Here is the hopefully happy ending to my problem of providing low power AC electircal
power to a small cabin using batteries, a generator (and now a small solar panel).

I bought a "smart" 40 amp solid state battery charger to properly charge the deep
cycle batteries (the battery charger gets AC from a gas generator) and the first
problem with this was that the charger took "forever" - like maybe at least 4 hours,
I didnt actually find out.. This was a problem since one of my goals with this system
is to minimize the amount of time the sound insulated but still noisy gas AC
generator is running. Second problem is that I burnt out the smart charger the first
day I used it by disconecting it while it was charging. It was a cheap one, I guess I
got what I paid for.

So I went back to 50 amp "Die Hard" transformer battery charger. However, this
charger can easilly damage battieries but if you are careful and monitor DC voltages,
it is a fast way to get the batteries to about 75% capacity - assuming the battery
bank can handle 50 amps (one rule of thumb says you need battery capacity of at least
250 amp hours to handle this charge rate). Using "Die hard" has two problems. First,
it is only safe to charge the batteries to about 75% capicity and batteries really
need to be periodically charged to 100% for longer life. Second problem is that when
it is cold (like for example if you just arrive at the cabin during the winter and
need to charge the battery) charging voltages increase a fair amount and you can
figure out the voltages required for 75% charging from graphs of charge voltage vs
temperature but this is just too complicated and error prone to do manually.

So what I did was aquire an 80 watt solar panel and associated smart battery charger
for the panel. The solar panel smart charger measures and compensates for temp so if
its near the battery, it should nicely charge the battery to 100% during the times
when the cabin is vacant. This is good for battery life and its also nice to have a
100% charge waiting for when you arrive and its fricken cold inside. My peak use is
somewhere around 150 watts so the solar panel also supplies a fair portion of the
load during the day.

I also purchased a battery voltage monitor so that I can tell when the battery energy
capacity gets to around 50% (only discharging to 50% about triples the life of the
batteries. If the capacity gets down to 50%, I can run the generator and use "Die
Hard" to dump 50 amps into the battery bank and get the generator back off as soon as
possible - like maybe 1/2 hour. I have to monitor the generator charging but this is
not such a big deal as by the time I need to use old Die Hard, the room has warmed up
(wood stove) and the battery voltage meter reasonably tells me when the charge
reaches 75%.

One problem I experienced with the solar panel route is mounting it on the roof. As I
understand it, the national electrical code (NEC) requires that roof mounted solar
panels have ground fault isolation. What they are worried about is that the wires
comming from the solar panel get frayed and if the roor is metal, it is supposed to
be grounded (by NEC code) and there could be a spark from the positive wire out of
the solar panel from shorting and igniting pine needles, ect on the roof. Or the two
wires from the solar panel could spark from just touching each other. The ground
fault isolation is fairly expensive to deal with (the hardware must be UL certified,
ect) so usually simple solar panels such as my application are mounted on a pole
rather than the roof.

The approximately 150 watts at 110V (from an inverter running off the battery 12
volt) supplies a 20 inch TV and video source, a 30 watt stereo system (consiting of a
20 gig MP3 player and computer speakers) and three 7 watt flourecant light bulbs. If
I need higher power (such as running a microwave oven, blender or table saw, ect),
the generator needs to be running which is a slight pain. However, you can get over
this fairly fast as this system is likely to cost as little as $1 a weekend in gas to
use and the alternitive wired in AC is $30 a month even if no electricity is used.
Its also nice being off the "grid". I think Ive got about $1500 into it not counting
the nearly $500 in stuff I burnt out and had to trash while paying my dues (some
expenisve AGM batteries and the smart AC powered charger which were ruined). I
believe I have saved a a fair amount by using the generator to supply the occasional
higher load requirements and supplement charging. Admitely somewhat of a cludge and
it requires some attention to run but what the heck..

wh

Walt wrote:

All responses are very much appreciated. I guess Ill have to get a "smart"
battery charger but I know my wife is going to miss the huge "Die Hard" charger
decor.

On the inverter side (supplies the 110V from the battery 12v), the one I use does
have a shut off at 10.5 volts which I beleive is normally defined as 100%
discharge. There is a graph he http://www.lifelinebatteries.com/graphs.asp -
click on life cycle performance. It shows the number of useful charge cycles as a
function of % dishcharge. For this partiuclar battery which is pretty high end,
you get (eyeballing the graph) 340 cycles at 100% dishcarge but at 70% discharge,
you get approx 660 cycles. This would imply to me that if I added an additional
42% capacitiy to get the same total energy capacity (because Im only discharging
70% rather than 100%), that I would double the life of the bank. Please correct
me if Im wrong but this seems an economical thing to do if the battery weight
doesnt matter. So I think I still need something custom that will alert me when
the battery dishcarge is getting near 70% for example based on voltage (or so I
can charge before the lights go out) and also possibly shuts down the 12 volt to
the inverter when the 70% dishchare is reached. There are certainly temperature
effects involved but my problem is only cold temps and all that I beleive happens
with voltage sensing only is that the batteries tend to get under-utilized a
little (which extends life) before they warm up.

"Wayne.B" wrote:

On Fri, 12 Nov 2004 00:37:49 GMT, Walt wrote:
Im leaning towards using cheap car batteries.. The West Marine cat has a
description of charging which has up to four unique charging phases but Im
thinking I will only use the first phase which is the "bulk" phase where you
dump a lot of current into the battery and just shut if off when the voltage
reaches a certain value (like 14.4 volts). This apparently only gets about
75% of the battery capacity charged.

Ill also be shutting off the battery output current when the capacity is
about 40 % used up (also based only on voltage - dont know this voltage
yet..). The reason for this is that I saw a graph of useful cycles for a gel
cell battery (like the one I ruined by overcharging - they now only put out
11.8 volts) and you get more cycles by not discharging as deeply. So Im not
really using the full capacity of the batteries - but no big deal since Ill
just get extra capacity and the weight wont matter (its in a cabin) - and Im
going for relitively cheap batteries. Id still like these batteries to last
so would appreciate any comments..

=====================================

Take a look at the Heart Interface inverter chargers. They do exactly
what you want on the charge side, and have automatic low voltage
shutdown on the inverter. Both charger and inverter modes are totally
automatic with nearly instant transfer switching.

Car batteries and el cheapo marine batteries are a bad choice for this
application, and will experience premature failure because of the deep
discharge/recharge cycles. The most economical batteries for this
type of service are 6 volt golf cart batts, either 2 in series to make
12 volts, or 4 in series-parallel for extra capacity. The best deal
I've seen on them is Sam's Club at $45 each but your local golf course
could probably point you to a good supplier also. West Marine has
them for about $85.