Living and working in sustainable environments: energy storage

There are many types of electrical energy storage; from capacitors, flywheel to rechargeable batteries of various technologies. However, in the home, office or factory, only a few are practical or affordable.

On the battery side, as the most practical form of storage, there are some challenges and tradeoffs that need to be taken into account. For example, the cost of batteries and their specific performance in terms of charge efficiency, discharge efficiency, and self-discharge performance versus price.

Solar or wind powered charging will be most economical for a deeply drained 12 volt flooded lead acid battery (10.7 volts no load) up to about 87% charged with ~ 91% efficiency. From 87% to 100%, the charging efficiency drops to 55% unless charging very slowly, which will not be possible for daily cycles. The initial cost of a Raw watt hour (R-Wh) for flooded lead acid batteries is approximately $ 0.24 .. $ 0.19. The size of the battery capacity also depends on how you want to use the battery. There will be two numbers of importance, the cost per Usable watt hour (U-Wh) and the expected life time in cycle days.

The U-Whs are significantly lower than the R-Whs advertised by the manufacturer and are dependent on a number of other factors, such as the general area of ​​operation between 55% discharge and 13% discharge. When operating in this area, charging efficiency is maximized by approximately 91%, but the life cycle of a lead-acid battery is greatly reduced and graphs from the battery manufacturer should be used to determine the life cycle. realistic. When used with solar generation or load balancing, a cycle day is mainly one day of the year. Lowering your power consumption on weekends can extend the life of your lead-acid battery. anti-sulphidation charge can be applied during that period.

Charging efficiency 91% (at U-Wh 32% of R-Wh)

Battery type / expected life cycle (days)
Supermarket chain store 12 volt / 300 car battery (less than a year)
Marine Deep Cycle / 400 (just over a year)
Flooded Sealed Lead Acid (250 Ah / 12 Volts) / 900 (2-3 Years)
Flooded controlled lead acid (1000 Ah / 12 volts) / 1400 (4-5 years)

When using flooded lead-acid batteries in the operating area between 90% charge and 70% charge (10% discharge to 30% discharge), the life cycle is greatly improved, but the charging efficiency not that good unless the batteries are charged. very slowly, in which case the charging efficiency can be improved to about 83%. In this case, the battery capacity is derived from the most efficient charge rate that results in very large battery banks.

Charging efficiency 55% (U-Wh 20% or R-Wh)

Battery type / expected life cycle (days)
Supermarket chain store 12 volt / 350 car battery (less than one year)
Marine Deep Cycle / 450 (more than one year … 1.5 years)
Flooded Sealed Lead Acid (250 Ah / 12 Volts) / 1000 (5-6 years)
Flooded controlled lead acid (1000 Ah / 12 volts) / 1600 (7..10 years)

Charging efficiency 95% (U-Wh 70% of R-Wh)

When you use LiFePO or also known as lithium iron phosphate batteries, the charging and discharging efficiency is significantly higher by around 95% at a higher percentage of U-Wh compared to 100% R-Wh, but the cost may be 2 to 3 times the cost of lead acid flooding which has a 2x long life at 80% of original capacity. The total economy turns out to be exactly the same cost over 10 years as the equivalent flooded lead-acid battery system for roughly the same storage and discharge performance. However, the storage space and weight of the same capacity favor LiFePO with 1/3 the volume and weight of lead-acid batteries and half the weight and volume of Ni-Fe batteries. Also, when you start looking for less than ideal conditions (cold, heat, unpredictable charge and depth of discharge), lead-acid battery performance drops dramatically and is worth the current cost of a LiFePO battery system.

Charging efficiency 90% (U-Wh 80% of R-Wh)

When space and weight are not so critical, a Ni-Fe battery is a solution, it requires a storage and operating environment of 0 ° .45 ° C (32 ° .117 ° F) and therefore must work indoors or outdoors. in a harsh climate controlled area or more sophisticated cooling built into the battery cell. However, the charging and discharging efficiency is high at 90% and the cycle time is more than 3000 cycles, giving it a service life of more than 10 years. Ni-Fe batteries are known to last 20 years at full capacity.

What this means for load balancing battery-based systems is that the cost of the equipment to charge the batteries efficiently, as well as the correct size of the battery to optimize charging efficiency and life time (replacement time) must be carefully considered to determine the system sizes that can produce a revenge of load balancing.

Both systems, flooded lead acid and LiFePO, you will benefit from each cell being fully managed by an appropriate charger controller rather than using multi-cell batteries that do not allow for individual cell management or replacement. Ni-Fe batteries are more tolerant of overcharging and undercharging, but require regular water filling or an automatic system to achieve automatic maintenance.

How often you have to change batteries or supplement with additional capacity will affect your ROI or cost per watt hour (Wh) stored and released. You may decide that it is more cost-effective to add 20% new batteries at the 80% end of life cycle if your battery type continues to reduce capacity at the same rate per cycle as it did to 80% capacity. point. In this case, you need to plan in advance the space required for the additional 20%. Then you can withdraw 20% when the full capacity has reached 80% again and replace with 20% new batteries and so on.

Disclaimer
No claims are made in this article as to the accuracy of the values, prices, or other information provided, as they are indications or rough estimates based on publicly available information and the author’s experience.

Tips for selecting a battery
Some battery manufacturers are adept at misrepresenting the true performance of their products.

Although a Supermarket chain store The battery is typically warranted for 1 year, has no Amp Hours (Ah) or Wh rating, and would only survive 150 deep discharges to full charge cycles, depleting its capacity to 50% or failing completely.

Do not use batteries for which you cannot obtain life cycle diagrams that show after how many deep cycles the 80% capacity mark is reached.

Starting a car or tractor once a day is not a deep cycle, cranking power and cranking cycles are irrelevant in a load balancing application.

A battery made for the app will have an Ah or Wh rating. Keep in mind that if your application is mission critical, design capacity with 80% of end-of-life capacity in mind.