In article , Paul Keinanen
writes:

On Tue, 24 Feb 2004 02:05:39 GMT, U wrote:

Solar panels and all that stuff is quite expensive, but old car batteries
("old" is when it doesn't get my Diesel started in subzero temps) are not.

While a cold diesel is hard to start, the capacity of a car battery
drops drastically at cold temperatures and assuming you are referring
to temperatures below 0 F (not 0 C), the capacity of the "old" battery
is well below nominal even in warm conditions.

Yet they DO start. :-)

I'm familiar with north country snow, ice, and freezing temperatures,
learned to drive in such environments. Automobiles are OUTSIDE
and have to take the entire change of temperatures. An emergency
power backup system does NOT have to have its battery out in the
cold...or hot.

So even if you can get those cheaply, you may have to have two to get
to the nominal capacity. The optimal float charging voltage may be
different for different old batteries and this can be a problem when
connecting multiple batteries in parallel.

Not absolutely necessary. 12 Volt automobile batteries designed
for LARGE vehicles have rather high ampere-hour ratings.

Do NOT - absolutely - keep batteries in parallel without series
diodes or other protections! One or more batteries in parallel can
effectively "cook" one that is low in voltage.

Batteries designed for car starter duty often expect some amount of
vibration, which they do not get when in stationary use.

Outside of bad design and poor quality control, the vehicle battery
source of failure is almost always excessive vibration. When plate
material is shaken off, it collects in the case bottom and can
short out neighboring plates.

Unless you get several batteries for free, it is not such a big
bargain as it would look like. I don't know about the car battery
final disposal costs, but if they are high, collecting a large number
of reduced capacity old batteries might be quite expensive when they
are finally disposed.

At most auto parts stores in the USA, one can get a medium-size
vehicle lead-acid battery for about $30 and trading in the old one.
Prices vary slightly according to region and store. Factory-new
batteries, such as Sears "Die-Hard" brand, are available for about
twice that, no trade-in. Auto dealership-service centers will
probably charge three times that (got stuck for that last year).
There is a great variation in lead-acid vehicle battery cost over here
so it is worth it to shop around.

My eye doctors' office uses two 12 V lead-acid vehicle batteries for
its servers (6 opthalmologists, 2 optometrists)...but only because
they have an old UPS designed for 24 V battery back-up. Last
year they moved that UPS and batteries to a concealed cabinet
instead of being beside the receptionists' desk where all who could
see saw it. Much better appearance. :-)

By the way, a small 12 V lead-acid battery designed for motorcycles
ought to be good for a heavy QRP rig.

Len Anderson
retired (from regular hours) electronic engineer person