Different rating panels in series

Re: Different rating panels in series

Welcome to the forum Jamaican Cherubim!

I would like a little bit more information to help... First what is the Vmp/Imp (voltage maximum power/current maximum power) ratings for your panels.

In general--Most of the "24 volt panels" have Vmp in the range of 30 volts (and that is when the panels are cold... they can easily drop another 10-20% when hot)... That is not really high enough to properly recharge a 24 volt battery bank which needs >30 Volts charging input.

So, if your present panels are in parallel--Then I would suggest you rewire the array and place them in some sort of series (all three panels in series) or series/parallel setup (and you may need another panel to get 2 sets in series, and then place the two strings in parallel).

But, don't do anything yet--Need the panel details first. Since your are using an Outback FM or MX 60 MPPT charge controller, that gives you some options that an "inexpensive" PWM controller would not.

-Bill

Re: Different rating panels in series

Well... There are lots of things going on here. And, it is, during parts of the day (and based on your battery bank state of charge/power usage), certainly possible for it to work--sort of...

Ok, but the reasons why things are the way they are. First, solar panels are (more or less) "Current Sources". The current output of the solar panels (between Vmp and Zero volts) will output a (maximum current) that is proportional to the amount of sunlight hitting the panel. In first light from the sun, the current is near zero amps. At high noon on a clear day, it is very near Imp (current maximum power) between zero volts and Vmp.

This is entirely different that a lead acid battery works. A battery will attempt to hold from 11.5 volts to 14.5 volts no matter if you are drawing zero, a small amount, or enough current to start your car. And it will also hold its voltage even if you start pumping current into the battery too. (and ideal battery would hold 12.000 volts... A real battery will have some variation).

So--Back to what you have. Your original array had two ~8 amp panels in parallel and they would output 16 amps at high noon (maximum Imp current). Then that group of panels in put in series with a single panel that only has an Imp rating of ~8 amps. Trying to get 16 amps through an 8 amp panel is not going to happen in the "normal way".

Now--We have to do a little debugging. First we have a charge controller in the system connected to your battery banks/loads. The charge controller has a variable input (the sun on solar panels) and a variable requirement (battery charging and loads). Just looking at the array current at a random time during the day or night is not going to tell us a whole lot.

What is first of interest for us--Is the solar array working correctly. Now that you have removed the one panel, what is the Imp-array current that you see when the sun is bright on the panel (say 10am-3pm) and the batteries are not fully charged (and you may have a heavy load on them too). Ideally, we should see somewhere around 80% of 16 amps or ~ 12.8 amps or so (or more). If, when the batteries are fully charged and/or there are no heavy loads in the middle of the day, the charge controller is going to decide that it does not need full power from the solar array, and the average current will drop (and Vmp array will rise somewhat).

If, you never see more than ~8 amps from your solar array (and the loads are heavy and/or the battery bank is significantly discharged), then I would worry that one or more of your parallel panels is not working correctly (bad panel, bad wiring, etc.).

The way you have your array wired is sort of difficult to diagnose. It would be easier if you connected your strings in series first (panel A to panel then connected those series strings in parallel. That way, if we find a string that has no current flow, we know that one of those two panels (or their wiring) has a problem.

And, when you put three or more parallel strings together, you should have a series protection fuse (or circuit breaker) in each series string to protected against a shorted solar panel (reduce the risk of fire). Anyway--That is for a later discussion.

For now--If the array is easy to get to. Take a piece of cardboard (or dark cloth) and cover one panel at a time in the middle of the day. You should see the current drop the same amount (roughly) for each panel covered (have somebody look at the charge controller and read the array amps to you as you cover each panel). Do not be surprised to see the current drop by 1/2 if just one panel is covered. The other panel in the string does not have enough voltage to charge the battery bank when its mate is covered.

I can go into more detail about the charge controller and how it works with your loads and such--But trying to keep post short and on point.

Regarding the 235 watt panel--Sure, you can put it on the C35 controller and wire that controller directly to the battery +/- bus just fine. Depending on the local temperature and state of charge for the battery bank, it probably will (on average) generate less power than it is capable of (single 29.5 volt panel is not enough to truly work at 100% with a 24 volt battery bank that needs ~20-30 volts to charge). Ideally, if you can get a second 235 watt panel and wire it back in the two series strings you currently have--That should work well on the Outback controller.

Second--The end result of your solar panels and your management of your off grid system is to keep the battery bank "happy". That means it should not be taken below ~50% state of charge very often, and should be recharged >90% State of Charge once or twice per week. (note, over charging is not a good thing either).

You can estimate how well a battery is doing by measuring its resting voltage (no charging/discharging for 3+ hours). But, assuming this is a flooded cell battery bank, you should have some sort of Glass Hydrometer to measure the temperature corrected specific gravity of the battery cells (and log the data). The solar panels, charge controllers, inverters and such are usually fairly forgiving. The Batteries are a different story. There are a dozen ways you can "murder" a battery bank. So, I would suggest that you get a good handle on your battery bank's state of charge and how much energy you are using (daily basis, or as a backup for unreliable utility power, etc.).

http://www.windsun.com/Batteries/Battery_FAQ.htm
http://www.batteryfaq.org/
http://batteryuniversity.com/

Also, I would suggest a less expensive DC Current Clamp meter (like this one--really a multi function volt/ac dc current clamp meter--If you go to the states is not very expensive from Sears) if you are going to do a lot of your own maintenance on your system. It makes it very easy to see how (for example) each solar panel is working--just clip the meter around one wire and read teh current (no cutting wires, no shock hazard, etc.).

