Jeff,

Mark

Mark,

As I see it, you have 2 possible reasons for this problem. You indicate the charging voltage is around 14 to 15 volts during charging which would seem to be good, but you did not say what the charging current was. For example, your inverter could be producing the right voltage output, but little or no current flow which would mean the batteries are getting little or no real charging amps going into them. My first check would be the generator and checking the amp loading of the charger.

Many low cost generators have a voltage regulator that drops the peak to peak voltage of the output waveform when more loads are added to the generator as their way of controlling overloading. Although this method of load control does not affect lights and motor driven tools, most battery chargers and inverters in charging mode will stop charging completely. These chargers are designed to use the full 169 peak to peak volts (120 VAC is the average of this voltage) of the generator. In other words, battery chargers only use the “peaks” of the voltage waveform and if these peaks are cut off or reduced, there is a major drop in charging amps output.

My second guess is the batteries. If these batteries are either old in age, or are newer, but have experienced many extreme total discharge cycles, or go long periods with only a partial re-charge, then what you describe is a classic indication of a failing battery bank. Assuming you do have a good charging process, when a battery is in the condition I just described, it will get what we call a “skim charge” and shuts down the battery charging because the battery voltage goes up real fast which tells the charger to stop charging. However, the aging battery plates usually have very little lead remaining in contact with the acid as most of the plate area is covered with calcium buildup and no longer in contact with the acid. This in effect is like making your batteries much smaller, which causes the battery voltage to rise very fast under the heavy charging, yet little real charging is taking place. As soon as the charging process stops, the battery voltage will drop almost down to a totally discharged level of around 12 volts since it was never really re-charged, and will quickly drop even lower when a load is connected, which also is what you have described.

I would not only suggest replacing these batteries, but I would switch to a deep discharge type of battery like a 6 volt golf car battery or larger L-16 battery. This will give you more amp-hours of capacity with fewer batteries wired in series. Its always better to have only 1 or 2 battery strings in parallel, as one string will always end up getting most of the charge and most of the discharge when you have 3 or more strings in series.

Good luck,

Jeff Yago

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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.

Gail:

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.

Jeff Yago

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