Upgrading Small Backwoods PV System

paulstamser · September 2010

Hello:

I am going to add to my small backwoods stand-alone 12 volt PV system with one additional solar panel. I would appreciate your opinions whether or not my plan is okay or do I need to modify it. I built the system myself but have no training or real knowledge whatever.

First I'll describe my existing system and then tell what I plan to add:

Existing 12 volt (stand-alone) home power system

Solar Panels
1 Arco ASI-16-2000 (early PV power plant take-off, 1970s?)
1 Kyocera 48 watt (1989)
1 Kyocera 50 watt (1992)
1 Photowatt 75 watt (1999)

Controller
"Solar Boost 2000E" (25 amp/12 volt)

Battery Bank
4 Napa 6 volt 220 amp hour wet-cell golf car batteries wired in 2 pairs for a 440 amp hour 12 volt system (new batts 2003)

Wire Runs
30 feet (or a little longer): One way run from PV panels to controller/batts
10 gauge - Size of wire in each run

(NOTE: My system has paired panels going through two separate 10 gauge wire runs: Two Kyocera panels (6.3 amps) are connected to one run, and the Acro and Photowatt panels (6.8 amps) are paired to the second run of wire. My amp figures might be a little off, but I don't think by too much).

2010 -- I plan to add:

1 Kyocera 135 watt PV panel (KD135GX-LPU or KD135-GX-LFBS - what's the difference between these?)
1 additional 30 foot run of 10 gauge wire

Note: I will also need a male/female cable to fit the "MC-4" locking connectors on the new Kyocera panel so I can splice into the new wire run, and also the disconnect tool for the MC-4 locking connectors.

I also plan to add a third run of wire for the new 135 watt panel to protect against excessive voltage drop.

Currently, my total PV system amperage is about 13.1 amps. If I add a new 135 watt Kyocera, total PV amperage will be about 20.7 amps. This is within the 25 amp capacity of the 2000E charge controller.

Am I missing anything? Is this PV addition upgrade going to work without any problems? I haven't worked on my basic system since 1999. That is how reliable it has been. (Did replace batteries in 2003 tho).

Any advice or tips you can give me would be very much appreciated. I'm self-taught in this and have forgotten most of what I once knew.

Thanks!

Cariboocoot · September 2010

Re: Upgrading Small Backwoods PV System

Bit of a mish-mash you've got there, eh?
You've already figured out that 173 Watts (+ whatever that first one is) of panels doesn't work for keeping 440 Amp hours of batteries charged, right? It's about 1/3 of what is really needed.

I know this will sound like the expensive option, but maybe it's time to redesign the whole thing?

Assuming that battery bank size works well for you, you'd want to get 40 Amps or so going to it @ 14.2 Volts charging. That's roughly 568 Watts of usable panel, or a 710 "nameplate" Watt array.

You may be able to "replace the used Amp hours" but you are definitely deficit charging that bank. Rough estimate with adding the 135 Kyocera ... 185+135 =320 @ 80% = 256/14.2 = 18 Amps of current which would still be under the minimum 5% recommendation.

How's the budget? Can you flog the old panels and scrape up enough $ for three Kyoceras? I think the batteries would be happier with that.

paulstamser · September 2010

Re: Upgrading Small Backwoods PV System

Thanks for the feedback.

Yes, I knew my PV system was too small but not by how much. As it is, however, it works pretty well for me. I have very small loads and my biggest load (computer) I mostly use during sunlight hours. I got 10 years out of my previous set of batteries and this set is 7 years old. So considering the system is undersize, it's not doing badly.

Altho it is a mish-mash, I can't see trashing the old panels and getting all new ones. If I can add the 135 watt Kyocera without problems that would be a decent increase over what I have now. What I need is more power on cloudy days; something in excess of 2 amps.

I guess I don't understand what is wrong with the panels I am using now. Is it because they are various sizes, makes, and ages? I didn't think that mattered...

BB. · September 2010

Re: Upgrading Small Backwoods PV System

There is probably nothing wrong with the panels themselves--it is the complication that a mixed set brings...

First, we need to know the Vmp/Imp of each panel... Ideally, the Vmp's should be within 10% of each other if connected in parallel (series Imp should be within 10%).

Next, the current "12 volt panels" have Vmp around 17.5 volts or so... Older panels tended to be designed for use without battery chargers and may have Vmp around 15 volts... Too low of Vmp and they will not do a very good job of charging 12 volt lead acid batteries on a hot day--particularly if you have a charge controller in the mix.

Lastly, with a bunch of old/small panels in parallel, a single failed panel/electrical connection can be missed and you could be running with a "crippled array"... Going with a few new high powered panels in parallel can make maintenance/aging issues a bit more obvious for repair.

