architecture tips

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

On the battery side, as the most practical form of storage, there are some challenges and trade-offs that have to be considered. For example the cost of batteries and their specific performance in terms of charging efficiency, discharging efficiency and self-depletion performance versus price.

Charging from Solar or Wind will be most economical for a deep depleted 12 Volt flooded lead acid battery (10.7 volts at no load) to about 87% of charge with an efficiency of ~91%. From 87% to 100% the charging efficiency drops to 55% unless it is charged very slowly which will not be possible for daily cycles. The initial cost for 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 the way 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-Wh are significantly lower than the R-Wh advertised by the manufacturer and depend on a number of other factors like the general area of operation between 55% discharge and 13% discharge. When operating in this area the charging efficiency is maximized to about 91% but the cycle life of a lead acid battery is greatly reduced and charts from the battery manufacturer have to be used to determine the realistic cycle life. When used with solar generation or load-balancing, a cycle day is mostly a day of the year. If your energy use is lower at the weekends this can extend lead acid battery life as anti-sulfurization charging can be applied during that period.

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

Battery Type / Expected Cycle-Life (days)
Supermarket Chain Store 12 Volt Car Battery / 300 (less than a year)
Marine Deep Cycle / 400 (little more than a year)
Flooded Sealed Lead Acid (250 Ah/12 Volt) / 900 (2..3 years)
Flooded Managed Lead Acid (1000 Ah/12 Volt) / 1400 (4..5 years)

When flooded lead acid batteries are used in the area of operation between 90% charged and 70% charged (10% discharged to 30% discharged) the cycle life is greatly improved but the charging efficiency is 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 charging rate resulting in very large battery banks.

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

Battery Type / Expected Cycle-Life (days)
Supermarket Chain Store 12 Volt Car Battery / 350 (less than a year)
Marine Deep Cycle / 450 (more than a year..1.5 years)
Flooded Sealed Lead Acid (250 Ah/12 Volt) / 1000 (5..6 years)
Flooded Managed Lead Acid (1000 Ah/12 Volt) / 1600 (7..10 years)

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

When using LiFePO or also known as Lithium Iron Phosphate batteries, the charging and discharging efficiency is significantly higher at around 95% at a higher percentage of U-Wh versus 100% of R-Wh, but the cost can be 2 to 3 times the cost of flooded lead acid having 2x extended cycle life at 80% of original capacity. Total economics work out to be exactly the same cost over 10 years as the equivalent flooded lead acid battery system for approximately the same storage and discharge performance. However, the storage space and weight for the same capacity is heavily in favour of LiFePO with 1/3-rd of the volume and weight of lead acid batteries, and half the weight and volume of Ni-Fe batteries. Also, when you start looking at nonideal conditions (cold, hot, unpredictable charge and depth of discharge), the lead acid battery performance falls off dramatically and it 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 has to operate in-doors or in a crudely climate controlled area or more sophisticated cooling built into the battery cell. However, charging and discharging efficiency is high with 90% and the cycle time is in the 3000-plus cycles giving it a life time of 10 years plus. Ni-Fe batteries have been known to last 20 years retaining full capacity.

What this means for load-balancing systems based on batteries is that the cost for the equipment to charge the batteries efficiently as well as the correct battery size to optimize charging efficiency and life time (time to replacement) have to be carefully taken into account to determine system sizes that can produce a payback from load-balancing.

Both systems, flooded lead acid and LiFePO, will benefit from each cell fully managed by an appropriate charger controller rather than using multi-cell batteries that do not allow individual cell management or replacement. Ni-Fe batteries are more forgiving to overcharge and under-charge but require water-refilling on a regular basis or an automatic system to accomplish their automatic maintenance.

How often you have to change your batteries or supplement with extra capacity will affect your ROI or cost per stored and released Watt hour (Wh). You may decide it is more cost-effective to add 20% new batteries at the end of the 80% cycle life if your type of battery will continue to lessen its capacity at the same rate per cycle as it did up to the 80% capacity point. In this case you have to plan the space required for the extra 20% beforehand. You can then decommission 20% when the total capacity has reached 80% again and replace with 20% of new batteries and so on.

Disclaimer
In this article no claim is made as to the accuracy of any values, prices or other information given as they are rough indications or estimations based on publicly available information and the author's experience.

Tips for Selecting a Battery
Some battery manufacturers are masters in misrepresenting the true performance of their products.

Although a Supermarket Chain Store battery normally has a 1 year warranty, it has no Amp hour (Ah) nor a Wh rating and would only survive 150 deep discharge to fully charged cycles depleting its capacity to 50% or failing completely.

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

Starting a car or tractor once a day is not a deep cycle, cranking power and cranking cycles have no relevance in a load balancing application.

A battery made for the application will have an Ah and or a Wh rating. Be aware that if your application is mission critical design the capacity taking the 80% of capacity at end of life into account.

Klaus Bollmann is a 30 year veteran in energy conservation, resource saving innovative products for a sustainable environment. Click the link If you are interested in Ringdale DC Systems high performance 48V DC products or ActiveLED 48V DC capapable lighting systems.

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