Solar + Storage Battery Considerations: Chemistry, Design & Charge

Expert instructor Wes Kennedy answers 3 battery consideration questions in his Advanced Solar + Storage course discussion board…

Student 1: One of the biggest hurdles we deal with on the commercial side of storage is the Authority Having Jurisdiction, or AHJ. Most often, these issues come up with the Department of Buildings (DOB) and local Fire Departments, as they can either approve or refuse to approve Li-Ion battery chemistries. They know and have the experience with lead acid, but Li-Ion is an emerging technology to them, and with minimal regulations and standards to go by, their reluctance to approve storage in many conditions using Li-Ion chemistry is quite evident. We are currently researching different solutions to suppress or stop any fire / thermal runaway events in the event it could happen.

My question is this: What is your experience with the regulatory AHJs with the Li-ion batteries and does any one have a solution they can share related to fire suppression / protection that has passed stringent AHJ scrutiny?

Wes Kennedy: This is the cutting edge issue in terms of implementation. The best strategy is to ask each of the battery suppliers what their position is, what support (with permitting) they offer, what jurisdictions they have product in, etc. Also, begin conversations with the AHJ very early in the feasibility study phase.

Where possible, externally sited, self-contained container type solutions, complete with fire suppression, seems to get the easiest time with permitting.

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Student 2: For any system with batteries, what is the best design strategy to ensure that the array is large enough to charge the array to a good SOC (State of Charge) in winter?

Wes Kennedy: The answer depends on your goal. If you are stand alone, or want the ability to island indefinitely, then you plan for stand alone operation.
1) Load profile in winter
2) PV production in winter

Crunch the numbers… It’s not pretty for a place with hard winters.

So, if you need 10 KWh in winter (load profile), and you only get 3 hours of peak sun, then that is 3.3 KW pv. Of course these are rough numbers, you still have to account for all the losses we detail in class.

Remember, sizing PV and Batteries always starts with the Load. 100 KWH battery bank in Alaska with 1 hour of Peak Sun in winter means a 100KW array would bring it from empty to full in one day in a typical day. But, the load determines how empty or full the battery is likely to be each day.

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Student 3:  1.) In a grid-tied battery backup (GTBB) system, can the inverter sometimes turn off due to low battery bank state of charge?

Wes Kennedy:  1.) Yes, when the grid is down and the systems are “islanding”. This is the achilles heel of AC coupling. The PV inverter needs the battery inverter to create the grid, in order to function. If the loads exceed PV production, and the battery gets low, then the battery inverter will stop. This stops the grid, which stops the PV inverter from functioning causing a dead stop of your system. One best practice for AC coupling systems is to add load shedding capabilities, so you shed loads without losing your mini-grid and PV charging.

2.) Nearly all the “classic” battery inverters: SMA Sunny Island, Schneider XW, Outback, Magnum, etc. have programmable relays, one of which is typically pre programmed to close when the battery SOC gets to a pre-determined level. If you run a generator start loop through that relay, then a low SOC starts (and then stops) a generator.
At SMA, we had two relays per Sunny Island, so if you had multiple SI’s then you had a whole group of relays that you would typically program to first, start a generator, and if it didn’t work (genset out of gas or out of tune), then a second relay would shed least important loads, then it would shed the next group of loads, then finally all loads.

Enroll in Advanced Solar + Storage Gain an advanced technical understanding of residential, commercial and utility-scale solar-plus-storage systems, as well as expert insight on policy, financial models, and emerging markets and opportunities.

Working a solar plus storage project? Unsure which battery to select? Check out Wes’ free Battery Selector Tool

About instructor Wes Kennedy

Kennedy was most recently the senior application engineer for SMA America’s Hybrid Energy Solutions group. In this position, project scope ranged from residential PV with battery backup to MW-scale stand-alone microgrids primarily in North America and the Caribbean. He has worked in the solar industry since 1996, when virtually all projects were battery-based. He began his career with pioneering solar companies Jade Mountain and Real Goods, cofounded Colorado EPC firm Namaste Solar, and managed the engineering staffs at groSolar and Abound Solar. His skill sets include engineering, design, training, education, installation, O&M, software modeling, sales, marketing and management. Kennedy currently resides in Boulder County, Colorado.