That all being said--if your panels are working well and you can match Vmp/Imp for your series/parallel connections--there is nothing "wrong" with keeping your older working panels unless you are trying to do a major upgrade. Then it can become an issue trying to get the right series/parallel matching of panels. In some cases, it is impossible to mix old and new panels on one charge controller--So people leave the old panels+charge controller setup and buy a new charge controller to run in parallel with the original setup to charge the battery bank (you can run charge controllers in parallel when connected to the same battery bank).

-Bill

paulstamser · September 2010

Re: Upgrading Small Backwoods PV System

Thanks Bill,

I'll get the Vmp/Imp of the existing panels.

The one I'm concerned about is the Arco ASI-16-2000. That thing is old. It was a take-off from a California powerplant (Carruzo or ??).

But I disconnected it one day and my output dropped a couple amps so it MUST be working.

I never let my batteries discharge too much. I don't think I've ever seen the battery voltage drop below 12 volts -- ever.

Cariboocoot · September 2010

Re: Upgrading Small Backwoods PV System

What Bill said.

Just from my POV, it's a matter that the mis-matched panels will cause a drop in efficiency and that would most likely be for the new Kyocera. So instead of getting 135 Watts at a typical 80% efficiency it might get "dragged down" to 75% or possibly worse.

Many people here have seen there panels continue to perform well over years without the often predicted severely reduced output over time.

I don't find a listing for the "LFBS" version of the K135. NAWS would know for certain, but I suspect that it's simply an older version with different frame or junction.

All said, simply adding the 135 Watt will work. It's just not as efficient as it could be and you'll still be a bit low on what you really should have for good charging. Aren't those "cheap" golf cart batteries nice? They'll take a lot of misuse without too much trouble!

I'd also be concerned about the parallel wire runs from the panels - easy for one to be disconnected and you may not notice right away if 50 Watts or so wasn't there.

icarus · September 2010

Re: Upgrading Small Backwoods PV System

One other factor to consider, if you have hardware that is bought and paid for, even if you don't get 100% efficiency (or whatever %) it really doesn't matter too much, since it is "free power" compared to buying new hardware.

I would consider all your factors, (loads/pv/controllers/batteries) design a system around the loads, and use the components you have in that design. If needed, build a new system to run in parallel with your existing, even if it is not fully efficient. Or, alternatively scrap parts and pieces that don't really fit, sell them if you can, and use the modern gear for a complete new set up. (I realize I am talking out of both sides of my mouth with this, but it illustrates what your options are!) Even the old ARCOs have some value, either in a system or to re-sell.

Hope this is clear as mud,

Tony

niel · September 2010

Re: Upgrading Small Backwoods PV System

don't forget the controller is only rated to 25a so you might want to start adding those up to see where you stand with that. my personal opinion is that 10% is too much of a spread and i say closer to 5%. i would suspect the arco of possibly going to far out of spec due to some aging and even possibly it was designed at a lower vmp to begin with.

dmiller · September 2010

Re: Upgrading Small Backwoods PV System

Why is a panel being added?

paulstamser · September 2010

Re: Upgrading Small Backwoods PV System

Thanks for the feedback. And yes, it is about as clear to me as mud, but I'm getting there.

For a start here are the specs for my existing panels:

Photowatt "75 watts" (bought 1999)
From spec tag:
Vmp = 17.0 volts (Voc = 21.6 v)
Imp = 4.4 amps

Kyocera "51 watts" (bought 1992)
From spec tag:
Vmp = 16.9 volts
Imp = 3.02 amps

Kyocera "48 watts" (bought 1989)
From spec sheet (tag on panel blank or faded)
Vmp = 16.7 volts
Imp = 2.88 amps

Acro ASI-16-2000 (bought c1990?)
No spec tag or sheet.
Voltage measured across terminals = 18.77 volts

To me it looks like the Photowatt and the two old Kyoceras are close enough to the new 135 watt Kyocera (Vmp = 17.7) to work okay. But the open voltage measured Arco (18.77 volts) when under load (Vmp?) may be somewhat lower. But if including it doesn't wreck the whole system I'll probably just leave it in. I did disconnect it one day and lost a couple amps, so it appears to be adding something.

I can't tell if I'm getting full output now or not. The highest observed output I've recorded was 10.2 amps at 1:10 pm (total array output should be 12.80 amps), but I think it's low because my system voltage was already relatively high (13.18 volts) and the controller was amping down and not giving full power to the batteries. At least that's how I think it works.

With the new panel added I will have about 19.97 amps total output, well within the 25 amp rating of the "Solar Boost 2000E" altho the specs say even it you overload it it doesn't matter but will only accept 25 amps max.

Yes, I do like the cheap golf car batteries. I figure if I can get 10 years out of a set I'm doing okay. I am careful about not discharging them too much and my stand-alone system is based on the theory and practice of small loads and simple but proven technology. I pump most of my water by hand and use underground cooling in combination with a 12 volt Coleman cooler in summer and the worlds best natural refrigeration in the winter.

