When I first installed the power system in our home 7 years ago I relied on a lot of folks for information and guidance. Most of it was good, but some of it has turned out over time to be not as good as it could have been, especially when you combine incomplete information with some poor equipment.
The standard technique for installation of a battery bank was (and still is) to use what is called series/parallel wiring. Shown at the left in a picture of our old battery bank of Trojan L-16s, series/parallel wiring is a way of combining batteries to get the correct voltage and storage capacities to match the rest of the components in a system. Each of the L-16s is a 6 volt battery and our system is a 24 volt system. Wiring batteries (or panels) in series will increase the voltage of the batteries that are wired together while keeping the capacity the same as the individual batteries.
Series wiring means to connect the positive terminal of one battery to the negative terminal of the next battery. By doing so you multiply the voltage of the individual batteries by the number of batteries interconnected until you reach the desired voltage. For example, in our system that meant we connected 4 batteries together in series to get one 24 volt series string of batteries.
This duplicates externally the way batteries are connected internally. Internally, batteries are a series of 2 volt cells, a 6 volt battery has 3 2-volt cells inside it, connected in series to each other to make 6 volts. A 12 volt battery has 6 internal cells. So by connecting batteries together in series you are making one large virtual battery of the combined voltages of the individual batteries. In a series string there is always one positive and one negative terminal left over that become the terminals of the string. Put a meter on the two unused terminals and you will see (in the case of our bank) 24 volts.
Parallel wiring multiplies the capacity (in amphours) of the batteries while the voltage stays the same. Wiring in parallel means wiring the positive terminal of one battery (or string) to the positive terminal of the next battery (or string). As you can see in the picture we have 3 series strings wired together in parallel for our battery bank. This is the technique used almost all the time for wiring battery banks in renewable energy systems.
One of the other factors that led to the wiring scheme for our original battery bank was the idea of keeping the length of the cable between the bank and the inverter as short as possible. At the time of installation I was told 5 feet. With the setup of our power room that meant that we could run the inverter cables (large 4/0 welding cable) to one end of the battery bank and connect to the positive and negative terminals of the series string on that end and keep the cable length to 5 feet..
Since the installation 7 years ago I have faithfully followed a regimen of maintenance and good usage designed to keep my batteries healthy and to ensure they would last a long time. I added distilled water every 3 months or as needed, cleaned the battery tops and checked the cables to make sure they were all strong. A few months ago I discovered my first problem. One of the parallel battery cables slipped out of its terminal end when I checked it, essentially meaining that there was a failure in the crimp that holds the end on the cable. I immediately replaced the cable with new one and figured it was just a one-time failure of the crimp caused by heating and cooling as current flowed through the cable. Boy was I wrong.
I checked and watered the batteries in late June of this year, adding quite a bit of distilled water. One of the cells had a water level quite a bit lower than the others but I didn't think too much of it. Not two weeks later I noticed a large and growing black stain, obviously liquid, spreading out from the bottom of the battery box, discoloring the wood of the box. I put baking soda on the stain and it immediately bubbled and hissed as the soda neutralized the acid. I had an acid leak somewhere in the system. Not good.
I immediately checked all the batteries and lo and behold the one cell that had been really low was now down again, this time to below the lead plates, leaving them exposed to air, definitely not a good situation. I put some more water in, though not filling it to the top, and started to consider my options. My first thought was that I had a cracked case. How else to explain the sudden loss of electrolyte from only one cell in such a short period?
I disconnected that series string from the rest of the bank and managed to stretch the inverter cables to reach the next series string. I now had two series strings of L-16s, my bank had gone from 1050 amphours capacity down to 800, all because of the failure of one two volt cell. The other 3 batteries in that series string checked out just fine. I started to hunt around for a replacement battery for that string.
I came close to buying 2 used L-16s from a battery dealer who had them sitting on a shelf but it turned out they had been sitting too long without charge and had gone bad. At first I was disappointed but it turned out to be a blessing in disguise. In my search for batteries I came across a source that had a quantity of good used sealed batteries. These were Dynasty 12 volt 125 amphour AGM/VRLA batteries. (that stands for Absorbed Glass Mat/Valve Regulated Lead Acid).
