12 volt battery bank on a hot summer day. Also, many of the new controller designs/functions are available with the "new" MPPT designs (and more expensive). More extensive logs, Ethernet/Web Server, additional charging options/set points/syncronizing of operation, etc...

More or less, for system solar arrays with less than 200-400 watts should be built with PWM controllers (from a price point/efficiency of view). For systems over 600-800 watts, typically it makes more sense to use a MPPT charge controller.

But there is no hard and fast rule. MorningStar has a very nice 15 amp MPPT charge controller (12/24 volt battery support) that can connect ~270 watts (max cost effective) worth of solar panels very nicely to a 12 volt battery bank.

-Bill

2Guido

Re: Blocking Diode help ??

Yes..
A bunch of different panels same voltage (35v) paralleled together and combined on the roof, then fed down 2gauge copper cables to the controller. Since I removed the blocking mega diodes, the current significantly increased today showing around 40plus amps...These charge 3 banks of four 6volt AGM's paralleled configured powering the Prosine 24v inverters. The Inverters auto cut-over each morning off the grid, then return at dark and the internal chargers top off the system...