Probably I'll go ahead with my original plan and add the new Kyocera 135 watt panel and a third wire run. Going into shorter days and cloudy weather now so this is the time when my batteries need the added electrons most.

PS: a panel is being added for more battery charging during short-day cloudy weather so I don't have to run the gasoline generator as much.

BB. · September 2010

Re: Upgrading Small Backwoods PV System

Yes, I would just add the other panels in parallel--It looks like your system is doing "OK" (yes, in quotes, more details why below).

First, your battery should charge up to 14.5 volts or so, then, once the controller has decided the battery is "Full"--then the controller can ramp back down to "Float" which is around 13.7 volts.

Your battery voltage at 13.18 volts while charging indicates one of several things... Either:

the battery is still way undercharged,
the battery bank is too large for the amount of solar power and it is not getting fully charged (roughly above 13.7 volts minimum), the output voltage of the array is too low (remember we want >~16 volts to get the batteries up to ~14.5 volts for full/fast charging an to allow for wire voltage drop and controller voltage drop),
you have a voltage drop from the controller to the battery bank (need heavier gauge wiring, shorter wiring, or there is a bad connection somewhere),
the controller is not allowing the battery voltage to go high enough (programmed voltage/battery type incorrect or controller failure).

Which of the above (or combination of the above), I am not sure.

Have you measured the Specific Gravity of the battery and compared it to the vendor's suggested readings? Is the battery fully charged or something less?

Basically, while charging the MPPT should pull as much energy from the solar array as it can until the battery reaches ~14.5 volts (bulk to float transition). At 14.5 volts, the controller will gradually reduce current until it has been several hours, or the current drops by 90% or more, or the sun goes down (depending on how the controller is programmed). Once the timer/minimum charge current is reached, then the controller will drop the voltage to Float (~13.7 volts) until the sun goes down or there are some loads placed on the system.

For a system that is used daily--I am not sure float is much of an issue. You probably would be happy to stay at ~14.5 volts for the whole day and get the battery fully charged.

How much distilled water are you having to add to the bank? If you are adding near zero water every month--you are probably undercharging the batteries. If you are near exposing the plates every month (lots of water), then you are probably over charging and can back off a bit.

If the system were a cabin that sat unused for several months of the year, then having the charge controller cut back to "float" is important. You don't want to boil the batteries dry--and you don't want to charge them hard (basically equalizing) all the time--Overcharging/too much equalization "wears out" the batteries too.

That you get 10 years out of a set of golf cart batteries--you are doing something right (or not drawing much power).

-Bill

paulstamser · September 2010

Re: Upgrading Small Backwoods PV System

BB:

Thanks for the feedback. I'll go with my plan of adding another panel to my system.

The 2000E controller has no float setting. Just Bulk Charge and Constant Voltage Charge, which is set at 14.01 volts. When it hits 14.01 volts it stays there until I add a load or the sun sets. But I was talking it being in 13.xx volt range before it got to that 14.01 volt set point.

Maybe I have a bad connection or a bad panel, because during bulk charge in full sun I'm not pulling the array's full amperage of 14 amps as measured on the controller display. Bulk charging amps vary but I've never observed it above 10.2 amps. I thought the charging amperage lessened as battery voltage rose, but maybe not. Booklet says 2000E delivers "as much charge current during bulk as possible" but if so, where is my 14 amps in full sun before the 14.01 volt cut off point is reached?

Maybe something is wrong with my system. I'll have to disconnect panels one at a time and see if something is amiss.

I only add water a couple times a year. Not often at all, altho my system is used 365 days a year. But my loads are light and I'm obcessive about turning off lights, etc. I haven't checked specific gravity for ages since my battery voltage is always pretty high and I get 7-10 years out of a set of cheap golf car batts.

BB. · September 2010

Re: Upgrading Small Backwoods PV System

Roughly, if everything is working well--On a day to day basis (average to warm weather), you will probably see around 70-77% of the rated panel wattage going into your battery bank during the middle of a "typical" sunny day.

256 watts of panels * 0.70 derating * 1/14.01 batt volt charging = 12.8 amps
256 watts of panels * 0.70 derating * 1/13.18 batt volt charging = 13.6 amps

So, if you are getting above ~1.25 amps--It would look like your system is working OK... (nothing major wrong).

Battery charging voltage--You might check the specifications for your battery bank--Charging at 14.4 to 14.6 volts (around 77F) is probably OK... Batteries will charge faster (assuming your solar charger can get the voltage this high) and more completely. You probably will use more distilled water too.

Note that once the battery voltage hits the set-point (14.xx volts)--the charge controller will start cutting back on current being dumped into the battery bank.

Also, check the solar array voltage--it should be at least 1 volt above the Vbatt charging voltage.

At 10 amps--You might have a panel (or several) performing at less that rated power... Disconnecting one panel at a time may isolate a weak panel/bad connection.

-Bill

Upgrading Small Backwoods PV System

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