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Explore lead-acid battery balancers

Posted: 25 Jan 2016     Print Version  Bookmark and Share

Keywords:Lead-acid batteries  uninterruptible power supply  UPS  effective series resistance 

Lead-acid batteries are extensively utilised 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 uninterruptible power supply (UPS) 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 stack will share charge evenly, some of the batteries in the stack might be severely overcharged while one of the batteries may remain undercharged. Both overcharging and undercharging lead-acid batteries causes battery life degradation.

Overcharging lead-acid batteries causes the electrolyte water to break into oxygen and hydrogen gas, which depletes electrolyte levels in the batteries. This has two effects. The concentration of the sulphuric acid in the electrolyte increases, which is damaging to the battery plates and reduces battery life. Furthermore, since the electrolyte level has dropped, a portion of the plates are now exposed to air, causing plate oxidation and reducing battery capacity. Sealed lead-acid (SLA) and gel batteries are particularly sensitive to overcharging since any lost water cannot be replaced. Undercharging lead-acid batteries causes plate sulfation in which the sulphuric acid reacts with the plates to form lead sulfate crystals. This reduces the ability of the battery to accept a full charge, and undercharging worsens. This leads to premature battery failure.

To increase battery stack life, individual batteries in a stack need to be balanced. Conventional wisdom is that overcharging a series stack of lead-acid batteries achieves balancing of the individual batteries in the stack, which in theory helps increase battery life. However, this is a flawed approach.

The only way to ensure that all the batteries in a stack are at the same voltage is by employing a balancing solution in which overcharged batteries shed excessive charge while undercharged batteries are given extra charge. An efficient battery balancing solution requires a switch network that can be used to move charge from one battery to another to achieve a balanced battery stack. The control circuitry is complex and a discrete implementation is large and costly. The LTC3305 lead acid battery balancer enables individual batteries in a series-connected stack to be balanced to each other.

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