They are high quality batteries used in telephone company switching stations and computer room power backup installations. Not normally used in off-grid application because of cost, they are often removed from service by the telco only a few years into their life for a number of reasons. Sometimes the system has grown to require a larger backup, which means getting all new batteries for the reasons I mentioned above related to aging, or a half a dozen out of a hundred or more batteries have failed in use, which means replacing all of them to ensure reliability. In a telephone or computer installation, no matter how much the batteries cost they are cheap insurance against failure and downtime, so they buy the best. Downtime on a phone system or massive data loss in a computer system can often cost thousands of times more than the most expensive batteries.
But for us off-gridders these slightly used batteries can be a bonanza. I first got wind of them from someone who dealt with telco installations who wanted to see batteries that were perfectly good, but technically used and slated for the smelter, get put to good use. And what better use than in renewable energy systems? By using these batteries, and being willing to forego warranties in exchange for getting batteries either free or at below bargain basement rates, off grid homeowners can have high quality batteries for very little investment. Even if they only last a few years they are well worth it.
The L-16s are a type of battery known as a "flooded cell" battery. This means that internally they have a series of lead plates immersed in a liquid electrolyte (dilute sulphuric acid). This liquid electrolyte will requires equalization (high voltage charges) to keep it from stratifying (the heavier acid separating from the water and settling to the bottom) and to keep sulphur crystal deposits off the lead plates, and also regular replacement of water (always use distilled water) that is lost in that process. They can also spill since the battery caps are not totally sealed, they must be installed with a venting system to clear out the hydrogen gas that is vented during charge and equalization cycles, and safety equipment should be on hand to handle potential acid spills and splashback during maintenance.
An AGM battery, on the other hand, uses a woven glass mat to hold the electrolyte in place against the lead plates, removing the stratification problems. They also have a higher specific gravity (stronger acid solution) which makes for more stable voltages and far less voltage drop over time. AGM batteries are sealed at the factory and have a valve (the Valve-Regulated part) that will allow any excess hydrogen created by accidental overcharging to vent and then seal itself again. AGM batteries are never equalized, they don't need it. In my 24 volt system I take the AGM battery bank up to about 27.4 volts and no more. AGM batteries can actually be installed upside down (though why you'd want to is beyond me, those suckers are heavy) and are not considered a hazardous material since they are virtually spillproof. I've seen an AGM battery with a completely broken case and I was able to easily pick it up and put it in my truck to take to the recycler, in fact it still showed good voltage. No acid spill, no dangerous electrolyte, it was all contained in the glass mat. (touching the mat itself would result in acid exposure)
Then I got to work dismantling the old battery bank, tearing out the box and getting rid of the acid stained wood (which required a fair amount of baking soda to neutralize). This was where I found my second problem that may have contributed to the premature failure of one cell. As I was removing the series cabling I found to my horror that when I removed the bolts holding the cables in place and picked up the cables, the ends promptly fell off. These were cables that I had purchased from a well-known national catalog company, made up by them in their office, when I had purchased the original batteries.
When they made them they used shrink tubing to seal the ends, shrink tubing that hid the extremely poor job of crimping they had done connecting the cable ends to the cable. Three of the series cables had the ends fall completely off and others were extremely loose. After 7 years even the shrink tubing couldn't hold them together. This meant that there was a high probability that there was a more resistance in those cables than there should have been, which meant more work for the batteries and charging system and less efficient energy availability to the inverter. They hadn't even been crimped enough to deform the ends of the cables (see the picture at left), on most of the cables not enough insulation had been stripped back to put the correct amount of wire into the cable end.
So I had a number of problems with my original battery bank, problems that had been covered up by shrink tubing, good battery maintenance and dumb luck. But that would change with my new batteries.
I decided to have my cables for the AGM batteries made up by a supplier from whom I occasionally bought supplies. They specialize in large battery system installation and sales (and occasional sales to small fry like me) and were more than willing to help me out. I explained to them what had happened, how my L-16s had been wired and that I was replacing them with the AGMs. They immediately set me straight on the problems of series/parallel wiring.
