August 18, 2017

Batteries for hybrid off-grid power – 5 questions you should ask

offgridpv

By: Anders Hansson and Thomas Rahkonen

In backup applications for telecom sites in developed markets with stable grids, battery operation is a well-understood topic with expected battery lifetime of more than ten years. However, for telecom sites in emerging markets with very unstable or no power grids at all, things are not quite so straightforward…

scenarios, batteries are typically employed in a charge-discharge-charge (CDC) pattern in order to minimise the runtime of diesel gensets. It is a strategy that generally works, but with a couple of significant drawbacks. First, such active use can often reduce the service life of batteries to just two to four years – even if special cyclic lead-acid batteries are used. And second, to keep diesel usage to a minimum during CDC operation, it is not effective to fully charge the batteries during each charging cycle, as battery power charge acceptance decreases rapidly as they fill up. The result is that gensets end up operating at low output power and therefore low diesel efficiency – thus defeating the object of using the CDC strategy in the first place.

Battery technologies

So what is the answer? Well, let’s first take a look at the main battery technologies available today:

Lead acid batteries: Lead acid technology has been the mainstay for industrial battery applications for decades. They are available in 12V and 2V varieties, both of which have their pros and cons. 12V lead-acid batteries have relatively few mechanical constraints, are easy to charge and can be configured in parallel 12V strings in order to meet an exact Ah requirement. However, string imbalance may develop over time, they do not tolerate thermal abuse particularly well and their cyclic life per cell is not the highest, especially not for front access blocks.

Meanwhile their 2V cousins are extremely robust, very tolerant of electrical and mechanical abuse, are able to approach the VRLA theoretical maximum for cyclic life, and have ‘built-in’ theft protection given that 2V power is useless for domestic applications. Further, they can be charged and discharged with exactly the same current at all times, with the result that the aging of all the cells in a bank will be uniform. But on the downside, 2V batteries require more rigid mechanical integration with the entire bank needing to be disconnected during maintenance or cell replacement. And they are also slightly heavier than their 12V rivals for any given Ah rating.

Critically though for hybrid power applications, whether 12V or 2V it is important to select batteries designed for cyclic operation. Batteries designed for good power grids (which use float operation) are typically far cheaper but will fail fast in more challenging applications.

Lithium technology: In comparison to lead acid batteries, many people assume lithium must be the best battery type simply because it is the newest technology. However, it is not as simple as that.

Lithium batteries have a number of clear advantages. Their small size with respect to energy storage capability is one, as is their ability to harvest and transfer energy irrespective of their charge. They can also be charged extremely fast and can accept large fluctuations in charging current. However, the use of a battery management system is mandatory, as lithium cells always need balancing. Lithium technology is also costly – at least three times more expensive per watt hour (Wh) as compared to VRLA. And lithium batteries can only be used where the ratio between energy storage and constant power to the load is small or moderate.

Hybrid power system selection: To further complicate the matter, any battery technology will only be as good as the hybrid power system using it. How precise is the system’s battery control? Has the hybrid power system vendor conducted appropriate testing and development with the battery vendor to make sure performance will be maximised? It is important that batteries are kept within the allowed temperature range as specified in the supplier warranty. So a good hybrid system will monitor this temperature and proactively warn the operator before problems occur. And with batteries constituting a significant portion of the cost of installing and running a hybrid power system, it should also safely log all usage and charge cycle data for potential warranty claims if batteries fail prematurely.

So which is the ‘best’ battery? What soon becomes clear is that there is not a one-size-fits-all answer to the question of which battery is best for hybrid power systems. Selection ultimately comes down to a number of different factors, including:

• Which battery type best complies with the mode of operation you need?

• Is the battery’s operational temperature range adequate at your locations? • Can you remain inside the limits of the maximum cyclic life or energy throughput for the service life you need?

•Is grid power available at each site and if so, how reliable is it?

• And how long will it take to reach any given site in the event of a grid or genset failure?

In an ideal world, you would choose the most appropriate batteries to install at each and every individual site, rather than making a generic decision at a network level. This may not be practical from purchasing or ongoing maintenance perspectives, so a trade-off will always need to be made somewhere. Overall though, for lead acid, 2V is often the preferred choice versus 12V alternatives but with lithium technology evolving at a terrific pace in the automotive industry, it won’t be long before it becomes a serious option for many hybrid power scenarios.

Five key battery questions you should ask:

  1. How do I choose the right battery for different site types?

Critical data points at a site level are the duration and frequency of any power interruptions at each site each day. Also important is how long it takes for service personnel to reach the site in the event of a power failure when the site will need to run on batteries only, as this will determine required battery autonomy time.

2. Is there a solid business case for deploying lithium versus lead acid?

Lithium technology is being increasingly adopted for sites with a reliable grid and limited power interruptions, but remains technically and financially challenging in pure off-grid applications where a larger battery bank is needed for battery autonomy.

3. How strict are extended battery warranties?

Most (if not all) warranties will typically be conditional if the batteries are being used in scenarios that are more challenging than their factory tests when new. Therefore, the exact charging and cyclic operation scheme must be disclosed and agreed upon in advance if the battery manufacturer is to commit to cover long-term performance in the warranty.

  1. How big should the battery bank be for a hybrid power system?

The size of the battery bank at any given site is typically driven by the size of the site load and the minimum battery autonomy time required. In pure off grid applications, an additional factor to consider is that the number of daily charging cycles must be kept below a certain value – typically four times per day – in order to prolong battery life. So the larger the battery bank, the fewer the charge cycles required.

5. What is the role of standards when selecting a battery?

International standards, IEC and others are very helpful, but only if a battery manufacturer’s compliance statement is accompanied with traceable test records confirming that all the test criteria in each standard have been met. Partial compliance is misleading and prevents fair and objective comparisons between manufacturers.

Anders Hansson is Power Solutions Manager, Flexenclosure and Thomas Rahkonen is Chief Technology Officer, Flexenclosure

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