Dual System from single PV array

homan7 · July 2009

I currently have a PV Array that connects to a Morning Star PS-30 which charges my flooded cell battery bank. I was given a large bank of AGM UPS batteries. Part of the reason why I took them was that I happened to have a spare PS-30 controller. The PS-30 will let you choose the type of battery but I can't do flooded and AGM at the same time. I did some searching here and came across this thread:

http://forum.solar-electric.com/showthread.php?t=5424

But, after doing some more searching, I found that you can run both controllers from the PV array by running two pairs of wires from the combiner box, each pair going to the respective controllers. I'm pretty sure that my PV array won't be able to handle charging both banks at the same time though.

I guess my question is, is it worth buying the switch and charging one bank at a time and then when the one bank is charged, switch to the other? Or would it be more efficient to just connect both systems to the PV array and squeeze as much juice out of them as I can until I can increase my array?

I'm ASSuming that it might be better to switch between the 2 but I'd like to see what suggestions/thoughts people have.

Cariboocoot · July 2009

Re: Dual System from single PV array

Knowing the size of the PV array and of the respective battery banks would help.

homan7 · July 2009

Re: Dual System from single PV array

Cariboocoot wrote: »

Knowing the size of the PV array and of the respective battery banks would help.

Sorry, the PV array consists of 2 130W panels, the flooded bank consists of 6 T-105s configured for 12 volts with a total of 675 AH and the new bank would consist of 10 100AH 12 volt AGM batteries.

Cariboocoot · July 2009

Re: Dual System from single PV array

The first thing I notice is that 260 Watts of panels isn't enough to keep 675 Amp/hrs of batteries charged, never mind adding any more. Do you have all those Trojan's wired in (2 series = 12V) * 3 parallel? Or are they set as three separate banks? At 675 Amp/hrs you should have almost three times the panels you've got now to provide a minimum 5% of Amp/hr charge rate (roughly speaking: 675 * 0.05 = 33.75 Amps charging current * 12 V = 405 Watts averaged array output [about 60%*] or 675 Watt array).

NOTE: There are engineers on this forum who can give you a much more accurate and sensible explanation of the above!

You are correct that the AGM's are not "charge compatible" with flooded cells. 10 100 Amp/hr 12 V AGM's in parallel would be 1000 Amp/hrs and, using the same rough calculation above, would want 1000 Watts of array.

Even though my calculations here are imperfect, it is clear you really don't have enough PV to charge either. And wiring ten batteries in parallel presents further problems: the wire lengths have to be carefully measured so that the resistance to/from any given battery is equal across all batteries or else they won't charge right. AGM's are very picky about charging.

To my knowledge, you should never put two charge controllers on one array.

I think your system needs a bit of a re-think. First thing: check those Trojans with a hydrometer and see how they're faring.

Photowhit · July 2009

Re: Dual System from single PV array

Looks like you already have more storage than you can keep healthy.

Do you have an outside source of electric to equalize the current battery bank?

Do you only draw electric on weekends?

BB. · July 2009

Re: Dual System from single PV array

From what I understand--The simple on/off and PWM controllers can be paralleled from a single set of solar panels (as long as the battery banks have a common negative ground.

MPPT controllers will most likely not function correctly with paralleled controllers on one solar array.

As always, you should verify with the controller manual/manufacturer before doing non-standard connections (and the company may just say no to avoid confusion and calls from unhappy customers and happy lawyers :roll: ).

-Bill

RandomJoe · July 2009

Re: Dual System from single PV array

Hey, that system is almost identical to mine. I also have 6 T-105s wired 12V. And I started out with two 135W panels as well. I know what the math says, but I was able to keep the batteries up fine with just the two panels. Of course, I couldn't draw them down much at all either.

I now have four 135W panels, and handily replace 10% each day by 1PM (what my regular load is), and can almost do 20% in a full day in summer (had a heavily overcast day a few days ago). But - again - I'm not drawing the bank down very much either. My typical daily total from the CC is 1.6kWh, max I've seen is around 2.5kWh.

