24v vs 28v power systems

Mike O'Dell mo at 131.ccr.org
Fri Jul 27 10:40:48 CDT 2012

i've always wondered why aircraft power systems are
always described as 28VDC instead of 24VDC like elsewhere.
is that because the aircraft engineers can evaluate
14 times 2 and everyone else is using 12 instead of 14
"just because"? but the automotive guys got it right
with 14 times 3 giving 42, so 14 times 2 is indeed 28VDC.

what I do know is that if your 6 cell lead-acid battery
is down to a real 12VDC at the terminals, you have probably
discharged it rather more than is good for its long-term health.
(the usual rule of thumb for a deep-cycle battery plant
to be sized for no more than 50% depth of discharge
and and 35% dod would be even better for long life.)

owning a big battery plant is a considerable pain in 
the be-hind because it only takes one screw-up to 
drastically shorten the life. and i know from painful
experience that a plant with eight size 8D AGM batteries 
is expensive to screw-up. it is a "24VDC" system wired as 
two banks of (2 strings of (2 jars in series) in parallel) 
with the banks normally operated in parallel. 

i'm in the process of implementing a new DC power monitoring
system, but alas, most "marine battery monitors" are designed
around using a low-side shunt and certainly not designed
for as many battery strings and charging sources as are involved.

i want to use Hall-Effect DC "Current Transformers" (DCCTs - don't blame
me, that's what they are called, by analogy to the AC devices)
so I can do it all high-side and not have the measurement thing
in the high-current path. that causes a lot of grief mounting
and packaging the shunts and most of them have the signal
conditioning *not* at the shunt, which is another big problem
of several dimensions. luckily the string voltage is low so
common-mode float is not a problem measuring the voltage
across individual jars. by contrast, in a large UPS system,
or plug-in EV for that matter, the strings are on the order
of 300 volts so the measurement hardware on jars at the top
of the string have to be able to float on top of that with
adequate isolation from the rest of the measurement system.

another issue is the dynamic range of measurements. since the
primary reason to measure the current is to integrate over time
to get total charge transferred into or out of the batteries
(time integral of current is coulombs), the DCCTs need to
be sensitive and low-noise but also withstand large spikes
without clipping. that means one must use the DCCTs that employ
more complex measurement techniques, but they are readily
available. That does mean, however that the Effective Number
Of Bits in the A/D converter reading the sensor output 
may have to be larger than it might first appear - simply because
of the required dynamic range - from a few milliamps to 
a couple of hundred amps. for bi-polar flow (charge and discharge)
200 amps full-scale really needs 16 bits - 15 bits + sign.

and i bet you're tired of this topic by now. (grin)


More information about the Tacos mailing list