Any and all fire extinguishers should be inspected at least once a month. If you live in an area of severe hot or cold temperatures, it is wise to inspect them more often.
Yearly
To recharge a fire extinguisher properly, you should take it to a professional fire equipment service company. They will empty the extinguisher, inspect it for damage, refill it with the correct extinguishing agent, and pressurize it to the appropriate level. It is important to have this done by a trained technician to ensure the extinguisher is ready for use in case of a fire.
When there's a fireWhen there's an inspectionYou should pretty much always have a fire extinguisher on a boat. This may not be necessary for some types of boats; it's kind of unlikely that an aluminium canoe or rowboat is going to catch on fire, for example, but on any boat powered by an engine you should have one (or more).
To properly refill a fire extinguisher, you should take it to a professional fire equipment service company. They will empty the extinguisher, inspect it for damage, refill it with the correct type of extinguishing agent, and pressurize it to the appropriate level. This ensures that the fire extinguisher is ready for use in case of an emergency.
You periodically check a fire extinguisher to ensure it will be ready and functional if you need it.
To properly recharge a fire extinguisher, you should take it to a professional fire equipment service company. They will empty the extinguisher, inspect it for damage, refill it with the correct extinguishing agent, and pressurize it to the proper level. This ensures it is ready for use in case of an emergency.
Fire extinguisher
Commercially speaking, in the UK they have to be checked annually, and signed.
Flammable metals often require special chemicals to extinguish, assuming there are any. So, in fact, you WOULD use a "chemical extinguisher", but probably not an ordinary dry chemical extinguisher.
A Class C fire extinguisher should be used for electrical fires.
the base of the fire
NFPA 1 (National Fire Code) requires that fire extinguishers be inspected every 30 days or less and maintained no less than yearly. The annual maintenance, if any, depends upon the type of extinguisher and the conditions under which it is kept, among other things.