Are you intending to USE both of those sets with just the two panels, or do you intend to just keep one fully charged while working off the other? If you want to use both, it won't matter which way you do it you probably won't be able to keep either set charged - simply not enough solar panel.

If you just want to keep (say) the AGM batteries maintained at charge, then I'd parallel the two (long as the controllers are happy with it) as it's not likely to use all that much power anyway, reaching float fairly quickly. (At least, my experience with AGMs shows they self-discharge very little over a week or month, let alone a single night.) I'd certainly give it a shot and see what happened, anyway.

Sounds like you now have a great excuse to get more panels!

Cariboocoot · July 2009

Re: Dual System from single PV array

See, that's the thing; it is presumed that if you have 675 A/hrs of batteries you need 675 A/hrs of batteries.
If you're not using the capacity, then everything is just sitting there being 'trickle charged' against self-discharge and it looks like there's nothing wrong. Eventually the batteries will sulphate, but this can take years. Some people probably wouldn't notice if their bank died in 6 years instead of 8.

BTW, 675 A/hrs is about 4000 Watt/hrs. I wish I had that kind of battery capacity! I run everything on about half that, with 700 Watts of panels and the bank gets cycled down and re-charged every day.

Dave Angelini · July 2009

Re: Dual System from single PV array

I learned this strategy from Dave Surrette (Rolls) in the late 70's. Pretty much the bible on how I design my systems for offgrid.

Assume that the system will never reach more than a 90% state of charge.
Try not to go below 50% SOC, ever! Complete absorption over 90% of the year
Use the energy stored from 70% to 90% SOC for your daily cycles.
Save the energy from 50% SOC to 70% SOC for aging to get long battery life.

I know Surettes has changed their reccomendations over the years but I also know they are in the business of selling batteries! If you do the above you will get 10 to 15 years on their batteries with decent maintenance.

The OP is making it complicated by mixing battery types and not really stating a lot of information that would allow decent specific advice.

Oh yea, I am really happy that Surrette and Trojan are making L16's with 1000 AH capacites @20HR. Been bugging them for many moons to do batteries less than 125LB's!

homan7 · July 2009

Re: Dual System from single PV array

Alright, I'm a little slow on the draw here. I read some responses before I left work yesterday and was going to respond to Cariboocoot something to the extent of what RandomJoe said in his post. I don't seem to have any problem getting the bank back to full (based on voltage only..I know bad..) with the current setup.

That being said, I have yet to check the batteries with a hydrometer to get readings between cells. I will report back when I do. The current draw on the batteries is very minimal. The system is fairly new so I haven't put any real draw on them. Being that this was my first setup ever with solar, I'd actually be really happy if the batteries lasted 6 years. I didn't really expect much more. After doing research on batteries that fit the system I'm trying to setup. I thought that having T-105s last 8 years was doing pretty good.

My current setup (minus AGMs) works fine for what I'm using it for. I really didn't have a purpose for the AGM batteries, but they were free and tha means I couldn't turn them down, so I was just proposing my initial thoughts on how I could incorporate them into the current system having the least impact to my actual system. It looks like I might be better off splitting the AGM bank into two 500Ah banks and connecting the panels to each of the (now 3) banks seperately to "top" them off as they discharge.

My (likely flawed) reasoning is that my PV array seems to be ok with 675Ah of battery, It would have a harder time charging 1000Ah at one time so if I split them into 2 banks then I could charge them seperately to keep them charged (no current draw besides discharge). That way they don't sit there and just discharge, unless that is better for them?

Dave Sparks wrote:

The OP is making it complicated by mixing battery types and not really stating a lot of information that would allow decent specific advice.

It is complicated because of the different battery types, this is why I'm asking for help. If there is more information that you need to know, please let me know what you're looking for. If I can answer it, I'll be happy to.

BB wrote:

MPPT controllers will most likely not function correctly with paralleled controllers on one solar array.

As always, you should verify with the controller manual/manufacturer before doing non-standard connections (and the company may just say no to avoid confusion and calls from unhappy customers and happy lawyers ).

