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.
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