A new charging algorithm addresses the problems of short genset battery life and sudden battery failure discussed above, while meeting the regulatory needs to deliver non-stop DC power to critical applications.

The principle is simple and straightforward. Emulate the beneficial intermittent vehicle charging regime for which SLI batteries were originally designed, but continue charger operation to support continuous DC loads. This “have your cake and eat it” approach reduces battery charging voltage to just above battery open-circuit voltage for most of the time that the battery would have been on continuous float charge. In this manner the charger remains available to power DC loads, preventing them from discharging the battery. To insure that the battery remains fully charged the charger periodically increases its voltage to insure that the battery remains at full capacity and capability to perform its duties. This arrangement thus emulates the intermittent type of charging that an SLI battery installed in a vehicle would see.

By significantly reducing the duration of separator-damaging float charging voltage, this charge regime reduces the rate at which polyethylene separators suffer oxidation degradation. This improvement in turn reduces the frequency of premature and catastrophic battery failures when SLI batteries are used in genset or fire pump applications. Reducing the time that the battery is actively charged also significantly cuts the amount of electrolyte lost due to electrolysis.

Four different charging voltages are employed in the intermittent charging system that is controlled by a microprocessor. Figure 1 and the descriptions below it describe operation.

At the start time of zero T0, we assume a discharged battery. The charger starts operation in the Boost mode VB. The charger maintains VB until T2, when the charger’s controller causes it to shift to the Float mode. Note that the correct duration of the Boost mode is variable, and depends on many factors.

Duration of Float mode at VF could be either fixed or variable. The only purpose of Float mode is to fully complete battery recharge. If the battery becomes fully charged during the boost charge there is no need to float charge the battery. Because the battery will spend very little time in Float mode the exact value of the float voltage, which in the case of stationary (not starting) batteries is all-important to battery life, is now of little importance.

When the Float mode ends at T3 the charger switches to a new mode, referred to here as Eco-float mode, VEF. The charger remains in Eco-float mode the time T3 to T4, which is measured and governed by the charger’s microprocessor.

When the Eco-float timer expires at T4 the charger switches to another new charging mode called Refresh charge, at voltage VR. The charger remains in Refresh mode for a predefined period T4 to T5. At the end of the Refresh time T5 the charger returns to Eco-float mode.

The alternating cycle of Eco-float to refresh mode repeats until an AC failure or battery discharge occurs, in which case the charger reverts to T0.

The ideal ratio of time the charger operates at the Eco-float voltage versus Refresh voltage varies depending on what the charger designer is attempting to optimize. Closely mimicking vehicle charging, for example, might result in a ratio of Eco-float to Refresh hours of somewhere around 18:1, depending on the user’s assumptions. Some SLI battery designers indicate that maximum flooded SLI battery life would be achieved using ratios much greater than 18:1. Regardless, both design goals agree that flooded SLI batteries should spend the majority of time at Eco-float voltage versus any other charging voltage, provided that there is regular refreshing charge.

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