The sizing example above for a home backup application using VRLA batteries would probably not be the same for an off-grid application where a shallower DOD would be used to increase the number of charge/discharge cycles over the life of the battery. A VRLA battery may have 850 cycles at 80% DOD, and 1800 at 50% DOD and as much as 3,000 at 30% DOD. Flooded lead acid (FLA) batteries may have twice as many cycles at 50% DOD, but FLA batteries require regular maintenance that includes replacing lost water using special safety gear, and a well-ventilated storage area so highly flammable hydrogen gases don’t build up.
Other applications include selling energy back to the grid, or storing energy for later use during high demand periods when escalated time-of-use charges may apply. In this case, a smaller battery bank could be used with a larger array to maximize the sell-back in areas where utility outages and peak demand periods are measured in hours, not days. If the large array with a small battery bank strategy is used, it’s very important the charging devices are capable of limiting the charging current to the batteries until full before releasing the full energy into the power conversion system, or there is a high risk of premature battery failure.
Charging batteries properly not only extends the life of the battery, but also prevents dangerous off-gassing and thermal runaway, either of which could run the risk of fire. FLA batteries off-gas the most, especially when running an equalize charge, which is a higher than normal charging cycle run periodically to de-sulphate and restore the battery’s capacity. The water level should always be checked prior to an equalization charge, with proper protective clothing, eyewear or face shield. The battery plates need to covered with water, but not completely to the fill level as the water will release gas bubbles that could cause the acidic water to spill onto the battery and surrounding surfaces. Ventilation fans should also be in operation during the equalization charge.
While off-gassing with VRLA batteries is very limited due to 99.9 percent recombination of hydrogen, the charge rate is important as overcharging can cause a permanent loss of electrolyte since they are sealed and no additives can be included. Also, undercharging a VRLA battery causes sulfating, which cannot be reversed since equalization charges at higher voltages is not recommended for this technology and can cause premature failure.
Having an accurate charging source with three-stage charging for VRLA batteries is very important. Most have a charging timer for each stage, but only a few offer charger termination controls, which sense the end of the charge and ends the charging cycle when the battery is full. This is important as the state of charge (SOC), or percent full, when the charging cycle starts can be highly variable and will dictate how long it takes to charge the batteries.
For example, a 200Ah battery takes two hours to charge at 50 percent DOD (same as 50 percent SOC), but takes three hours at 80 percent DOD and only 1 hour at 20 percent DOD. If a two hour charge cycle is always used, the batteries will often be both under and over charged. With charge termination control, the battery current is monitored for what is called return amps, or end amps, which is the lowest charging current the battery will accept, as the charge current never drops to zero when the battery is full; usually about two percent of the AH rating of the battery. By monitoring the return amps, charge termination control can detect exactly when the battery is full no matter what the SOC when the charging was started.
Having a charging system compatible for Li-ion batteries is also very important. Li-ion batteries are intolerant of being overcharged. Regulation of the charging of a Li-ion battery is the job of a battery management system (BMS) and without it, thermal runaway and the risk of fire is almost assured when overcharged. With a good BMS that has failsafe controls, concerns about hazardous exposure or fire can be greatly reduced.
OPTIMIZING PV SYSTEMS - PART 2: ENERGY STORAGE
Energy Storage eFeature | January 2015