On 26/02/2016 10:59, David wrote:
This first post is a place holder in part, so that I don't forget to ask
the question, as I usually do once I get involved in stuff during the day.

The Motor Home has mains powered habitation air conditioning - that is an
ELectrolux slug on the roof.

As soon as we know the spec, it might be possible to assist.

So for it to work you need to be on a site with mains electricity.

There is also the issue of the power surge on start-up compared to the
power demand on normal running.

What I would like to be able to do:

(1) Run the A/C whilst driving - that would I assume involve an inverter
which could take power through the 12V electrics buffered by the
habitation batteries (unless the demand needs a direct connection to the
alternator charging circuit instead of via the charge controller which
charges the habitation batteries). This also allows starting the engine,
firing up A/C and then stopping the engine and letting the A/C run on
using the habitation batteries.

For a narrow boat I installed a 3kW inverter (with surge capability),
150A alternator and 4 100Ah batteries. It can be done though
challenging. I doubt you'll have the space to fit a second alternator.

(2) Run the A/C when away from mains power - using a small Honda generator
which might be able to meet most of the demand apart from start-up. I am
envisaging perhaps the generator pushing power into the system whilst the
A/C takes power out so that the use of habitation battery charge is
slowed. This probably equates to running a UPS (that is, power in to UPS
for charging, power always out of UPS for running the device) and using a
small generator to keep feeding some power into the UPS during a power
cut. The generator may not fully meet the power demand but it slows the
rate of discharge.

Again without knowing the rating of the air con, and the size of your
portable gen, we're in the dark.

This does seem to demand a lot of inefficiency, though, potentially with
the generator input being converted from 240V to 12V then back again. It
would be nicer if the battery 12V power could be used to boost the 240V
input from the generator so that most of the power comes directly in at
240V.


Bottom line is that one way may require blending two 240V inputs into a
single output, with obvious (I think) requirements to lock the wave forms
of the 240V together.

You cannot "blend" 2 AC voltages.