Thanks for the advice regarding MPPT controllers, I hadn't thought about it. The instructions for connecting the 2 controllers to the PV array came from MorningStars tech site so I'm fairly certain it will work in that setup. I was really trying to find out if setting it up that way made better sense than switching to each bank (manually). But it seems that I am going to have to switch between banks as there doesn't seem to be any chance that my array could keep up with all of the batteries connected at once.

BB. · July 2009

Re: Dual System from single PV array

AGM's will in good shape with little to no load will probably do just fine on 1% charging (at least in terms of "storage"). Just monitor the resting voltage (2-3 hours after any loads/charging has been removed) and compare with the mfg. resting voltage information.

Flooded Cell batteries actually use the "stirring" action of the hydrogen gas that forms once the batteries are fully charged--hence the normal recommendation for 5-13% (of the ~20 Hour Rate / battery capacity) or so recommendation for charging current.

Perhaps, the simplest thing for you right now is to get a nice heavy duty switch will can allow you to switch loads (up to 350 amp current--I would probably only switch it when near zero current--switching at 350 amps would probably fry the switch contacts in short order.

Obviously, to just switch the charge controller from one back to the other can be done with a smaller switch...

Or, pick a battery voltage that works for both (say 14.2 volts) and charge both together--and disconnect the AGM bank when you need to equalize (once every 2-8 weeks, depending on your vendor's recommendations).

There are probably several "good enough" ways to solve your issues... And a few really bad ideas that can cause problems (over charging AGMs, not watering Flooded Cell, storing Flooded Cell batteries below 75% capacity--may also apply to AGMs, and cycling below 50% state of charge).

Deep Cycle Battery FAQ

Fully charged AGMs can usually be stored for months (I have seen some Mfg. state 6-9 months) between recharging. Probably just check them with an accurate DMM once in a while to make sure all is OK and recharge when down near ~80% state of charge.

-Bill

Cariboocoot · July 2009

Re: Dual System from single PV array

Batteries prefer to be kept charged. Letting them self-discharge or drawing them down and leaving them will shorten lifespan considerably.

It sounds like you're running along on a small percentage of your battery capacity. In this case, it's good because that's what's kept them going so far: you manage to put back what you take out because you're not taking that much out. The only potential problem here is the inability to create sufficient power to equalize the cells. There would also be some concern about equal resistance in the battery cables so that each set is drawn on and recharged the same. This is why it would be a good idea to check them all with the hydrometer.

Do you intend to replace the Trojans with the AGM's? Or are you looking for supplemental power?

So you have to think about the future: if your usage increases you'll definitely need more panels to keep up. You're quite right about the panels not being able to keep up with both: the charge current would be slightly more than 1% of the total Amp/hrs and probably wouldn't keep ahead of self-discharge.

Try reading the Battery FAQs http://www.windsun.com/Batteries/Battery_FAQ.htm and see if it makes any more sense to you than my ramblings.

Gee, nobody ever gives me any free good batteries!

NorthGuy · January 2013

Re: Dual System from single PV array

I understand this is an old thread, but I found these ideas to be different from most recommendations. So, may I ask you to clarify them. I think this will really help to understand better how batteries operate.

Dave Sparks wrote: »

I learned this strategy from Dave Surrette (Rolls) in the late 70's. Pretty much the bible on how I design my systems for offgrid.

Assume that the system will never reach more than a 90% state of charge.

Does this mean that you never ever try to charge the batteries to the point where SG is at the max level specified by manufacturer?

Dave Sparks wrote: »

Complete absorption over 90% of the year.

Does that mean to complete absorption on 90% of the cycles?

BB. · January 2013

Re: Dual System from single PV array

My interpretation is that if you set your "floor" at 50% discharge--Assume that the battery is never (on average) more than 90% full...

If you were trying to get the battery to 100% full every day, it would take several extra hours of charging (which you may not have the sun for)--Plus the last 5-10% charging is inefficient and a bit hard on the batteries (sort of like equalization every day).

And, the batteries, on average, at very happy if they are >90% SOC a couple times a week.

-Bill

Dual System from single PV array

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