Selecting the Right Battery for the Job
Feb 1, 2000 12:00 PM, By Glenn Alber and Mark Lamendola
Battery selection is no easy task. Using the same battery in two different applications may give you excellent performance in one instance and a complete failure in another.
Selecting the right battery for the job doesn't always mean selecting the cheapest option. If you want to select the battery that gives you the lowest total cost of ownership (TCO), it's going to take some research on your part. No single battery design works best in all situations, and new battery designs hit the market often. To help you keep up with the latest offerings, we've put together some selection guidelines to steer you in the right direction.
Battery designs. Today's designs make it hard for you to inspect and test your batteries. Cell terminal posts are either difficult to connect test instruments to, or they're simply inaccessible. This leads to performing tests inconsistently, unsafely, or not at all. If test results are inconsistent, then trend results are meaningless.
Figs. 1 and 2 (original article) show typical 12V battery modules used in low power applications. Though these modules illustrate maintainability problems, they are by no means the worst in the industry. Some battery modules do not even have terminal posts. They simply have leads extending out through the battery case and then terminated in a two-pin plastic housing connector. This may be a great design for the battery and cabinet manufacturer. However, it is a disservice to you; as it prohibits any kind of useful testing.
IEEE Standard 450 details the recommended testing and maintenance for large, stationary, flooded lead acid batteries while IEEE Standard 1188 describes the recommended practices for Valve Regulated Lead Acid (VRLA) batteries. According to IEEE Standard 1188, VRLA batteries are at least four times more maintenance intensive than their flooded counterparts. Therefore, VRLA batteries are a poor choice for holding TCO down when other factors are equal. Even in larger sizes, VRLA cells are usually difficult to test for internal cell resistances and intercell connection resistances. The main advantage of VRLA batteries is their low initial cost.
Unfortunately, purchase price takes precedence over TCO today. You pay for bad choices with lower system reliability and higher maintenance costs. One arrangement you don't want is the paralleling of individual cells in a string. This installation makes it impossible to identify problems without disassembling the intercell connections.
The battery modules in Figs. 1 and 2 have no direct access to the terminal posts of the battery. This means you can't correctly perform IEEE 1188's internal ohmic measurements. Since you must connect test leads to the stainless steel hardware, you must also include the resistance of the hardware in the measurement. For low ohmic batteries, the resistance of the stainless steel hardware can represent a large portion of the total, therefore making it nearly impossible to trend changes in the internal resistance as the battery deteriorates.
For example, a 1000A-hr, 2V VRLA cell has a typical internal resistance of 140 micro-ohms. If you don't place the test leads directly on the terminal post (and thus must include stainless steel hardware resistance), you may measure a total resistance of 340 micro-ohms. If the internal cell resistance increases to a failing value of 175 micro-ohms, the total measurement would be 375 micro-ohms. Since the 375 micro-ohm value reflects only a 10% change, most of us would fail to recognize we have a failing cell.
The typical battery cabinet is a maintenance nightmare. Not only are the batteries difficult to access, but reaching into these cabinets is also unsafe for you and the system. Because of the access problem, you may tend to do the maintenance poorly or not at all.
You may feel you can solve battery selection and maintenance problems by contracting out the battery maintenance services. However, this may compound the problem and is typically the wrong approach.
Base your battery selection on real-world criteria for lowest TCO. Then, write your performance specifications to get the best battery for your money. This is much more satisfying than simply creating problems by always going for the lowest price.
Sidebar: Battery Selection Considerations
• Physical case and terminal design of each cell
• Type of venting
• Type of electrolyte
• Location for maintenance accessibility
• Type of service
Sidebar 2: Battery Sense or Battery Cents?
Getting tired of "saving money at any cost?" Even if that cost is far more money than what we save? Can companies make their battery cabinets safer and easier to work in? Can battery makers redesign terminals for maintainability?
Major manufacturers in this industry would rather compete on value and total cost of ownership than on initial purchase price. But, the decision ultimately rests with those who spend the money. Today, we have a gulf between what the buyer specifies and what works best for the owner. For example, consider the isolation transformer. Why did manufacturers reluctantly remove the isolation transformer in the last few years? Because specifiers began awarding the UPS contract to the lowest bidder. They demand lower initial costs, but end up paying far higher total costs.
How can you reverse this? Set minimum standards for performance and safety, and document the rationale behind each. When you prepare a purchase requisition, think of labor savings, fewer safety problems, and increased system reliability. These are criteria that must make sense to those who approve your purchase requests.
When you consider all the costs of owning a battery backed-up power system (even over just a few years), it should be easy to justify doing things right. That inexpensive VRLA battery installed today will require replacement at least once, and most likely twice, over the life of a slightly more expensive flooded battery. And the maintenance required to assure a reliable system is at least four times more for the VRLA battery.
Sidebar 3: Maintenance Services
Many of you outsource battery maintenance. Typically, the contract for these services does not list the qualifications necessary to perform such services. Therefore, you may be allowing people to work on your equipment without licenses or credentials that guarantee proper service. The average battery service company has battery technicians with little formal training and not even a fundamental understanding of batteries. These people also may not have any formal training in safety nor an understanding of electrical theory as basic as Ohm's Law.
Remember, the liabilities of not setting formalized minimum requirements for maintenance personnel are there. The chances of improperly trained people getting hurt are real; they may also hurt your power system, and the likelihood they won't identify problems before it's too late is almost a guarantee.