Balancing Lead-Acid Batteries

January 13, 2016 // By Aspiyan Gazder, Linear Technology Corp.
Lead-acid batteries are widely used in a broad range of industries and applications. The telecom industry uses a series stack of four lead-acid batteries to provide a 48V stack. Energy storage solutions (ESS) use lead-acid batteries in a variety of series and parallel configurations to store energy generated by renewable sources such as wind and solar. Series-connected lead-acid batteries find extensive use in the UPS (uninterruptible power supply) industry to provide backup power when the mains power is lost. Golf carts and other industrial electric vehicles are typically powered by a stack of series-connected lead-acid batteries.

In all the examples mentioned above, two or more lead-acid batteries are connected in series. When a single lead-acid battery in the stack fails, all the lead-acid batteries in the series stack need to be replaced to maintain battery stack performance. This is a considerable expense.

When batteries are manufactured, they conform to tight specifications for parameters such as energy capacity, ESR (effective series resistance), leakage current and number of discharge cycles to ensure quality, guarantee a minimum lifetime and meet various standards. Furthermore, these specifications only apply to a single battery. There are variations in battery specifications due to limitations in the manufacturing process, and when multiple batteries are stacked in series these specifications no longer apply to the battery stack. Batteries connected in series will drift over time due to unequal leakage currents, and capacities of individual batteries may change over time.

Figure 1: Top of stack voltage is not divided evenly across the batteries in the stack

Extreme operating conditions and frequent discharge cycles further exacerbate these problems, which eventually cause one of the batteries in the stack to fail. At that point, the entire battery stack is deemed to be bad, and all the batteries in the stack require replacement. Replacing a failed battery itself does not solve the problem since the replacement battery’s characteristics would be very different from other batteries in the stack and stack failure would recur. This problem is true for battery stacks made with batteries of any chemistry, not just lead-acid batteries.

In most series-connected battery stacks, only the voltage at the top of the stack is measured, and it is assumed the batteries in the stack are matched and hence share charge equally. Figure 1 depicts a scenario in which the top of the stack voltage is programmed to be 53.2V, but the individual battery voltages are unknown and may not all be 13.6V. Since not all batteries in the

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