Your site plan might call for having two chargers, connected to the battery in parallel, to provide redundancy. As you know by now, any two chargers with the same output voltage can be used in parallel, and both will provide load current as needed. The rule on redundancy is that each charger must be rated to supply the entire standing load, plus about 10% more to charge the battery when needed. That means that if you have two differently rated chargers, the smaller must stand by itself, in case the higher rated charger takes a vacation.

This applies even in the case where you have redundant batteries. If you have one charger per battery, treat the charger sizing for each battery as if it were a stand-alone system.

Some sites call for load sharing. Battery chargers don’t need to share the dc load current to work successfully in parallel. Some users require load sharing, though, because it allows them to confirm at a glance that both chargers are operating normally. It also prevents that annoying (and possibly disruptive) false zero-current alarm, which might occur if one charger isn’t contributing load current. There is also a slight improvement in transient response in the event that one charger should fail, but with a battery on line, this isn’t so important.

Check back at the description of the charger failure alarm in the CASM in SECTION 6.3.5. If you have an SCR/SCRF charger, with a CASM installed, and select the True CFA alarm mode, you will be immune from false zero current or charger failure alarms, even without load sharing. For more information on sharing the load between chargers see SECTION 6.4.2.

Note, though, that IEEE Standard 946 recommends that forced load sharing “should” be specified for parallel chargers.