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Stone Marmot Goes Solar: Part 7, Installing The Battery Backup Portion

By Sid of Stone Marmot

Mar. 11, 2010

So, we now had a working grid tied solar electric system. But at the time Harrimans installed the grid tied portion, Harrimans hadn't received the battery backup inverter, an SMA SI5048 yet. So, a couple of weeks later they received the SI5048 and proceeded to install the battery backup portion of our system.

Figure 1 shows the inverters and related electronics mounted inside the garage by the main breaker panel. Notice it got a lot more complicated when compared to the original grid-tied-only system shown in Figure 3 in Part 5 of this article series. The light gray Sunny Boy SB3000 and the DC disconnect that were installed previously are still there. Added are the big yellow Sunny Island SI5048 inverter, the smaller black Outback PSX-240 autotransformer mounted below the SI5048, and a small subpanel above the light gray SB3000. The SB3000 converts the high voltage direct current (DC) from the solar panels into 240 VAC and feeds the subpanel above it. The added subpanel contains all the critical circuits in the house that we want to maintain power to, such as the refrigerator, lights, etc, if the main utility power grid goes down. The SI5048 inverter charges the battery bank, senses if the main power grid goes down and disconnects our house from the grid if that happens, and converts the 48 VDC power from the batteries to 120 VAC to run the AC loads within the house if the main power grid goes down. The autotransformer matches the 120 VAC output of the SI5048 to the 240 VAC output of the SB3000.

Inverters and related electronics mounted inside the garage

Figure 1 - Inverters and related electronics mounted inside the garage.

Remember that, with a strictly grid tied photovoltaic (PV) solar electric system, when the main power grid goes down for any reason, you lose all power, even if it is a bright, sunny day and your PV panels are still generating lots of electricity. This is because your grid tied inverter gets it frequency reference from the grid and won't work without it. This is done on purpose to assure good matching to the grid and to avoid backfeeding the grid if the grid power goes down to protect any maintenance people working on the grid. You need this added circuitry to allow you to use your solar power when the grid goes down and still keep you isolated from the grid to protect the maintenance workers. This extra complexity basically allows your solar system to act like a giant uninterruptible power supply (UPS) for your whole house, similar to the UPS you may already have for your computer. And this system will switch automatically from grid to backup and back to grid as fast as the UPS used for your computer, keeping steady power to the critical AC circuits in your house.

Two additional power disconnects and the battery cabinet were added on the outside of our house by the power meter. The disconnects are same as shown in Figure 2 of Part 5 of this article series, one for disconnecting the DC output of the solar panels from the inverter and the other is an alternating current (AC) disconnect between the solar electric system and the main AC power grid. This last disconnect allows the power company to disconnect the solar electric system from the utility power grid in case they have to do maintenance on the local grid, though the power company will probably just remove the power meter to absolutely guarantee that the solar installation can't back feed the power grid during maintenance conditions.

The battery cabinet that Harrimans typically used for installations the size of ours was discontinued by its manufacturer. So they substituted the Midnight Solar MNBE-B cabinet instead. Midnight Solar makes good products and the MNBE-B is a quality cabinet. Unfortunately, it is rated for indoor use only, which isn't to code for our outdoor installation.

Also, the batteries barely fit in the enclosure. The data sheet for the cabinet stated that it could hold eight golf cart batteries. But golf cart batteries come in a couple different sizes. They must have meant the smaller golf cart batteries, not the larger ones we were using. These larger batteries were very difficult to install and virtually impossible to maintain in this cabinet, which again violates most building codes. Note that even though these are sealed "maintenance free" batteries, there is still some light maintenance needed, like checking for corrosion on the terminals and making sure the cables haven't loosened with time and temperature.

I pointed out these problems with the cabinet to Harrimans. The owner agreed, saying his own people complained about this substitute battery cabinet. So they replaced it with the Midnight Solar MNBE-D3R cabinet. This cabinet is aluminum instead of steel, is outdoor rated, and is the same size as the MNBE-B cabinet except about eight inches taller. Consequently, there is much more clearance for the batteries.

The eight batteries, incidentally, are each the Discover EV305A-A. They are absorbed glass mat (AGM) valve regulated lead acid (VRLA) "maintenance free" batteries rated 6 VDC, 305 Ampere-hour each. They are wired in series for a 48 VDC, 305 Ampere-hour battery bank. Total power stored is about 15 kWh, which should run all our 120 VAC loads under normal use for about seven days without any sunlight. Of course, you would not want to run the batteries down that far as battery life is dramatically reduced for lead acid batteries if they are very deeply discharged. This would rarely be a problem as it is very, very unusual to go more than three days here in Florida without any significant sunlight, which would recharge the batteries if the grid were down for an extended period of time. Also, if the grid were down for an extended period of time and we were going through a series of cloudy days, we would probably go into an more energy conserving mode of operation.

After the battery cabinet was changed out, the system passed its electrical inspection without any problems. So we now had a complete, fully functional grid tied, battery backed up solar electric system. But we had not received our new power meter yet. Remember from my last article that the digital power meters most power company customers have can't tell which direction power is flowing, so we get billed for both using electricity and for generating electricity until we get a new bidirectional power meter. So our solution was to run off-grid until we received our new power meter. This is easily done by either switching off the outside disconnect or the breaker that is in series with it inside our main electrical panel. This also provided a good test of the battery back up system.

We ran off grid for 15 days until we got our new bidirectional power meter. Everything worked pretty much as expected with one small exception. When the battery charge was low, the SB3000 would run continuously when there was sufficient sunlight to charge the batteries, as expected. But as the batteries approached being fully charged, the SB3000 would be kicked off by the SI5048, even if there was plenty of sunlight and the PV panels were still generating lots of power. Often, the SB3000 would cycle on for about ten seconds, then off for about five minutes, as the batteries got close to fully charged. It seemed to take forever to get that last 10 % of charge into the batteries.

This "bang-bang" type of control of the battery charging seemed rather crude, rather wasteful of solar energy, and was not expected. But, after thinking about it, there needs to be a way to taper off the charging of the batteries for maximum battery life. This means all the solar energy is not going to be able to be used when the battery charging is being tapered off. To use all this solar energy, the excess not needed for charging would have to be sent to some diversion load, such as directly to a water heater. Doing this would dramatically complicate the design of the inverters and the solar installation, significantly increasing the cost of both.

A suggestion for the inverter manufacturer, SMA, would be to include a control signal in the SI5048 that would be activated whenever the battery charging got into this tapered "bang-bang" mode of turning the Sunny Boy inverters on and off. This signal could be used by the customer to turn on diversion loads, such as a secondary water heater element, when in this tapered charge mode. This signal should be rather cheap and easy to add to the SI5048, if it isn't already there. Most installations probably wouldn't presently use this feature. But this may be important for people who live either off grid or in areas with a very unreliable grid and need to squeeze as much power as possible out of their systems.

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