When they install a large battery bank in a computer system room or telco switching station they often install hundreds of batteries. Wiring them series/parallel would be impractical to begin with and make for harder maintenance. But the big surprise for me was their method of installation and why they did it. They use buss bars as connectors for all series strings (see picture of my buss bar installation at left), there are no parallel connections across batteries in their systems. Parallel connections of multiple series strings can cause all kinds of problems in large battery banks. The batteries in the strings with the main connections (in my case the inverter connection) take all the current load while the middle strings contribute voltage and a little bit of current to try to balance out the bank capacity.
This means that the batteries closer to the system connection will fail sooner and the bank will overall function less efficiently because of internal imbalances. By using a buss bar all series strings in the bank are connected individually to the main buss bar and the inverter cable is also connected to the main buss bar. Each battery in the bank receives the same charge current and discharges the same amount of current without having to pass that current through other batteries in the bank. It also allows for any number of series strings to be included in the bank, which means you can use lots of smaller batteries to get large system capacity rather than a few larger batteries.
So the folks at my supplier not only sold me cables they set me up with a pair of buss bars (one for positive and one for negative) that would work for my system and allow for the addition of more batteries in the future if I so desired. As you can see from the picture of my installation, there are extra holes available in the bars for more pairs. Each hole holds two cables, one on the front and one on the back. The main inverter cables each have their own hole on the bar. I also installed a divider between the two bars to minimize the chance of accidentally crossing the positive and negative terminals while installing or removing cables.Using 12 volt batteries instead of 6 volt also means that if any of the batteries fail I will only lose two batteries and a smaller percentage of my capacity than I did with my L-16s where I lost 1/3 of my bank capacity when one battery had a problem.
Compare the picture to the left to the picture of my original batteries at the top of the page. With the flooded cell batteries, space must be left above the batteries to allow for easy watering and regular maintenance. I can install the AGM batteries on shelves, since they don't need watering I don't have to access the tops except to wipe them down occasionally to remove dirt and dust or to check the cable connections. I can still easily reach the tops of the batteries in this configuration to check connections and add or remove batteries. It is space efficient. Rather than being spread out all on the same level I can stack them. The buss bar allows me greater freedom to place the batteries since I don't have to connect series pairs to other series pairs. I only need access to the buss bar. I can also use smaller battery cable. In this installation I am using 1/0 cable instead of the 2/0 I needed for my original bank. The cables only need to be sized for the load being carried by that pair of batteries (in my case, the total system load divided by 12 since I have 12 series pairs in my bank). Even at the maximum capacity of the inverter (60 amps at 120VAC) that's only 25 amps per battery, and that only for short periods of time (the inverter will only put out approximately 33 amps continuously (just under 14 amps at 24VDC per series string).
Just because an installation of AGM batteries is easier and cleaner doesn't mean you can scrimp on safety. Though you don't need eye wash, or baking soda or rubber gloves you do need to protect yourself and your system from the power that is stored in all those batteries. There is a reason welders use DC. Accidentally crossing the terminals of even one battery (connecting the positive to the negative of the same battery or series string) can unleash some hellish amounts of energy in a flash (literally). Whenever I am working with batteries I follow a number of precautions...and I use a lot of electrical tape, you can never have enough electrical tape.
I couldn't be happier with my new battery setup. I have more space available in the power room with the new batteries in cabinets along the wall. My installation is cleaner and neater. I have 1/3 more capacity than with my old batteries at less cost. With the AGM batteries I find that my system holds its voltage far better than with L-16s.
I ran an informal test by turning off the breakers controlling the PV input so there was no charge and running all sorts of things in the house to see how the batteries would stand up. I did laundry (everything I could find to wash), made lots of toast, ran my small computer network 24/7, left lights on (that was pretty hard to do, I've got myself trained), watched TV and generally acted quite decadently.
After 3 1/2 days with no charge and constant use my system voltage finally dropped below 24 volts. I had used 900 amphours from my 1500 amphour battery bank in that 3 1/2 days and I could easily have gone another day before I might have needed to fire up a backup generator. Remember, this was with no incoming charge from the PV panels. Even in a stormy situation we would get some incoming power.
I figure that the kind of unusual usage I subjected the bank to during the test was the equivalent of 7 days usage during bad weather. Having a week's worth of power available in the batteries is a pretty comforting feeling.