Comments inserted where appropriate below:
"Peter Bennett" wrote in message
news.com...
On Sat, 26 Mar 2005 20:48:42 -0800, Stephen Trapani
wrote:

Peter Bennett wrote:



Forget about electric heat - it just isn't feasible with the batteries
you have.

Yeah, I've got a Force 10 propane. I'll get that working soon,
hopefully. I just thought maybe I could use the floor electric heater in
a backup situation if I needed to, on the 850watt setting if I need to.
Looks like it won't be good for much else.

If I recall correctly, Group 27 deep cycle batteries are rated at 100
- 110 Ampere-hours, so you have about 200 AH capacity in total. It is
generally recommended that deep cycle batteries should not be
discharged more than half-way, so your usable capacity it only 100 AH.

Your 850 watt heater will draw about 7 amps at 120 V, and the inverter
will draw perhaps 75 - 80 amps (allowing for inverter inefficiencies).

The AH rating of a battery is calculated assuming the battery will be
fully discharged in 20 hours - for your 200 AH battery bank, this
would mean a 10 amp discharge current. If you discharge the battery
at greater than the 20 hour rate, the total energy you can extract is
reduced. Your heater would require about 8 times the 20 hour rate.
The table I have for this only goes up to 5, where the effective
capacity is reduced to 72%. You could probably expect about 60%
capacity, so you could only run the heater for about 40 minutes. (and
then you might have trouble starting the engine.)

I'd go a little further on this estimate. Yes, you shouldn't go below 50%
charge on your batteries which reduces your 200 AH battery bank to an
effective 100 AH total as stated above, but you also will find it difficult
to get your battery bank charged much above 85% by running the engine. An
AC charger plugged into shore power running overnight is about the olny way
you'll do it. It takes too long to trickle those last few amp hours in. So
you've really only 70 AH total to work with in your two group 27's. I'd
forget about the electric heater unless you can stay below 400 watts with
the engine running. The rest of the things you suggested running off the
inverter should be OK due to their intermittant use as explained by others.

You asked a couple of questions in another post:

"And then if I keep running the over-use after the batteries run down,
the inverter overharge alarm will go on? No big deal I can just turn off
the large draw and charge up the batteries before I turn off the Yanmar?"

and:

"One more question, is this sort of usage hard on the batteries at all?
Wear them out faster or anything?"

Running the inverter until the low battery voltage alarm comes on will take
your batteries to a charge level far below 50% which will shorten their life
quite dramatically, perhaps ruining them in just a few of these very deep
cycles.


The microwave, power tools (and possibly an electric
kettle) are OK, as they are short-term loads, and the total
ampere-hours used by them should be fairly low. The TV and boom box
would be on for longer periods, but are fairly low draw, so may also
be OK - but I would definitely want a separate starting battery!

A separate starting battery wouldn't have to be very big, perhaps one the
size used in a lawn tractor would do? It shouldn't be hard to find a place
to put one that small.


So in order to do without the seperate starting battery I can manage
with careful planning, etc?

Stephen

Yes, maybe...

You should look at the current required for each device you want to
use, and how long you will use it, to determine the ampere-hours you
will consume between recharges. A battery monitor like the Xantrex
Link 20 can help you monitor the state of charge of your batteries.

I still strongly recommend adding a separate starting battery - it is
valuable insurance in the event you do discharge the house batteries
too far.

You should also replace the stock voltage regulator on the alternator
with a "smart" or three-stage type - this will allow the engine to
recharge the batteries much more effectively than it does now. A
three-stage AC charger would also be a good investment (there may be
one built into the inverter...)


--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
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