-Bill

Re: Different rating panels in series

Jamaican cherubim wrote: »

Thanks again, I learnt a few things today, I was thinking if a solar array has say 3000 w coming from it then the charge controller would have displayed 3000 as its output I but you are saying that it would depend on the load and the depth of discharge of the battery.

That is correct.... There is available energy from the solar array (how much sun is hitting it). And there is how much power the battery bank+loads will absorb.

The charge controller, more or less decides if the battery is not charged, it will attempt to hold ~28 volts when the sun is up. If there is not enough sun, the controller will simply let through as much current as it can.

If there is enough sun, and the battery+loads need less power than the solar array has avialable... The Charge Controller will "hold" 28 volts until it thinks the battery bank is fully charged (more or less if it can stay in "absorb" stage for 2-6 hours--depending on your settings).

At that point the Charge Controller will drop to "float" voltage and attempt to hold ~27.2 volts until the sun goes down.

Note that the charge controller also will change the "charging voltage set points" based on temperature (ideally, you should use a remote battery temperature for best controller of the battery charging voltage).

my battery bank is often times full and I don't allow it to go below 50 %, I don't hav any instrument to indicate to me the % of battery charge that have but based on extensive reading here and other forums I get the idea that 50% charge on my"24v" battery bank would be some where around 24v and 100% wud be around 25.2-25.3v.

Yep, battery voltage vs current flow vs state of charge vs temperature vs age--etc.. is a real mess. If you pay attention to all of the major items (direction and amount of current flow, the present state of the charge controller)--You can do pretty well guesstimating the state of charge of your battery bank. But, at the very least, you should use a good quality glass hydrometer to measure the pilot cell every day to once a week--And check all the cells around once a month (when you check water levels, before you add the water). Here is a set of charts that give an example of how the parameters relate:

New poster "leaf" has a really nice set of charts that compare battery voltage against different rates of discharging and charging (as well as resting voltage readings).

leaf wrote: »

Am trying to upload the charts I am using...

Attachment not found.Attachment not found.

I don't quite a agree with the resting voltage line (I think the voltage is a bit low)--But it shows how to estimate a battery's state of charge while operating.

Note, where the charts "flatten out"--the room for error estimating state of charge is pretty high.

Seeing it that am a bit closer to the equator than you guys in the states I get more sunlight hours and its basically summer conditions all year long, although the imp says close to 8 amps on each panels I keep getting 9, even 10 amps from each panels at peak hour, maybe multimeters are not that accurate on checking amps.

If you can, either discharge your battery bank and/or turn on a bunch of loads and ensure you are taking all of the current that the Outback controller can supply (around 11am-1pm). At that point the controller should be pulling all of the energy possible from your array. And you can log the array voltage and current, as well as the battery bank voltage and current (from the charge controller). At that point you should see Varray*Iarray=Parray--Which usually will be around 80% of your array rated wattage.

If your array wattage on a bright/clear/cool day (with sufficient current into the battery bank/loads) is less than 50% of array wattage--You know that something is wrong.

If the reading is less than ~80% -- Perhaps something is wrong or not (array clean/not dusty, battery bank taking all current, etc.).

If the reading is >80% of array wattage, you are probably fine.

It sounds like something is not right--But we don't know about the "down stream" side of the system at the moment (battery state of charge, lack of heavy loads, etc.).

Regarding using a multi-meter... The meter in the Outback should be accurate "enough" (it may average reading 5% high on output current).

I would not even bother using your DMM as a current meter--Most current flow in your system will be (at times) over 10+ amps and will fry most meters (using a DMM set to 10 amp scale is good for measuring Isc--short circuit--It is a good way of seeing if a used solar panel is probably OK--if it reads near Isc in full sun and has near Voc voltage open circuit--when you have to do a quick check before you buy).

Anyways, I will be getting another 235w panel on friday to balance things and then I will be hoping to double my battery bank's size, its such a pittyy I didn't get a bigger cc, I could beat myself, adding more than 6 panels to the outback 60 doesn't make much sense and the xantrex c series is basically garbage, I don't know if its broken or what because it never showed me the constant green light that it states on the manual that should indicate that the battery is fully charged nor has it ever gone in float mode with that cc, thanks again Bill, greatings from sunny Jamaica.

A 24 volt battery bank and a 60 amp Outback controller can "cost effectively" support a solar array of:

60 amps * 29 volts charging * 1/0.77 panel+controller derating = ~2,260 Watt array

MPPT controllers will not be damaged by a larger solar array--They just limit the current in the middle of the day (a little bit of wasted energy).

-Bill

Re: Different rating panels in series

That looks good. Your array as presently configured seems to be working well.

Getting a second panel to add it should be a nice addition.

Just to calrify terms... Watts and kWatts are a "rate"--Like gallons per hour.

Watt*Hours and kWH are an "amount"--Like gallons pumped

So, 800 watts from your solar array for two hours will be: 800 Watts * 2 Hours = 1,600 WattHours

We are so used to Miles per hour (or Kilometers per hour for the rest of the world), etc... That Everyone wants to call it Watts per Hour or I used 500 Watts today (instead of Watt*Hours).

Similar for Amps and Amp*Hours... Amps is a "rate" and AmpHours is an "amount" or total amount used.

And, yes you have to be very careful with DC power... DC is much better at being an "arc welder" than even AC power. Cover solar panels with cardboard/blanket before making/breaking connections is always a good idea.

-Bill