We have a 1 kw off grid system. The actual electrical use is very low, but the system constantly fails- that is, it gets so low we have to shut everything down and run the generator. I think it may be the design. The house is in Kauai. There are 8 L16 6 volt batteries, all wired in series. I do not know how the panels are wired, but we are told that they total 1050 watts- I suspect they are also all in series. We use fluorescent lights- 4 in all, plus propane refrigerator and stove and solar hot water.
Other than lights, all that is powered off the system are a cd player, charging cellphones, a hairdryer in the morning for about 3-5 minutes, and a shop vac during the day occasionally for about 15-20 minutes. We cannot run the washer w/o the generator.
Surely there has to be a better way to live. We have called in two solar guys. We have replaced batteries. We have added panels.
I have done a great deal of reading and it seems that there should be some batteries in parallel, plus the panels need to be wired some in parallel. Before I challenge the solar guy, I wanted some advice. Unfortunately, I do not think that the experts on Kauai really analyze load, etc. No one has asked us what we need; they simply tell us what the system will be.
Thank you for any advice or direction you can give me. We are desperate.
There is no difference in total battery capacity regardless of wiring in series, parallel, or series-parallel. Now before all those other “experts” out there read this and start sending nasty email back (yes, it happens all the time even knowing this is a free site!), I need to explain.
Batteries absorb energy measured in amp-hours, and they supply your loads by sending these amp-hours back out. A 6-volt deep cycle battery will have the same storage capacity regardless of how it is wired. It makes no difference. Lets use an example, a typical L-16 battery will have 350 to 425 amp-hours depending on weight (lead) and manufacturer. Lets say 400 amp-hrs to keep it simple.
If we wire 8 batteries in series, we will have a 48 volt battery with a total amp-hour capacity of — 400 amp-hours! In series, the amp-hours do not change. This will have a total capacity to power ( watts = amps X volts) = 19,200 watt-hours ( 48 X 400).
Now lets take the same batteries and re-wire as a 24 volt battery. This will be two separate strings of 4 batteries to make 24 volts each, then wired in parallel. Again, watts = amps X volts = 24 X 400 = 9600 watts capacity per EACH parallel string, so in this case the amp-hrs are added, or 2 X 9600 = 19,200 watt-hrs.
The same would be true if we re-wired them for 12 volts, you will still have the same total power capacity. Having proven that fact this does not mean they are not real differences with each layout. We always try to use the highest voltage battery as possible, as this results in the lowest amp current flow, which allows using the smallest wire size, and inverters designed for higher voltage can use smaller components as the currents will be lower.
Another problem in wiring batteries for smaller voltages is you need multiple series strings. Like our example above, it would take 4 separate “strings” to wire your 8 batteries if you wanted a 12 volt battery bank. The problem is, the more strings you have, the more risk there is of un-equal charging between each parallel string. For example, if one battery in one string is starting to fail, or just does not charge as well as the others, this will cause that entire string to receive less than its share of the charging current. It is also possible for a single battery to fail, then all the other strings will “see” this as a load and start moving massive currents into that string, just like they were chargers. This can even cause the failed battery to get so hot it will melt down, so less strings is better.
Again, things are never simple as you think, because our desire to have only one battery string and a higher voltage may not match your loads. Maybe your inverter can only operate on 24 volts, or you have a bunch of 12 volt lights. Several manufactures for DC refrigerators only offer a 12 volt or 24 volt model, not 48 volts, so this is usually why some systems must have lower voltage batteries.
They say if you ask an engineer what time it is, they will tell you how to build a watch. I hope I have not done that with your question, but we need to first dispel this battle you are in about battery voltage. If your inverter and other DC loads are 24 volts DC, then your battery must be wired for 24 volts DC. If you have a few 12 volt DC loads like lights, you can still use a 24 volt battery and install a DC to DC converter. These are low cost and will allow an equal loading across all individual batteries that would not be the case if you wired them to only part of the large voltage battery bank. Due to high cost, your inverter voltage will determine the battery voltage.
Solar Array – Most solar modules under 100 watts are 17 volts output (ideal voltage for charging a 12 volt battery plus some efficiency loss). This means the they are wired just like the batteries in the battery bank, you wire them to give the output voltage to match the battery voltage, and this may require several separate strings. The recent introduction of MPPT solar charge controllers now allow using a higher voltage solar array than the battery bank as it will adjust the voltage and current as required for the best charging output. However, this advantage was not intended to allow using a high voltage solar array to charge a 24 volt battery. Most people use a “slightly” higher voltage solar array when there is a very long wire run between the solar array and the batteries to off-set the voltage loss of the long wire.
Again, there are many technical reasons for using different voltage arrays, different voltage battery banks, and different voltage loads, but you can’t say one way will have much less capacity than another as long as you are comparing the same components and just wiring them differently.
Your Case – You said your batteries were replaced, but If I were there, I would first check each battery to make sure they are all good. If you have 7 new batteries and 1 dead battery, the battery bank will never charge right. You can use a digital volt meter for this, but these readings need to be taken when the battery is under a modest load. A much better way is to use a hydrometer. If you are adding water to your batteries more than once every 3 or 4 months, either your charger is programmed to high or your batteries are getting old. Frequent watering indicates failing batteries if the charger is working correctly.
I would next dis-connect the solar array from the batteries and check their output voltage, then check again after they are connected to the charger. Not connected, most 12 volt nominal solar modules will have an open circuit voltage around 21 volts. If this array is wired for a 24 volt battery, you should see 42 volts, and for a 48 volt battery you should read 84 volts. After re-connecting to the battery, it will be more like 27 to 29 volts into a 24 volt battery, and 54 to 59 volts into a 48 volt battery. Your solar charge controller may have a meter to provide this information.
I would next check charging set-points. You appear to be in a warm climate, and this requires “lower” charging set-points for both your solar charge controller and your inverter charger set-points when using the generator.
The last thing I would check is the charging current when using the generator. I have found that most “contractor” type portable generators do not work well with inverters when charging batteries, its just they way they are designed to control their output voltage. If there is even a slight drop in generator output voltage your inverter will not just have a lower charging output, it may not charge at all. I have watched low cost generators run for hours and not do any charging at all. Assuming you have the worlds greatest generator, if your inverter is not programmed wrong it will take for ever to re-charge.Assuming you have the worlds greatest generator, if your inverter is programmed wrong it will take for ever to re-charge. When using an inverter that has programmed charging set-points, I try to increase the charging current amps up to the point I almost stall the generator, then back off a few amps. Generators use almost the same fuel at half load as almost idle, so loading them up provides the best fuel economy, fastest re-charge time, and less run hours.
If all this checks out just fine, then you may have the classic solar home problem I see every day – the small system was designed to run a few lights and appliances, then each year you add more load or use them longer hours each day and pretty soon you start complaining the system just does not provide the power it did when first installed. If this is the case adding more batteries may not help if your solar array is too small for your loads.
Sorry you are having so many problems, it should not be that way,
I hope this helps.