Stumbling along ten years off the grid

[jdc]

I have three separate 'lines' of solar panels in my array. The first line is two 'strings' of 4 x 80 watt panels (12 volt each) for 640 watts of 48 volt power. They are wired in series (48 volts) and parallel (the two groups) for one 'older established' circuit running to a Trace PWM controller. That current runs into the 'back' end of the battery bank. But, really, each panel is rated at 17.5 volts so, when the sun is shining I am getting over 60 volts.
I am adding 2 more strings of 3 x 240 watt panels (24 volt panels each) putting out almost 90 volts or close to 1500 watts of power. This second 'grouping' runs to the second charge controller. It is an MPPT OUTBACK and I am wiring it into the 'front' end of the battery bank.
I have 12 x 220 ah (8D) batteries forming three banks of 48 volts each. The larger 2-string (240 watts) section is feeding an OUTBACK inverter/charger.
So the question is: any problem with a PWM charge controller putting in juice from the back end of my battery bank while another charge controller (MPPT) goes in the front?
Admittedly, I know nothing. But that is what the forum is for, right? Please be kind....

[ZoNiE]

What do you mean by "Front" end and "Back" end of the battery bank?

[jdc]

front and back

Imagine twelve 12v batteries. Wired for 48 volts, that means three groups of four. But then all three groups are then wired together to increase amperage. Ergo, three groups of four forming one group of twelve with one being directly connected to the MPPT charger. Let us call that 'first wired in' batch group A.
Now imagine another charger supplying current from a different set of panels but this charger is a PWM (Trace). Different characteristics, it seems. So, to me, that means that I should NOT put it right beside the first source of charge (the MPPT) at group A. Instead, I should put it back on the last group of four batteries that I am calling group C. The idea is that any conflict in 'wave forms' or pulses or even levels of charge are moderated by the mass that is the twelve batteries especially with group B in the middle.
I really have no idea what I am talking about but I tend to visualize my batteries as three connected swimming pools and I am currently MPPT'ing into one pool and proposing to PWM into the last one. The middle pool wouldn't feel a thing as it filled.
How stupid is this idea?

[inetdog]

Imagine twelve 12v batteries. Wired for 48 volts, that means three groups of four. But then all three groups are then wired together to increase amperage. Ergo, three groups of four forming one group of twelve with one being directly connected to the MPPT charger. Let us call that 'first wired in' batch group A.
Now imagine another charger supplying current from a different set of panels but this charger is a PWM (Trace). Different characteristics, it seems. So, to me, that means that I should NOT put it right beside the first source of charge (the MPPT) at group A. Instead, I should put it back on the last group of four batteries that I am calling group C. The idea is that any conflict in 'wave forms' or pulses or even levels of charge are moderated by the mass that is the twelve batteries especially with group B in the middle.
I really have no idea what I am talking about but I tend to visualize my batteries as three connected swimming pools and I am currently MPPT'ing into one pool and proposing to PWM into the last one. The middle pool wouldn't feel a thing as it filled.
How stupid is this idea?

That you have the idea is not stupid, but the details of your solution are far from optimal, and your analogy just does not apply.
Look at the battery wiring information at http://smartgauge.co.uk/batt_con.html for a very clear introduction. For two or four strings the "diagonal' method is best. For three strings, it is hard to avoid going to a bus design. In either case, you will get the best efficiency by connecting inverter and CC at the same battery post or bus points.
(With fuses as appropriate. Your battery bank can make a very fine unintentional arc welder or arc cutting torch.)

[jdc]

Hmmmmm.........

THAT makes great battery sense! In that way there is a balanced charge. Thanks for that. It helps a lot but not 100%...... and that is because the 'strings' of panels are so different that I am loathe to connect them all at the MPPT charger. Two strings coming in from 3 x 24 volt panels wired for 72 volts (the Outback sorts it out for optimization) and the old pair of strings two strings of 4 x 12 volts configured for 48 volts. To my mind, I have panlels and I should be able to use 'em. But the panels are so different.
Still I really like that info.....should have done that years ago.

[inetdog]

THAT makes great battery sense! In that way there is a balanced charge. Thanks for that. It helps a lot but not 100%...... and that is because the 'strings' of panels are so different that I am loathe to connect them all at the MPPT charger. Two strings coming in from 3 x 24 volt panels wired for 72 volts (the Outback sorts it out for optimization) and the old pair of strings two strings of 4 x 12 volts configured for 48 volts. To my mind, I have panlels and I should be able to use 'em. But the panels are so different.
Still I really like that info.....should have done that years ago.

You are absolutely correct that connecting one 72 volt and one 48 volt string to the same MPPT input of a CC is not going to turn out well. But if the current outputs of the panels are close you could put them into a mixed series configuration (leaving one or more panels out, sigh).
The rule of thumb is that you can put panels (or strings of panels) in series if the currents are within 5% and in parallel if the voltages are within 5%. In some cases 10% match is good enough.
Your most efficient solution (other than getting an additional MPPT-input CC) may be to take just six of your eight 12V panels and make up another 72V string. Use the remaining two panels as spares or for another project.

