![]() The LTC3305 provides two modes of operation, which are programmable via the MODE pin and four termination thresholds, which are programmable via the TERM1 and TERM2 pins. The maximum amount of current permitted to flow during any connection is limited by an external positive temperature coefficient (PTC) thermistor element. This sequence continues (1, 2, 3, 4, 1, 2, 3, 4) until all batteries in the stack (and the auxiliary cell) are voltage balanced to within a specified threshold, as shown in the curve in Figure 2b. The LTC3305 then commutates to the next battery in the stack. If the voltages are different, current will flow in the appropriate direction until the voltages of the individual battery and the auxiliary cell are equal. Each battery in the stack is individually and sequentially connected in parallel with an auxiliary cell using a network of 10 external low RDS(ON) NMOS transistors controlled by the LTC3305. The LTC3305 lead acid battery balancer is the industry’s first and only active lead-acid balancer that enables individual batteries in a series-connected stack to be balanced to each other.įigure 2: Complete 4-battery balancer (a) and related voltage curves (b)įigure 2a shows an application in which a single LTC3305 is used to balance four series-connected lead-acid batteries. The control circuitry is complex and a discrete implementation is large and costly. 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 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. 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. To increase battery stack life, individual batteries in a stack need to be balanced. This reduces the ability of the battery to accept a full charge, and undercharging worsens. Undercharging lead-acid batteries causes plate sulfation in which the sulfuric acid reacts with the plates to form lead sulfate crystals. Sealed lead-acid (SLA) and gel batteries are particularly sensitive to overcharging since any lost water cannot be replaced. Furthermore, since the electrolyte level has dropped, a portion of the plates are now exposed to air, causing plate oxidation and reducing battery capacity. The concentration of the sulfuric acid in the electrolyte increases, which is damaging to the battery plates and reduces battery life. ![]() Overcharging lead-acid batteries causes the electrolyte water to break into oxygen and hydrogen gas, which depletes electrolyte levels in the batteries. ![]() Both overcharging and undercharging lead-acid batteries causes battery life degradation. 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. 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. 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.
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