GL: Battery-inverter-charger questions

[David&Joan]

Jim:

You would only have to run your genset at night if you have 115 V AC loads at
night. The only load that I can envision is air conditioning. Air conditioning
loads are generally too much to run with an inverter. The other potential
loads are a hair dryer and a microwave. These are very short term and don't
require that much DC amphours to power an inverter to operate them. But it
does take a big inverter to operate a hair dryer. You can time your usage of
these loads to coincide with your genset running, so you may not absolutely
need an inverter. The Freedom 25 would run a hair dryer or microwave (but not
at the same time) and also has a 100 amp charger built in. This or even the
Freedom 20 would be a good choice for your system. The inverter/charger will
cost $1000 or more.

So, you will run your genset to charge batteries, not necessarily to run AC
loads. And to charge batteries quickly you need a big charger, so the Freedom
25 inverter/charger is a very good solution. Another solution is the Xantrax
MS2000. This is a true sine wave inverter which you probably don't need.

First figure how much DC loads you have. Refrigeration and lights are the two
biggest loads. With an efficient and well insulated refrigerator, you should
need 100-150 amphours per 24 hours. So, assuming that you will run your genset
or your propulsion engine to recharge batteries once each day, then you will
need about 2X your daily load in amphour capacity. This is to keep from
discharging more than 50% for best battery life.

A house bank of four 6V golf cart batteries wired series/parallel will give
you 440 amphours of capacity at 12V and should work fine for your needs. These
should cost about $400. You could do the same with two 8D AGMs at three times
the price.

Flooded cell batteries like golf carts can accept a charging rate of 25% of
the rated capacity. So you can charge the 440 amphour bank stated above at 100
amps with the Freedom 25 with the generator running for about two hours each
day (that you don't run your propulsion engine).

You need to think about how to wire your DC system to protect the engine and
generator start batteries from being discharged along with the house bank. A
good scheme would be to charge the house bank directly from the
invereter/charger and isolate and charge the starting battery from the echo
charge feature built in to the Freedom. The genset probably has its own
charger that will keep its starting battery charged. Make sure that it too is
isolated from the house bank. This type of system where the start batteries
are isolated and charged by the echo charge output is a set and forget system.
There are no switches to forget to turn.

You probably are going to need a marine electrician to install this system for
you. Find one and talk through the issues noted above to get his ideas.

David