T&T: How Does an Inverter work

[Peter Bennett]

Tuesday, April 15, 2008, 6:35:40 PM, Ken wrote:


KB> 5.) Your battery bank will be rated in amp/hours.

To be pedantic, a deep cycle battery is rated in ampere-hours (amps
times hours) not amps per hour.

KB> This is a bit complicated
KB> as it varies depending on the rate that the amperage is "taken", but for
KB> first approximation the following will be given as example.  I will assume
KB> you have 4 golf-cart batteries.  These I will assume are wired as 
KB> series/parallel.  This will give you about 440 amp/hours total capability.

Again, the rating is ampere-hours (but Ken's calculations are
correct).  Deep cycle batteries are normally rated assuming they will
be fully discharged in 20 hours.  If you discharge them much faster
than the "20 hour" rate, you will get fewer total amp-hours than the
rating would suggest.

KB> 6.) Since you should never discharge your batteries below 50% of full charge
KB> (it hurts them) you really have 440/2 = 220 amp/hours available.  So, you
KB> could run the coffee-maker for 2.2 hours (actually considerably longer since
KB> when it gets to temp after brewing it cycles on/off) so rather lets say you
KB> could dry your hair for 2.2 hours (might have the frizzies, but what the
KB> heck).

KB> This ought to give you an idea of how to predict how long the system can
KB> stay up.  Likely, if your system is like mine (I have a normal household
KB> fridge) then you will have about 3 to 4 amps at 120 VAC running (about 50%
KB> duty cycle, i.e. on/off) so there is 360 to 480 watts right off the bat.  Go
KB> to http://tinyurl.com/4j73s3 for a nice PDF explanation that is more 
KB> in-depth.

KB> Hope this is as clear as mud, and helps,
KB> Ken.

I feel that inverters are good for supplying small AC loads (like a TV
or computer, or a few lights) for a moderate time, or large loads
(kettle, coffee maker, electric frypan) for short periods.  Unless you
have an unbelievably large battery (and corresponding charging
capacity) you shouldn't use an inverter to run an electric heater all
night!

>> Hi all. I recently bought a 37ft Roughwater, my first boat, that I am 
>> living on. I have lots o questions but esp how the inverter works? When I 
>> am anchored I cannot quite figure out (its 2500watts) when and how it 
>> drains? What load I can put on etc.........any help??
>>
>> Lori 

Whatever you run from the inverter, you _must_ have sufficient
charging capacity to replenish the batteries so that they will be
ready for the next use.

-- 
Peter Bennett, VE7CEI    Vancouver, B.C., Canada
Ennos 31 "Honeycomb"
GPS and NMEA info: http://vancouver-webpages.com/peter 
Vancouver Power Squadron: http://vancouver.powersquadron.ca