Canon SPEEDLITE EL-1 Advanced User's Manual page 30

Caution
When you disconnect the charger from the outlet, do not touch the charger plug for
about 10 seconds.
When the remaining battery level is 90% or more, charging will not be performed.
Note
Battery storage
Store in a cool, dry and well-ventilated place.
Even when the battery is removed from the Speedlite, a very small amount of
current may flow inside the battery. This results in over-discharging when the
battery is left in this state for a long time, so you may not be able to use the battery
even if you charge it.
When storing the battery for a long time, charge it about once a year to about 50%
before storing it.
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