Newby question

[Allen Maclean]

Guys and gals I have 2 240 watt canadian solar panels. One 2000 watt inverter. 2 100 amp hour batteries and one 90 amp hour battery. These are all wet lead acid. This is all set up in series. 12 volt system. The panels are connected to each other then go directly to a cheap mppt Chinese controller.

My question is I'm having to charge my batteries daily with my Gen. This system is hooked up to my 30 amp plug direct to my rv. I'm only running my freezer during the evening. I shut off everything accept my rv fridge which is 12 DC. What can I do to reduce my generator charging down to a minimum.

Because I'm still learning all help would be appreciated. Thanks

[Glen Johnson]

240watt is a max rating under perfect conditions, 25°c and 1000w/m directly facing sun usually... almost impossible in real world sinarios.. so derating it to real world, 200watt per panel after losses with regulator and wiring etc( 400 watt total) to be conservative, over say 8 hours of sunlight, 3.2 kwh, so 266 ish amps at 12v ... thats a realistic guess of available power over a day..

loads, you say only the fridge runs at night, how big is said fridge ?? also what loads are on it during the day ??

thirdly, what tells you , you need to charge batteries?? are you going primarily on voltage or are you using a meter ?? what condition are the batteries in ( new ish, 5 years old ) ??

just trying to get a bigger picture to assist..

[Allen Maclean]

Batteries are new. 2 months old approximately. I'm going based on actual voltage. I've metered them and I also used my voltage tester. The fridge is a small rv fridge. Its an older fridge but works on dc as well as ac. Freezer is a 5 year old that if memory serves is 4.1 killowatts annually. I forgot to mention I have 50 feet of pv cable feeding the regulator from the panels. During the day I have been running the generator to recharge the batteries so no loads until night time.

[Glen Johnson]

hmmm... 4.1kw annually seems a little low, can you get some kind of measurment on it to work out exactly how much?? what is the lowest voltage they getting to under load ?? REALLY difficult to measure available capacity from voltage with lead acid, as it will vary with load (peukert's law applies) , with that as a load it should breeze through with the above..

when you say shut off everything , you mean only the freezer and leave the dc fridge on or other way around??

you have total, 290 a/h , with 50% depth of discharge safety factored in that leaves 145ah available.. which is 1.7KWH ish... not a whole lot, but as a guess should be fine.

example, my off grid setup is 48v , with 900 ah total LA storage ... under heavy load, it can and does regularly drop to say 45-46 volts pulling 5+ KW and this is a X forklift battery ... why I got a proper AH meter ...

https://a.aliexpress.com/_mM1Cn7v

this is the one I have, works very well and reasonably accurate... oh and cheap to boot. takes all the guess work out

[Rob Beckers]

Hi Allen & Glen,

Allen doesn't mention where he's located, and if those solar panels are nicely south-facing with good sun exposure at a decent tilt-angle. For our general area here (Ottawa) 480 Watt in solar PV would reasonably produce 1.3 kWh per average day from spring-through-fall, and half that in winter (if kept snow-free, at a steep tilt-angle, and south-facing). Those numbers are real-world, if somewhat on the pessimistic side, and take conversion losses of inverters and batteries into account as well.

So, if you're over here that's what you have to work with, 1.3 kWh/day. Now let's look at the loads.

To start with, any dual-fuel fridge or freezer (propane and AC/DC) is ATROCIOUS when running on electricity. When on electricity all they do is run a heater to simulate a propane flame, and they use tons of electricity. Don't know if what you have falls in that category, but thought I'd mention it.

A regular semi-decent fridge or freezer clocks in between 0.5 - 1.3 kWh per day. A little bar fridge is not much better than an 18 cu. ft. regular fridge, and freezers use about the same as fridges (they have better insulation). Most are around 0.8 kWh/day. You have two such devices, so that's 1.6 kWh/day to run them.

The inverter itself takes power to run too, even if nothing is connected. The general number we use (a bit pessimistic, reality may be a little better but not by all that much) is 0.5 kWh/day for the inverter.

In short, without anything else (lights, TV, computers) you seem to be up to about 2.1 kWh/day in energy use, with about 1.3 kWh/day coming in (unless you're in the tropics with sun galore, production would be better there). You're likely short on energy.

For the batteries, get a decent hydrometer (we sell a cheap one that works well at Solacity, the German-made HydroVolt (https://www.solacity.com/product/compaselect-hydrovolt/)). That will tell you what's going on inside the batteries. Even though they're pretty new they're likely already sulphating and you have less capacity than you think. With 12V 290Ah you have 12 x 290 = 3.48 kWh in energy storage, and we consider 50% usable for regular day to day use, or about 1.7 kWh. That's likely not even a day's worth of your use. Normal off-grid sizing for systems that are used in winter sizes for 3x daily use (the wording is "days of autonomy", and with 3 days in winter they'll still need a generator to pick up the slack during dark times). For summer use 1 - 2 days is reasonable, though you will at times fall short.

