Consulting-Specifying Engineer - November 2007 - (Page S4)

on-peakperformance Best Battery Selector By Mike Benson and Bill Kaewert President Stored Energy Systems Longmont, Colo. >>>>>>>>>>>>>> in Engine Starting Using a BBS in engine starting systems adds up to reliability when one battery fails. >> A best battery selector (BBS) in engine starting systems enables use of two independent batteries to increase starting system reliability. In other words, if one battery fails, there is backup. In this case, the term “battery” means multiple cells connected in series. There are two reasons for using a BBS and two separate batteries—here we’ll call them “Battery A” and “Battery B”: 1) The engine can be started even if one battery fails (dead, short, or low voltage). 2) The failed battery is automatically isolated from the good battery, preventing a discharge path from the good battery into the failed battery. What’s a BBS? A properly engineered BBS consists of two highcurrent diodes, specialized heat sinks, voltage surge protection, and high-current terminals assembled and mounted in a metal housing. Each diode Normal Operation Batteries each provide ~50% power allows starter-motor cranking current to flow in the forward direction, but blocks current from flowing in the reverse direction. The BBS is so named because the best battery is the one that provides the majority of current to the starter. Because a BBS is a passive device, it does not perform any switching or control functions, so the best battery is selected in a passive manner, based on the principle that current flows from the source of the highest voltage. In a system that has well-matched batteries and cabling, under normal operating conditions, two good batteries will each provide about 50% of the total power delivered to the starters. In the case of a partial battery failure, the stronger battery will supply the majority of the starter current. But with total battery failure, the remaining battery becomes the “best battery” and provides 100% of the power delivered to the starter. Figure 1 below shows a starting system drawing 2,000 amps. Battery B FAILED Current flows to the starter and NOT into failed Battery B (Battery A current is blocked by Battery B’s diode) Battery B Battery A Battery B Failed + - Battery A BBS BBS - + + - - + 1,000 amps 1,000 amps 2,000 amps 0 amps 2,000 amps Starter 2,000 amps Starter current is blocked here Figure 1 - In this system that draws a starter current of 2,000 amps, when Battery B fails, current flows into the starter and not into the failed battery. The BBS couples good batteries and isolates a failed battery. 4 november2007

Table of Contents Feed for the Digital Edition of Consulting-Specifying Engineer - November 2007

Consulting-Specifying Engineer Contents Editor's Viewpoint Letters In The News M/E Roundtable Emergency Power for Healthcare 2007 Products of the Year Industrial-strength Lighting Hazardous HVAC Codes and Standards Management Report Jobs/Classifieds Specifier's Notebook On-Peak Performance Contents Best Battery Selector in Engine Starting Applications NEC 708: Practical Impact on Backup Power Systems Protecting Power: Specifying Outdoor Generator Enclosures Investing in Backup Power Systems Coes, Consultants, Manufacturers, and Standby Power Systems

Consulting-Specifying Engineer - November 2007

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