Instead of polyethylene, AGM type SLI batteries typically employ separators made of fiberglass. Because fiberglass does not oxidize like organic polyethylene, float charging at the battery manufacturer’s recommended float voltage is not inherently harmful to AGM type SLI batteries. Therefore, the intermittent charging method discussed is unlikely to increase the life of AGM type batteries. The differing tolerance for float charging between flooded and AGM types of SLI batteries raises a question whether AGM batteries would be harmed by the intermittent charging plan presented. The manual for a recently purchased AGM starting battery recommended a relatively low voltage for float applications, and a higher charging voltage when employed for engine starting. Both charging schemes appear to be viable. Because intermittent charging system presented in this paper substantially emulates vehicle battery charging, and because AGM starting batteries are sold for vehicle starting, it is reasonable to conclude that the method discussed in this paper is acceptable for use with AGM type SLI batteries. Put another way, arguing that this intermittent charge regime harms AGM batteries would be tantamount to declaring that AGM SLI batteries are inherently unsuited for engine starting duty either in gensets or vehicles. From this we reach the conclusion that this intermittent charging system clearly benefits flooded types of SLI batteries, but will not harm AGM type starting batteries any more than installing them in a vehicle would.

Some makers of SLI batteries provide recommendations on how to float charge their products. Such recommendations conflict with the intermittent charging regime discussed here.

The likely explanations for this conflict are as follows:

-Although SLI batteries were never designed for float charging, battery manufacturers needed to provide some sort of guidance to the group of customers including genset suppliers that are required by regulatory authorities to float charge the battery;

-That in the cases where battery makers recommend float charging flooded SLI batteries that this is done solely to satisfy customer demands for float charging guidance;

-That the apparent lack of teardown analysis differentiating genset batteries from vehicle batteries has kept battery manufacturers from thoroughly understanding why the same batteries live shorter useful lives in gensets than in vehicles;

-The lack of understanding of these mechanisms has until now inhibited the development of battery charger technology that could meet the various competing requirements of battery performance, longevity and regulatory compliance.

Float-type battery chargers: Float-type battery chargers, including those with “multi-rate” float and boost charging modes - even when the chargers are adjusted to the battery manufacturer’s specified charging voltage value - cannot solve the separator oxidation problem. This is because the requirement to operate continuously in the float mode is the root cause of polyethylene separator failure. Float-type battery chargers therefore contribute to, rather than solve the problem of premature separator failure.

On/off type battery chargers: One potential solution for emulating automotive duty would be to apply the crude battery charger regulation technique of on/off charging. This technique regulates the charger’s output by turning the charger full on when voltage drops below a preset value indicating partial discharge, and then turning it off when battery voltage rises above another present value that represents the battery achieving full charge. One potential benefit of this scheme is that charging is shut down periodically, as it would be in the vehicle application for which SLI batteries are designed. There are, however, two significant problems with on/off regulation schemes. The first problem is that such schemes cannot be classified by regulatory authorities as “float” chargers, since they turn off some of the time. A second, more significant, problem is that such chargers perform poorly when tasked to support periodic or constant DC loads that are typically connected in parallel with the battery and charger in emergency generators. When faced with continuous DC loads on/off type chargers would cause the battery’s active materials to wear prematurely because of repeated charge and discharge cycles.

In contrast, the intermittent charging mechanism discussed in this paper enables useful reduction in the rate of separator oxidation by emulating the charging duty cycle of the average vehicle, while allowing the charger to deliver continuous DC to power connected loads, and also to be classified for regulatory purposes as a float charger.

Highly configurable battery chargers: Some chargers offer sufficient programmability as to enable battery charging at voltages near battery open-circuit, similar to the Eco-float mode discussed above. This apparent similarity, however, to the intermittent charging system presented herein is only superficial. Without clear guidance about how to program highly configurable chargers, non-expert users are unable to exploit such chargers’ adjustable function to achieve useful benefit. In contrast, a commercially available charger employing the intermittent charging discussed contains all necessary operating rules necessary to enjoy the benefits of intermittent charging. Operation is fully automatic, and no user programming is necessary.

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