Glen has a good point with the shunt-based battery monitor. Those that measure know what's going on!:cool: I stopped advising shunt-based monitors for people with lead-acid batteries because they rely on a known Amp-hour number for the battery bank. In reality, unless you take really good care of your batteries, that number fluctuates (a lot) and when batteries sulphate the monitor will indicate artificially high. Instead we've moved to just suggesting a MidNite Solar MNBCM (https://www.solacity.com/product/midnite-solar-mnbcm/), which is Voltage based. It's not accurate, but less to go wrong and it does give a decent idea of what's left in the tank.

There are two articles I wrote that may be helpful too:

https://www.solacity.com/how-to-size-a-solar-system-that-really-works/
https://www.solacity.com/lead-acid-battery-care/

Hope this helps!

-RoB-

[Allen Maclean]

After reading everything I guess I need at least 3 more panels. Is this correct. Also you suggest that I have no more than 4 batterys in a string. They should be each at 100 ah. I have thrown in a 90 ah so this may cause me problems in the future. The batterys may sulphate sooner if there not equalized. I can double this by changing to 24 volts but I will need a 24 volt to 12 volt stepdown for my inverter or should I get a 24 volt inverter. Up to now everything I have is setup for 12 volt for my rv.

Also just to clarify I run my fridge on auto. So when I'm on generator it runs on ac power. When the generator is off it runs on dc with gas. My freezer is on at all times.

Sorry for the jumbled thoughts.

[Glen Johnson]

don't know where you got the " no more than 4 " in a string... lol, one of my banks is made up of 2 " strings " of 6v batteries ( to make 48v on both respectfully) thats 16 " batteries " in total, as long as you look after them, won't matter if you have 30 batteries in parallel, just equalise when needed, monitor regularly and if something amiss, fix...

as for voltage, at your power levels ( that you've stated) 12v is perfectly fine, if you decided to increase that, increase of voltage would be recommended as current would drop ( all things equal, watts are watts, current needs thicker cabling)..

as stated, absorption fridges are incredibly in-efficient on " power " , great on gas.. compared to a compressor based fridge.. that may be your biggest downfall.. you will need to weigh up whether to increase storage and panels or replace fridge with a more efficient design...

as for reading state of charge with voltage on a lead acid battery, as I said in last post, incredibly difficult too.. google it if don't believe me.. all the voltages posted are at resting, so if ANY loads are on em, immediately inaccurate.. why i suggested using a ah meter, then you will know EXACTLY how much juice you've used out of your bank and whether charging is necessary..

[Rob Beckers]

Glen, the general rule is "no more than 4 parallel strings" for lead-acid batteries. Not 4 in a string.

It gets progressively harder to keep all batteries in parallel strings doing the same thing at the same time. The odds of getting one's money's worth (i.e. batteries living as long as they are supposed to) go down rapidly beyond 3 or 4 parallel strings. Not that it can't be done, I know a guy that has two dozen 12V batteries in parallel, and it seems to work for him. He only cycles them very shallowly though, and they're essentially nearly always at float. For regular use, stick with two stings, 3 if you have to, and if you really, really have no other choice make it 4.

-RoB-

[Glen Johnson]

Ive personally broken that " rule " comfortably plenty of times, thats parallel strings, my last system had 7 strings of 48v and was cycled 30-40% every night for about 7 years ... left it working and as far as im aware its still going...

my experience with this issue has almost always been a) lack of maintenance b) unforeseen occurrence c) incorrect settings d) all/some of the above ...

just really bugs me when these " rules " are set , no one dares break em... but they can be broken... why i said previously about the strings, it matters little if 16 are in parallel or series , they still charge and discharge at slightly different rates, which can cause the " weaker " cell to die and the rest to take the slack , which can and does happen.. however if you are maintaining and checking semi regularly you will quickly pick up the issue and deal with it accordingly... lead acid batteries are incredibly tolerant to " abuse " , more so than people realize, however low discharge and temperature are the 2 biggest killers...

just please don't put blinders on with set numbers if the system NEEDS more than 4 paralleled strings, just be aware it will require a closer eye on it... not that you " cant "

[Allen Maclean]

Thanks very much. I'm getting a better picture here. One thing I'm not clear on is cycling the batteries. I'll have to look this up. I'm in the Smith falls area. Panels are due south at 37 degrees as my tilt. When sun is out I am getting good power but this regulator sucks. Waiting for a newer one. I may stop in at solacity and pick up a good hydrometer though. I will keep my system at 12 volt for now because it's easier for me and my rv. I am gonna add more panels as I can afford them and more batteries for winter months. Thanks for the help. It's very infomative and confirms my wanting to go off grid as the right choice at least for me.

[Glen Johnson]

sorry Rob, i didn't mean to come across rude... and i understand you said as a general rule...