 
If you want to improve reliability add a DC Generator rather than increase the Battery Bank.
When optimizing a power system for efficiency and reliability too many engineers fail to create a balance between Power Generation and Energy Storage. Too frequently the battery, which is a storage device, gets placed in the role of a power generation device; compromising the efficiency and reliability of the system.
For instance, in computer UPS systems, increasing the battery bank size is a solution to provide reliable power in the events of longer utility power outages. Increasing the battery bank size also compensates for problems of bad cells within the battery bank.
In traditional Telecommunications Systems, BTS site batteries are used to back up the grid and follow the same example in computer UPS systems. Battery banks are increased in capacity to provide power for power outages of longer duration, to compensate for problems within the battery bank itself, or to allow sufficient time for service personnel to arrive at site with a generator.
In Solar Photovoltaic Systems the battery stores the surplus energy produced by the solar and the stored energy powers the load at night and during poor weather conditions. Battery banks are increased in size to match the worst-case weather conditions, and to compensate for problems within the battery bank itself. The typical design principle on solar systems is to size the battery bank for 5 days of autonomy.
It should be appreciated that increasing battery bank sizes to provide sole energy storage is very expensive and decreases reliability. Design engineers are now appreciating that, Diesel, LPG, or CNG can reduce the need for larger battery banks. Fuel is also an energy storage medium and together with a smaller battery bank will create a more efficient, reliable, and cost-effective system.
Adding a DC Generator to the system is the most efficient and reliable means to improve system efficiency and reliability while reducing both the capital cost and operating costs of the system. The DC generator allows a system integrator the ability to reduce the size, maintenance, and cost of the power system.
The DC Generator/Battery Bank System
With this improved system, the battery bank and DC generator are used in combination to complement each other and to greatly reduce cost. The battery bank is used to improve the performance of the DC generator in applications where the loads vary in power demand. At low loads, where the DC generator would operate outside its peak efficiency, the battery bank saves fuel by allowing the DC generator to cycle off.
Application Example
In solar hybrid systems the battery bank is used to absorb the excess energy from the photovoltaic arrays and wind generators. For example, if the load is 1.5 kW and the solar array produces 5 kW, the battery would absorb 3.5 kW averaged over the peak sun period (5 hours x 3.5 kW=17.5 kWH). The battery would start to deliver the stored 17.5 kWH late in the afternoon when the sun’s energy declines and the array output drops below 1.5 kW. This would be one cycle using solar energy. The next cycle is generator powering the load and charging the battery at the same time.
The sizing of a battery bank is optimized by is combining the following requirements:
- Being able to absorb all of the excess energy from the solar array, wind turbines.
- Having sufficient storage capacity to power the system at low loads; thereby, supporting the off cycle of the DC generator. Start your optimization with an 8 to 12 hour run time on the battery alone.
- Optimizing the battery service life by minimizing the number of charge/discharge cycles. Increasing the storage capacity reduces the number of charge/discharge cycles thereby increasing the battery service life; but at the same time more battery increases the cost of the battery bank. For off grid applications start with 20% discharge for lead acid batteries with 1 or 2 cycles per day. For lithium-ion batteries start with that 60% discharge with 2 or 3 cycles per day.
Increasing the size of the battery bank does not help to increase reliability as the battery is only a storage device not a power producing device. The solution for increased reliability is at the power producing / energy conversion sources and not at the storage medium. Increasing reliability means improving the availability of power; therefore, to improve site reliability means adding a second DC generator and possibly more solar.
Improving storage reliability requires the proper selection of battery chemistry and construction. Many applications have been using batteries that were optimized for standby applications and not cycling applications. And some battery manufacturers have exaggerated the performance of their batteries and cycling applications.
Having an oversized battery bank on site simply means that you buy more time for the technician to arrive on site. And in rural areas it may take a week to get the technician on site. Once the technician is on site he is faced servicing a large dead battery. Solution is to keep the battery small and to make the power generation sources redundant.
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