Posted by Marc F Hult on September 17, 2005, 4:33 pm
FWIW, here's what I do (use fixed-spacing font to view ASCII diagram):
Home Automation Control System
28vdc NG Genset -->| Switch over------+
AC line --+--60amp charger/converter -+-- 14vdc batteries ---o 28vdc
| | | |
+--80amp charger/converter -+-- 14vdc batteries ---o 14vdc
| | |
+-------+--------+-------------o vdc 'grnd'
A bank of deep-cycle lead batteries is series connected to provide 14vdc
(nominal 12vdc) and 28vdc (nominal 24vdc).
Most loads including home automation computers, UPS, and low-voltage light
dimmers are connected to the 28vdc output.
Home control and other devices that can run off 12vdc (security panel, home
automation controllers, video modulators multiplexors, cameras, audio
equipment) are connected to the 14vdc rail.
Devices that need other voltages are supplied via DC-DC converters (eg 5vdv
for microcontrollers, router, switches and so on) or DC-AC inverters (eg
24VAC for HVAC)
The 'lower' (0-14vdc) bank of batteries is recharged and powered by a
UL-listed 80 amp Inteli-power (one "l") charger/supply
The 'upper bank' (14-28vdc) is recharged and powered by an 60 amp
Together they provide nominal 60 amps at 28vdc and 20 amps 14vdc =~2000 watts.
A Baldor natural gas 28vdc genset has been purchased (not yet installed) to
kick in during power outages that has additional capacity for additional 28vdc
and 120VAC UPS needs. Controller is a Bouchette A120
The choice of 28vdc was driven in large part because this the highest nominal
voltage that meets the requirements of the under-30-volt sections of the
National Electrical code (NEC). A 14vdc system would require conductors
(wires) with twice the cross-sectional area to have the same distribution IR
This provides isolated, UL-listed, NEC-compliant single-conversion power for
the most/all of critically needed devices including lighting. There is no
interruption whatever if line power goes down because the system is connected
directly to batteries at all times. A power outage simply changes the charging
source from line AC to genset.
The system can be expanded to higher voltages or currents with additional
batteries and chargers. and additional charging sources (eg photovoltaic)
DC Grounding and maintaining isolation of subsystems that need to have
separate "grounds" is _very_ important and a whole 'nuther topic.
HTH ... Marc
Posted by Frank Olson on September 17, 2005, 9:31 pm
Sealed lead acid batteries don't vent hydrogen like a standard wet cell
(that only happens if the charger malfunctions). In fact they're designed
to "recycle" it within the battery. Here's a link to a fairly well known
GS Batteries has been around for many years. They're the only batteries
Edwards installs in their fire alarm systems (in Canada at least). The Rep
for Western Canada used to show a video of the manufacturing plant in Japan.
The building was huge (covered many acres), and there were only seven people
working there because everything's automated (at least back in 1987). I'm
not entirely sure that most of the processes weren't controlled by earlier
versions of Elk Magic Modules... :-))
Posted by John Beardmore on September 17, 2005, 9:49 pm
How much hydrogen do you think will be evolved ?
Posted by Marc F Hult on September 17, 2005, 10:22 pm
Depends on how smart the charger is, condition of battery, how quickly you want
to recharge the battery after it has been drained, how deeply you drain cycle
it, how anxious you are to prevent stratification of the electrolyte and
sulfation -- among other things ;-)
If it is a standby systems that is seldom actually used (as opposed to a
diurnally cycled system with eg photovoltaics) the use of a smart charger like
in the url below should keep H2 concentration in even lightly convected air to
below the level of concern for explosion unless the space itself is too small.
There are hydrogen sensors one could use for monitoring. My now-dated experience
with them was that they were more trouble than they were worth. Presumably they
My qualitative experience on what is "too much hydrogen" is limited to having
one largish battery blow up in my face 20 years ago. It hurt (including pride)
but miraculously I suffered no permanent damage.
Posted by Marc F Hult on September 18, 2005, 2:11 am
Cheers? Now you've got me fretting/thinking again ;-) .
Years ago I built a mobile environmental research lab that carried a couple
hundred cubic feet of H2. The lab was quite air tight and I'd work and sleep in
it during Minnesota winters. The H2 was being continually used for scientific
instrumentation to create a flame so it wasn't immediately discernable whether
some was escaping into the lab atmosphere. The lower explosive limit of H2 is
only about 4% ...
If someone has authoritative information on current best management practices
for hydrogen from small battery banks such as we are discussing, I'd appreciate
knowing. Also reasonable priced H2 sensors. A quick f/google showed me that
they are still an obstacle to progress.
TIA ... Marc