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Backup Battery Charging - pros and cons

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Posted by Curbie on July 8, 2009, 10:41 pm
Say someone has 48V battery (bank) and inverter and their primary
charge is some alternative energy source (you pick it), what are the
pros and cons of backup charging between properly matched:
1) AC household voltage generator  
2) I.C.E. with an automotive DC alternator?

Ulysses does something like this and I've reading through BobG's posts
on OtherPower along the same lines, my overall question is why?

1)    Aren't most inverters designed for AC household voltage
2)    If you go with AC household voltage generator, doesn't this
provide additional temporary backup against an inverter failure?
3)    Is the I.C.E. with an automotive DC alternator just so much
more efficient for battery charging that it justifies the costs and

I don't know maybe I just missing the "Big Picture", could explain
this to me?



Posted by Jim Wilkins on July 8, 2009, 11:29 pm

What's wrong with a 120VAC transformer-rectifier charger that runs
from the power line or a small generator? I build Variacs and ammeters
into my simple chargers and manually set the current to whatever the
batteries need, according to the temperature / voltage chart on the


Posted by Curbie on July 9, 2009, 12:24 am

You got me by the tail!

I was thinking that the reason someone go through the hassle and
expense of coupling an automotive alternator to an I.C.E. for battery
backup was purely for I.C.E. to electrical output efficiency?

What are the pros and cons going your route vs. just hooking the
I.C.E. household AC generator to the inverter (both options require
backup gen & inverter)?

Please, go easy on me I'm already confused by all this.


Posted by Jim Wilkins on July 9, 2009, 1:39 pm
The charger I use most is an old 12V 6A non-automatic one with a 3A
autotransformer added to feed the transformer. I put in a lighter
outlet for a voltage monitor and for safer connections when
experimenting. The transformer in that charger tolerates the
overvoltage boost from the autotransformer, which lets it equalize an
old battery with high-resistance cells at 16 - 18V.

Most everyone around here has a generator or two, and gas or wood for
hot water and cooking, so they provide the redundancy. My generator is
just large enough to run a saw for storm repairs, small enough to
carry to the damage site. Last time I helped fix a neighbor's larger
genny, then got power from it through a long extension cord.

I've uncovered my drained solar water heater in midwinter and it
barely rose above freezing. There's only a short period in late Feb -
early March when an air collector would help, but the sunny side is
exposed to storms and falling branches, and a nice lilac bush fills
the best location and helps shade that window in summer,

Once daytime highs reach ~40F in mid March the existing passive solar
gain through the windows is nearly enough if I watch the forecast and
open and close the insulated curtains.

Here in New England we can go a month without seeing the sun, this
past one for instance.


Posted by Curbie on July 9, 2009, 3:27 pm

It seems options are real important when playing with alternative
energies and since I'm not quite sure where I'll be with some of these
backup systems, I find very helpful to look at these issues from every
possible perspective, thanks for sharing yours.


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