Following the April 22, 2014, Webinar “Battery Backup for Grid-Tied PV Systems,” Solar Power World’s Managing Editor Kathie Zipp conducted a Q&A session with presenter Doug Pratt, Senior Engineer at SimpleRay Solar. Here is a recap of some of the listener submitted questions.
Kathie Zipp: Are purely off-grid solar arrays as expensive as on-grid backup systems?
Doug Pratt: Generally, no. Now, much of that is simply because people who are going into an off-grid situation are usually a lot more careful about their electric loads. They know that electricity’s going to be expensive and limited, so they choose appliances with a bit more care. We see off-grid houses simply consuming less electricity, which makes the systems less expensive. However, I have done some off-grid systems that were enormously expensive, so we make the system as big as the customer wants.
Zipp: Are there any particular safety standards for battery-backup systems?
Pratt: Batteries have to be enclosed so the terminals are protected. Sealed batteries don’t vent, but the building code does not recognize that. They just see all batteries the same, whether they’re wet cell or sealed, so the batteries need to be in an enclosure that vents to the outside.
Zipp: Do battery back-up systems work with microinverters?
Pratt: Absolutely. Not only will they work with microinverters, but the microinverter manufacturers generally support them.
Zipp: Do the microinverter systems require going back through a DC inverter to the batteries?
Pratt: No. Once you come out in AC, that AC from the micro-inverters is going to land on the critical loads breaker panel, AC breaker panel, and it’s all AC from that point on. It’ll pass through the add-on inverter, but it passes through as AC, not DC.
Zipp: In a grid-tied battery system, are the batteries only being charged by PV or can the utility grid charge the batteries too, such as in applications in rural areas where the grid is unreliable?
Pratt: With the direct battery-based systems, the inverters won’t let the utility charge the batteries unless there’s been a power failure or for some other reason, the batteries have been allowed to run down quite a bit. So, under normal conditions, the system is going to let the PV recharge the batteries. It’s just an overnight thing; don’t recharge it with PV the next day. But if the power goes out for any period, then the first thing that the inverter’s going to do when utility power comes back is run a float charge on the batteries, pop them back up and be ready for the next outage.
Zipp: What happens once batteries are 100% full, and what are the dangers of overcharging?
Pratt: As long as utility power is available with these systems, the inverter’s going to hold the batteries to a standard float charge level, a voltage that the batteries can stand 24/7. If the utility fails, we don’t have any place to dump that excess power and the charge controller will limit how much power goes into the batteries. Then it will bring the batteries up to 56 volts, or so, on these systems. But, then the charge controller cuts in and it starts limiting how much power goes to the batteries to prevent overcharging.
Zipp: What about hybrid systems, when you have a combination of PV and wind or hydro for example? Are there any different considerations for those?
Pratt: Each power source is probably going to have its own sort of charge control for it. They will operate in parallel, and as long as they’re all designed to charge a 48-volt battery pack, that’s fine. We can have multiple inputs to the battery, and as long as they’re all programmed to maintain the same voltage level, that’s fine as well.
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