[jdc]

heartbreak

OOohhh...the pain....that is something I thought of and just found too distasteful so I bough the other CC. But there is no point in asking for advice if you aren't going to take it so maybe that is what I have to do. Darn!
Just to be clear: the old 'strings' rated at 12-17.5 volts crank up, as a rule to about 60 volts. I know the rating is 17.5 (times four is 70.5 but I have never actually seen that).
The new panel strings are rated 24 - 30 volts or (times 3 = 72 to 90 volts).
Outback will handle (as I recall) up to 140 volts.
I do not see any configuration that gets within 10%. If my older string is putting out 60 to 70, then the newer ones would be up around 90 (the sun shining on them all at much the same intensity). If they are down to as low as 48 then the lower output on the new strings would be 72.
If I do as you suggest and use the parts from the old strings and make up a new/old one of 6 x 12 or 72 volts (which puts out 6 x 17.5) then I am also getting out of the range at 105v. Five of them would be 87.5 and that is within 5%. So, you are saying that 5 panels makes a balanced string? And so I 'store' three? Oooh.....the pain..

[inetdog]

If you have a second MPPT CC, then you are fine connecting both to the same battery bank. Just never connect two MPPT devices to the same set of panels.
Are you having a problem with the second CC?

[jdc]

kinda

Not really. Only have one. The second CC is Trace (PWM). Right now - after our chat - I am thinking of running the Trace CC to four batteries and having them 'isolated' so that they are NOT in the system. Then I would use only 8 batteries in the MPPT managed system and, using a BIG RED marine switch, switch over to the smaller system once in a while to 'spread the load'. In that way, I can use all the panels and actually 'store' power for a while in a bank.

[jdc]

kinda part two

I am in the process as I write of re-configuring. I just took down the old 640 watt strings and I have the whole bunch of panels (old and new) awaiting my decision - which WAS doing the back and front charge - but I decided to check out my theory at the last minute and now I am re-thinking it at the eleventh hour (typical). One thing for sure....because of the physical difficulty of installation (see offthegridhomes.org) and my being 66, I am going to have do this right the first time. I just can't see myself swinging about from the solar frame (about thirty feet up) more than once.

[ZoNiE]

Imagine twelve 12v batteries. Wired for 48 volts, that means three groups of four. But then all three groups are then wired together to increase amperage. Ergo, three groups of four forming one group of twelve with one being directly connected to the MPPT charger. Let us call that 'first wired in' batch group A.
Now imagine another charger supplying current from a different set of panels but this charger is a PWM (Trace). Different characteristics, it seems. So, to me, that means that I should NOT put it right beside the first source of charge (the MPPT) at group A. Instead, I should put it back on the last group of four batteries that I am calling group C. The idea is that any conflict in 'wave forms' or pulses or even levels of charge are moderated by the mass that is the twelve batteries especially with group B in the middle.
I really have no idea what I am talking about but I tend to visualize my batteries as three connected swimming pools and I am currently MPPT'ing into one pool and proposing to PWM into the last one. The middle pool wouldn't feel a thing as it filled.
How stupid is this idea?

Thanks JDC. I kinda figured that's what you meant, but wanted to be sure. Not sure if that really makes any difference since you are all bussed together anyway, But the "Pool" analogy makes sense. From what I can tell, all the different charge sources usually play nice together so long as they are roughly the same voltage., IE mains/genset powered charger and a solar powered MPPT controller. I would *think* the PWM plays nice too, but then maybe the pulsing of voltage may confuse the battery monitor? or the MPPT charger? (Are you using a trimetric or anything like that?)

[Sunking]

There are only 2 reasons to parallel batteries.

1. To make a specified AH rating at a given voltage. Batteries come in sizes from anywhere from 2 AH up to 4000 AH. So unless you are above 4000 AH there is no reason to use parallel batteries.
2. Redundancy for Mission Critical systems so one set of batteries can be taken off-line for maintenance purposes like communications centers, hospitals, military, etc.. Household systems are not normally Critical Mission.

The problem is shorter battery life, maintenance, and installation issues.

[jdc]

Confusing..

Yeah, Zonie...that is the idea. Put in juice at the back end (batteries 9,10, 11 and 12) by way of a PWM charger that is managing a string of solar panels comprised of 2 stings in parallel of 4 strings in series or 8 x 12 volt panels wired for 48 volts. AND put in juice by way of an MPPT charge controller managing two strings of 3 x 24 volts each. That means the two charge controllers 'gather' different voltages from their respective arrays but -after having done their job - put in similar charges to the battery bank.

One HAS to parallel groups of batteries to keep the voltage in the 48 - 52 v range. I can't very well store all the juice at say 144 volts and let the inverter sort then. Is that what you are saying..?

[Sunking]

One HAS to parallel groups of batteries to keep the voltage in the 48 - 52 v range. I can't very well store all the juice at say 144 volts and let the inverter sort then. Is that what you are saying..?

Huh? You do not have to or can you parallel batteries to get 48 volts. To acheive 48 volts requires you to series the batteries. If the batteries are 2 volts, it takes 24 batteries in series to get 48 volts. if you have 6 volt batteries it takes 8 in series, if you have 12 volt batteries it takes 4 batteries in series.

The question is AH, not volts. If you need say 48 volts at 500 AH, then you need to be looking at 6 volt 500 AH batteries and buy 8 of them to wire in series to get 48 volts @ 500 AH. No parallel required or would it be possible to use 8-6 volt batteries in parallel to get 48 volts. It is impossible.

Problem is you are stuck in a 12 volt battery box and need to get out of it. Lead acid batteries are not 12 volts, they are 2 volts.

[jdc]

12 batteries

Three groups of 4 each x 12volt batteries. 3 groups wired to produce 48v each. THOSE three groups are wired in parallel so as to maintain the voltage and increase the amperage. Each single battery has 200 ah. Each group has 200 ah. The three groups together produce 600 ah. Sorry if I was confusing. Trust me, I find this confusing. But I also got the 'go ahead' from Outback. "No problem going in front and back with two different chargers. Go for it!"