We still need to do some “stress” testing on the system, by pulling energy out of the batteries under controlled conditions—such as intentional grid loss with and without PV and wind inputs. We want to test how well the inverter responds to changing generation levels and house loads, including EV charging. Art’s team is developing Web page interfaces for both the Li-ion battery management system (BMS) and DRI inverter so we can monitor and tweak the systems. The ultimate goal is to simplify and make the whole system as robust, reliable, and manageable as possible.
Selecting & Using Lithium Batteries
Although lead-acid batteries are typical for battery-based RE systems, I wanted to try lithium-ion batteries. My research indicated that they can be superior in many ways if properly cared for, offering about twice the energy density of a lead-acid battery; four times the storage capacity in the same footprint; a longer cycle life; and half of the weight. Lithium-ion battery management systems continuously monitor every cell’s voltage and temperature and actively manage each cell’s state of charge (SOC) with a “balancing” circuit.
Our 15.4 kWh GBS battery appears modest compared to the estimated daily energy use of 20 kWh. However, having more than 16 kW of generating capacity guarantees that the battery bank will not be stressed by daily deep-cycling—even if the utility grid goes down for an extended period. I prefer to invest in ample generating capacity rather than massive storage capacity. If generation cannot keep a large battery bank charged, then the batteries will degrade faster, requiring premature replacement. Wind and solar energy are complementary—there’s wind at night and in winter, and sun during the day and in summer. This characteristic allows a smaller battery without sacrificing energy reliability.
We expect battery maintenance to be essentially zero. We also expect them to last two to three times longer than an equivalent lead-acid battery. The GBS lithium-ion batteries require no “watering” or electrolyte monitoring. If a cell in the series string goes bad (there are 120 cells), it will require replacement, costing less than $100. Individual cell monitoring lets us know if a problem develops in any cell.
The DRI-10’s high-voltage battery input is attractive for the same reason that high-voltage PV strings are attractive—smaller wire can be used without increasing voltage loss. We used #8 AWG to handle up to 40 A at 400 V (16 kW), which is surely more attractive (and cheaper) than conventional bulky 2/0 or 4/0 low-voltage battery cables.
An Added Load
A day after the PV system was in, Art showed up in a 2103 Nissan Leaf electric vehicle (EV) and let me drive it. I was in need of a new car, and I liked the idea of fueling my car with energy from the wind and sun, so I decided to buy one!
I routinely charge my car’s battery after each trip, even though I could easily make two trips. However, if the battery is fully charged when I set out from my home atop a hill, I cannot capture the benefits from regenerative braking. This means I’m using electricity to charge the battery, and then using the conventional braking, which adds wear and tear to the car. Instead, I charge the battery to 80% so I can use the regen braking, and possibly extend the life of the battery and the car’s conventional brakes.